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Or I have this Mercedes and I have questions, part 2.
I have a 2013 133K mile CLS Shooting Brake with the 3L V6 and the fuel economy is dire. The best I've ever managed was 42 on a long steady drive back from Gatwick. It would do mid 30's with bikes on the tow ball rack and high 30's without.
Yesterday I did a steady 70 mile drive, mostly A road and Mway and it did 33! It's been worse since the last service, but I can't return to the garage for reasons that aren't worth wasting many pages on, so don't bother asking...
I've checked the obvious, like tyre pressures and I don't think the brakes are binding; the centre of all the wheels feel similarly warm to the touch after a long drive and there's no burning smell.
I used a mate's Snap-On code reader and there were no codes that would indicate any issues.
Where do I go from here? Am I just looking at huge bills while a different garage chases their tail hunting for an issue?
What was included in the service?
Edit: I'm surprised the fuel economy is as good as you say it is to be honest. That's quite impressive.
The best I’ve ever managed was 42 on a long steady drive back from Gatwick. It would do mid 30’s with bikes on the tow ball rack and high 30’s without.
Yesterday I did a steady 70 mile drive, mostly A road and Mway and it did 33!
what more do you expect from a 10year old 3l V6?
Other than lose as much weight as possible (empty the boot, remove spare tyre etc) and done have thing’s attached to the outside. The only other thing I can think of is gentle acceleration and use the brakes as little as possible by lifting off the accelerator early when slowing down
other than that the only way of reducing cost I can think of will be use the car less.
Sounds like you're getting bang on the average?
https://www.honestjohn.co.uk/realmpg/mercedes-benz/cls-2011/cls350
It's a 3.0l, turbocharged, V6, it's never going to be economical, just slightly more economical than a petrol one.
The only service related thing I can think of is check the oil level, having the oil at the max level can have an impact on the MPG as the engine does a not-negligible amount of work just sloshing the oil around and pumping air back and forth under the cylinders.
Your fuel economy looks good to me!
But if you're confident it used to be better and you don't think it's just down to weather/traffic/ different routes, then I'd start by looking at the air filter, it's a bit clutching at straws,but check it's clean and check it's a genuine Mercedes Benz one.
My gut feel is that there is nothing wrong, but that's where I would look if I thought fuel economy had got worse after a service.
3L v6? hmmm..
My V70 2.4t struggled to get more than 30mpg mixed driving, 40 on the motorway (under the speed limit, diriving super peogressive).. It would return about 20mpg avg when 'making progress'.
How fast are you driving on the motorway? If you drop the speed from 70mph to 55ish it will make a significant difference to the economy...
The only turbo car I had would change from 32mpg (ish) to 18mpg (ish) just by ‘cruising’ 5mph faster.
The key to getting the best mpg was staying below the revs where the turbo kicked in.
Om642 lump or petrol?
My om642 7g is about the same. Cruise set at 60ish obc reads 42mpg, reality its 38mpg.
Cold starts seem to hit the mpg really hard. Mines got aux cabin heater and i think thats a drain on the battery and alternator. Plus 9c amd its 4mpg better straight away
Headwind? That's can make a decent difference especially if you're doing 70mph or more.
It's still better than the mid-20's mpg I used to get from my 3.0l, flat 6 AWD Subaru Legacy!
You're trying to beat physics, I'm afraid - high 30's to 33mpg isn't that much, as above a headwind, or a heavy right foot and it'll easily done.
How many miles are you actually doing a year? 33 Vs 38mpg is £300 a year difference if you're doing 10k miles a year.
My best with that engine in my old Vito was about 32mpg
A carbon clean or terraclean might help.
Folks used to get 35 out of their 350cdi. E class coupe
For 240bhp it was never very inspiring and was quite choppy on the road so not even a good cruiser.
The c coupe 220cdi they had before would get 45 mpg and both handled better and was comfier somehow.
How are you measuring it? What the dash says or full tank to full tank?
Take out a couple of pistons? 30 doesn't seem at all bad for a car of that type, and it's pretty common for economy to fall off a little as cars age. A carbon clean <might> help, but I'd not put much faith in it, and the cost of the clean buys a fair amount of petrol. It's probably nothing to do with the service that you mentioned, the oil used can make a little difference but not much at all.
Tyres can make a big difference- pressures too, but a sensible efficient tyre can give good performance and add a few mpgs compared to a sticky or just less efficient tyre. Ovicously expensive, one for when they need replaced.
In the end there's generally not a lot you can change, that's cheap enough to be worth it when you offset it against the price of petrol, except for thngs like tyres where there's no real additional cost, or things like driving style. Like, a good remap can often get efficiency improvements but takes ages to break even.
Is it just the difference between winter & summer? I get worse mpg in the winter.
How are you measuring it? What the dash says or full tank to full tank?
What the dash says, but I'm comparing all readings from that.
Is it just the difference between winter & summer? I get worse mpg in the winter.
That has been suggested, but it was the sudden change after the service that has me worried.
Om642 lump or petrol?
Diesel.
What was included in the service?
Edit: I’m surprised the fuel economy is as good as you say it is to be honest. That’s quite impressive.
Merc schedule B service which is basically oil and filter.
It used to be better...
what more do you expect from a 10year old 3l V6?
It used to be significantly better. If something is wrong, I'd like to get a handle on it before it becomes terminal.
Headwind?
It was an out and back trip, so unlikely.
I've asked the exact same question on the owner's page on FB and most people are claiming much better figures.
Edit, you've just said diesel 😀 i was guessing petrol...so what i wrote below is from that angle
Sounds ok, much better than people are getting in their golf r's etc
I don't believe its that good either... Im going to say you are going by the computer and not tank to tank
Auto or manual? for reference my old 530d wouldnt beat 32mpg in auto form, i got 10mpg by converting to manual, had so much of an impact
anything you get back will literally be 1 or 2 mpg which is going to make bugger all difference on your wallet.
In perspective, my old 525 petrol wouldn't beat 23MPG (mixed) iirc and that was after me spending a shed load of time tweaking...
Oh, and switching to premium diesel made no discernible difference, other than to my credit card bill.
More clutching at straws from me ,but the wrong type of oil can make a noticeable difference to fuel economy,so if the fuel economy genuinely is worse after the service and all they did was change oil and filter then it could be the oil.
Thermostat on the way out? When I bought our Galaxy it was shot, and that knocked 10mpg off the car with it stuck open and cold engine.
One other variable was new front tyres at the time or service. It had Continental Sport Contacts on before and Avon ZV7 after.
The Conti's are rated C for fuel efficiency, the Avons are rated D, could the tyres really make that much difference?
Can you define significantly better? What is the drop we are talking about?
It went from high 30's on a run without trying too hard, to 33 while trying really hard to maximise efficiency.
Really taking a stab in the dark here perhaps wrong diff oil or transmission fluid? Failing lock up clutch in the torque converter (I’ve never heard of this happening but I suppose it could lead to really poor fuel economy).
Oil burner you say, oooh also consider partially clogged cat or dpf. In your diagnostic report does it state ash and soot levels?
Another thing; before the service, there was a noticeable difference to fuel economy when using the tow bar rack (3-5mpg) now it's 1-2mpg lower with the rack on.
could the tyres really make that much difference?
Possibly, in the absence of anything else I'd blame the tyres,but it's an expensive experiment to swap back and see if things improve.
It's more likely to be due to tyre than the wrong oil
Is the tow bar rack electrics properly integrated? On VWs plugging anything into the towbar electrics changes DSG engagement routines and shift points - effectively the gearbox and engine switches into a different, sloppier map. My how we laughed at my brother when his (seriously fettled) superb lost 100hp and many torques from its custom map after plugging in his bike rack. Has it become stuck in towbar mode?
Is the tow bar rack electrics properly integrated?
No idea, it was fitted before I bought the car.
Has it become stuck in towbar mode?
No idea if it even has a towing map to be honest.
i think merc had to fix a lot of cars because of dieselgate, so it might be that? has it been in a merc dealer recently?
my 2010 v6 viano which is a 2t van with windows and seats etc can do mid 30s but you have to drive very carefully.
my wife gets 30 out of her v70 d5, but if i drive it on a motorway in granny mode i actually have managed 50mpg. i was tooling along at 60 though.
if it wasnt a main dealer, what exactly did they do? just a oil and filters?
is the lock up clutch lokcing up?
OK ,
Its unlikely to be the oil as long as its 5w-30 fully synth. B service wont have touched gearbox , that 40,000 for a partial change . Diffs are another long interval and wont have been done on a B .
The Batwing mass air sensor is a tricky bugger to refit so I would take a good look at the airbox and the PCV. The diaphram fails on these so it may be related to that. On the E you have to remove the airboxes to refit the filter although its above the MAF if its been put back wrong it might make a differnce.
Tracking done with tyre swap ? That might not help.
I dont think the Daimler 7g tronic has torque converter lock up , at least either mines borked or it drives with the TC in suish mode permanently.
DPF full ? My car dosent have a DPF so cant comment on the longevity, but with the chassis and box all contributing to wafting its exactly the type of car that will soot up.
has it been in a merc dealer recently?
Merc Indy.
just a oil and filters?
Pretty much. Plus looked at a load of stuff.
is the lock up clutch lokcing up?
No idea, how could I tell? The same garage changed the gearbox oil several months before the service.
Edited to add - most of these have been covered above.
Mass air flow sensor working? I'm guessing on yours it would flag a fault code but my HDI Berlingo MAF was dead and it resorted to a default fuelling map that overfuelled.
Tracking? I do my own tracking by the string method and set my cars up with no toe in or out. Again my little Berlingo rolls so well.
Tyres? Again....my stupid little Berlingo, it's on Continentals and they must be rock hard as they last for ages yet still manage to grip and must have good rolling resistance.
Oil viscosity - has the garage filled it with 5-40w or something equally more viscous than should have been used?
Deiselgate update could be a good one.
Similarly could it already have had an aftermarket tune that has been wiped during an update?
There are sometimes sensors that can be out of spec without bringing up a fault, a decent auto spark or mechanic might be able to run through them to check.
Things like the engine cooling fan permanently running can also cause a drop in efficiency, I've heard the one on my van can pull about 20-30bhp from the engine at high revs (it certainly makes a difference to engine braking when it switches on and off)
May require dremel:
I've got a 2011 CLS (same model as yours but the saloon) 350 cdi 3lv6 with 64K on the clock and my average over thousands of miles is 38mpg. On the motorway I get low to mid 40s if I do 70 ish, nearer 50mpg if I do 60, I managed 55 by sitting with the trucks at 55mph on a 200 mile round trip...but only the once, mpg has been a bit off lately as driving with the heater on burns more juice.
Your mpg sounds a bit meh.
Plan your journeys to be downhill.
That doesn’t seem very good. My old 350cdi estate would easily do +40mpg on a run. Even my last e43 (3.0 petrol) would do mid 30’s on a run.
I’d get it to a Merc indie with the right software to get it checked over.
One other variable was new front tyres at the time or service. It had Continental Sport Contacts on before and Avon ZV7 after.
The Conti’s are rated C for fuel efficiency, the Avons are rated D, could the tyres really make that much difference?
There's your answer!
New tyres are probably 6-7mm more thread, and probably have more air to boot. And car tyres, like bikes, are woefully inconsistent between brands which could compound it further.
13mm on a tyre is about 2% of it's diameter.
Old ones underinflated by 5 psi or a slow puncture? maybe another 5%?
The bigger the wheel, the faster you actually go for the same speedo and odometer readings. The fuel economy is dictated by the actual speed though. You could very well be driving quite a bit quicker than you were.
Does it have “learned driving style”? Could this have been reset during service. If so it could take a few hundred miles to sort it out/settle down.
Does it have “learned driving style”? Could this have been reset during service. If so it could take a few hundred miles to sort it out/settle down.
I've done nearly 3000 miles since service.
TINAS might well have it, I was purely thinking about rolling resistance but rolling radius could entirely mess up your maths.
The ZV7's supposed to be a decent tyre with decent wear, but tbf continental make really bloody good tyres.
I have the exact same car but with the twin turbo 4 cylinder, and I get 55mpg on general motorway driving, although driving to my work office (2hrs) repeatably gets about 59mpg.
All I do is put the cruise control on 70mph and leave it there as long as possible.
ISTR the OP's car is the AMG Sport spec, same as mine, which usually only means sport suspension, a different bumper and sporty wheels which are probably 255/35/R19 on the front and 285/30/R19 on the back. I could only get 50mpg at best on the ones it came with which were Hanook something/Pirelli P-Zero. I changed the wheels to 18s and the tyres to Goodyear Eagle F1 Asymmetric 5 in 255/40/R18 and it gained me about 10% in fuel economy. I think partly due to size and partly because you can only get really sporty tyres in those sizes. When I went to Scotland with three people, a full load of gear and one bike on the back I averaged 50mpg. However, around slow suburban driving on streets it is only just over 33mpg.
The best premium tyres for fuel economy on that kind of car seem to be Bridgestone Turanza T005 and yes, I have looked into this a lot 🙂
It does sound like something's not quite right with the car - the drag of two more cylinders shouldn't account for 30% fuel efficiency difference and unlike with a traditional petrol car, larger displacement doesn't require more fuel to move the car.
As said there could be things like MAF which may be functioning but giving poor readings. That would normally lead to smoke, if it's over-fuelling, but you wouldn't see that because of the DPF. That would get blocked quickly, and then lots more fuel would be used to clear it out. So I'd be looking there. If you can get some diagnostic data, try and figure out how many regens it's doing. A small boost leak could have the same effect.
Where are you, by the way? I was considering getting a diagnostic tool for mine.
No idea, how could I tell?
When mine had an inlet manifold leak it went to limp mode and it also prevented the torque converter lock operating. With the lock, the gear changes weren't rapid but they were positive and solid, it was ver much 'in gear'. Without, the engine didn't have a fixed connection to the wheels so it felt like a slipping clutch all the time and the gears sort of blended into each other.
Another common fault is boost hose splits.
Iirc its between the intercooler and inlet
Theres another pcv inlet which lets a tiny bit of oil in the pipe.
This causes a weakness and at boost they can split.
Undertray off, 8mm self tappers, gloves on, squeeze pipes whilst shining your mtb lights on the hose.
Other wise, when you do a 7g plus gearbox oil it only changes 3/4 of it. The oil in the tc and cooler are still old and its blended together, best thing isc2 x changes or a torque converter drain , and cooler flush.
I think the daft system is why these boxes fail, 3 changes in at 120 000 miles you will still have some of the original oil 236.15 in there.
I asked specifically for a torque converter drain when I had mine done. It's not that hard, there are videos for it, it's just an extra few steps.
How quick is the car? It should be fairly rapid by normal car standards. When mine was in limp mode it was still quite driveable unlike the Passat which would only do about 40mph on the flat and much less uphill. It's just about possible that with that much displacement it's been in limp mode all along and he hasn't noticed if he's not sure what to expect from it.
According to Parkers, the fuel economy is 46 for the 350 and 52 for the 250, so approx 13% difference. I could just about get 52 on my original wheels, so the OP should be getting around 46 if he's careful.
How quick is the car?
Very. If I hoof it it squats it’s arse down and takes off like a scalded cat.
I’ve been in limp mode once (long before the service) so I know what it drives like then.
I was thinking air filters as neither schedule mandates changing them but that wouldn’t explain the marked difference that started at a specific time.
I was thinking air filters as neither schedule mandates changing them but that wouldn’t explain the marked difference that started at a specific time.
It would explain it if a hose or sensor plug has been removed and not put back properly.
On the Passat if you unplug the MAF it uses a default value, so it still drove apparently fine but not optimally. That's a possibility. There are procedures you can do to test a MAF apparently, but I've only tried this on VAG.
Air and fuel filters are on a separate 45k mile schedule so they don't necessarily get done at the regular service.
I do recommend looking for new wheels in a different size next time you need tyres. It's expensive though, since they are a rare size and you then also need to change all four tyres at the same time to go with. 18s are much more comfortable though.
I missed the part where you said it was getting 33 after the service. I would take it to somewhere with STAR to get a full readout - generic code readers may not give you all the information.
I don't know Merc's, but does it have a DPF? Assuming it does, how frequently is it regen'ing? As the filter fills with soot, it'll regen more frequently, which increases fuel consumption.
Usually an expensive fix though ☹️
OM642 owner here in an E Class Estate, done 37mpg average over more than 30'000 miles from when I changed the battery and it reset.
Your figures are not 'abnormal' at all however theres a few things to consider
- Dieselgate software scam - it is widely reported that on cars with OM642 and 7GTronic, post your 'free software update' that gearbox does not now change into top until well over 70mph. Which is appaling. And once you've had it done, they wont undo it (coz emissions, init).
- 7Gtronic does have lockup n every gear, but it could well be due a fluid change (They are NOT good for life!)
- Thermostat is a common failure, sometimes not so obvsious due to heavily damped temperatue gauge. Should be rock solid once up to temp, and fluctuation at all indicates duff stat. Very easy fix, about £100 for Genuine Merc part. Front and centre on the engine.
- Check the batwing, the Orange turbo seal and the smaller PCV seal should be replaced at every service as they fail but crucially its very easy to badly fit them. Turn your radio and heater off, boot it, and see if you can hear a faint boost leak noise from just in front of the dashboard... Common! Again, dirt cheap from Merc.
- Fuel filter is every B service. Make sure its been done. Make sure its a decent component.
- Tyres can and do make a difference.
- Fundamentally, we get the same figures. For me, 37mpg over >30000 miles is brilliant, frankly. Its a 2 tonne car once loaded. It'll hi tthe limiter at 155mph, and launch off a sliproad onto the motorway faster than my Lotus does. You can sit at 70+a bit for 5 hours down the length of England no problem and not a hint of fatigue. I think you might have slightly unrealistic expectations, buy a miserable 4 pot Octavia DI if you want mega MPG but it wont do any of that stuff as well.
ETA - in the short term, relating to your recent drop post service - check the batwing. Theres 2 MAFs whch could be damaged or not plugged in correctly (although this was flag a CEL), and turbo seals is an obvious one. Check the coolant running temp with an OBD on a long run if you can too.
And please tell me that it didn go to Merc for the service, as if so, your probably a victim of a 'software update' and guess what, your stuck with it.
@snotrag I'm in Evesham, Worcestershire.
Re Dieselgate, the service was done at an indy, not a main dealer, so I would hope it hasn't been done. However, I have noticed that it seems "reluctant" to shift into 7th on A roads, but OK on Mways.
Temp gauge is rock solid.
Gearbox serviced last year.
Batwing? Is that the intake piework from the dual air boxes?
It's less the mpg figure that concerns me, but the sudden change in efficiency.
Yes - theres two air filters tucked down the side, then the 'batwing' has a Mass Air Flow sensor each side and the flow joins, feeding the turbo which is in the middle at the back. Pop the plastic engine cover off its pretty obvious.
To one side is a smaller conection for the PCV system.
Both have orange silicone rubber seals which A)fail and B) are easy to badly fit.
See here: https://forums.mbclub.co.uk/threads/om642-cls-350-turbo-intake-gasket.271288/
The two MAF plugs ar an obvious potential maintenance error, along with the seals. I'd have a noseround for anything obvious if I were you.
Not local unfortunately otherwise I'd have a look for you.
Sounds a bit low but there could quite possibly be some weather related influence.
I did this (chester - London - Chester) trip last summer but I get about 37 on a long steady cruise control run at this time of year (3L v6 Diesel)
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And this was when the car was about 75k miles younger but a long motorway trip (Glasgow - Chester) in colder conditions - so it's not necessarily an age thing!
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The only turbo car I had would change from 32mpg (ish) to 18mpg (ish) just by ‘cruising’ 5mph faster.
The key to getting the best mpg was staying below the revs where the turbo kicked in.
Similar here .. my old BMW 3l flat 6 could jump from 50mpg to 40 mpg (or lower if I tried) just based on almost imperceptible changes. The key for me was watching the instantaneous MPG and changing the acceleration and speed .. there was a proper quantum step (actually 2) that were probably not just "Turbo on/off" but the more complex engine management modes call them (dump/normal/lean) and the key was sticking in "lean". (Pump mpg also tied well to trip)
If you have an instantaneous maybe try that .. if not then maybe try an ODB2 reader that will give you that and more...?
The other part was winter/summer and journey direction....
In winter the big 3l block took a while to heat up and I had a fair but of non motorway/dual carriage way on the way out and almost straight onto the motorway on the way back (2 different locations but similar characteristics).
On the way home it was still warmer after 8hrs than it would have been in the morning and got to the better efficiency quicker.
My car goes into top gear at around 58mph. It drops from 1600rpm to 1400rpm, because of the silly adapting throttle it takes a minute of steady state cruise after a boot to get onto a mway or dually to change up, or it will jold at 2000rpm then change up.
Tyre pressure can make a difference on the E, 32 to 34 seems to be a sweat spot, with lower softer and 36 plus spoiling the ride, mine on comfort suspension tho with 16in wheels
The other part was winter/summer
Warmer air has less mass and drag.
Previous posters have listed the many other factors that could affected the MPG. It's probably a combo of the tyre change (have you double checked pressures?), fuel variation, driving into the wind on your worst MPG example and even the computer calculating incorrectly. Have you tried manually calculating MPG from a full tank? I used to get around 35mpg from a 3L diesel BMW, so you're in the right ball park.
Aside from trying to find the issue, it's worth noting how MPG is sold to the public as a figure to obsess over, the difference in actual running costs for 33mpg v 40mpg is £350 a year for 10k miles. Considering running a car like that Mercedes is likely to be £2500+ per year in servicing, insurance, tax and depreciation alone; a few MPG is a minor difference.
42 to 33mpg is much greater drop than any of the factors listed here apart maybe from boost leak.
Usually an expensive fix though
The fix for a DPF becoming blocked is not a new DPF. It's fixing the thing causing the excess smoke.
Dieselgate software scam – it is widely reported that on cars with OM642 and 7GTronic, post your ‘free software update’ that gearbox does not now change into top until well over 70mph. Which is appaling. And once you’ve had it done, they wont undo it (coz emissions, init).
I searched for downsides to the emissions fix and there are a few furious posts reporting this with the 9sp box that the OP doesn't have. But it seems to be because the fix resets the shift points, and they re-learn with time.
I've had the fix, it made the throttle response nice and nippy for a while, but it went back to normal. No discernible difference in economy, power, noise or anything. It's not like the VW fix; Merc weren't pulling the same scam.
Point you all seem to be missing is that the economy dropped after a service. His method of calculating economy hasn't changed, his driving style hasn't changed, etc etc. He's had the car a while so should know what to expect.
Assuming 10,000 miles a year, 33mpg will cost £4700 (£2200 fuel + £2500 minimum running costs), 40mpg will cost £4300 (£1800 fuel + £2500 minimum running costs). All of a sudden £350 is not significant, especially when realistically a 3L diesel isn't going average 40mpg all year. So if a realistic 'good mpg' is say 37mpg, we're talking £200 on a near £4.5k spend to run a Mercedes per year. This all assumes no other running costs/breakdowns/tyre replacement.
If you want low running costs, a 10 year old 3L Mercedes isn't a starting point, so a couple hundred pounds is irrelevant.
Also bear in mind that a "headwind" isn't canceled out by a round trip.
Drag is proportional to the square of speed so to put numbers to that
(70+20)^2+(70-20)^2=10600
70^2 + 70^2 = 9800
(figure is just illustrative, you'd need to multiply by Cda and time as well)
So a round trip on the motorway on a windy day is using ~8% more fuel.
I'm doubting there's anything "wrong", you're just driving a massive engine and comparing the best day (worn tyres, low pressure, windless (or even a one way tailwind), hot weather, etc. Vs New tyres, full pressure, freezing weather, new + inflated tyres, etc.
If you want to feel better, do/replace everything suggested, wait a few months for the ECU to adapt, and it'll be better, because it's summer.
All the 'it's a 3.0l diesel' comments make no sense to me. In a diesel, it only injects the fuel it needs to power the car at the speed you request. The displacement doesn't make much difference, unlike in a traditional petrol car that has to fill the cylinder with a minimum amount of fuel. That's why bigger petrol engines are less economical.
The two extra cylinders and the V configuration add more friction. And there may be other airflow factors, I don't know. But regardless of any of that - he's driven it in for a service getting low 40s, and driven it out getting low 30s. The displacement of the engine and the number of cylinders haven't changed....
Warmer air has less mass and drag.
Not so you'd notice at the speeds to get to the motorway/dual carriageway... but the other thing I'd notice is the heater came on "properly" (as in switched) as the instantaneous mpg went up... presumably in the fairly sophisticated engine the ECU is trying to get to operating temp ASAP using every "trick" ???
Once I noticed/realised the two seemed linked it was even more noticeable.
Point you all seem to be missing is that the economy dropped after a service. His method of calculating economy hasn’t changed, his driving style hasn’t changed, etc etc. He’s had the car a while so should know what to expect.
*Insert The Office “Thank you” meme here…*
To be fair, this has been more useful than the responses to the exact same question on the FB owners page, not that that is particularly surprising… 🙄
The computer could be calculating the MPG incorrectly, and a service/ECU reset (or battery disconnect) could have affected that. I have previously noticed car MPG gauges aren't consistent and the best way is to check on a full tank to empty. The last time I did that the car was out by 10 % and there was even a hidden menu to override the gauge offset. This offset could have been reset at the service.
Also for reference, I looked at buying a 2014 E class with the same engine a couple years ago and didn't because the reported MPG from several sources showed realistic MPG of just 35mpg. Which is partly why I'm suggesting the original MPG might have been inflated.
Not so you’d notice at the speeds to get to the motorway/dual carriageway
So what's the explanation for my two examples above?
Two long motorway trips with very similar average speed but with 15-20c difference in air temp. The journey in warmer air was actually 5mpg more efficient even though it included much more urban driving and for half the trip was carrying more weight.
To be fair, this has been more useful than the responses to the exact same question on the FB owners page, not that that is particularly surprising
Unless you are doing the same journey in the same weather/time of day etc. I think it's really hard to say and its easy to cherry pick "I did x mpg on that trip last year" and potentially spend money that doesn't make a significant difference.
I know you said you've done 3000 miles since the service but on the van I noticed (or thought I noticed a drop in MPG after the service) that didn't seem to be reflected at the pump and was perhaps a "reset" occurred in the service???
he’s driven it in for a service getting low 40s, and driven it out getting low 30s.
No, he said he used to get high 30's and now it's mid-low 30's - so it's about 15% which could be explained by a combination of factors mentioned above.
All the ‘it’s a 3.0l diesel’ comments make no sense to me. In a diesel, it only injects the fuel it needs to power the car at the speed you request. The displacement doesn’t make much difference,
I wonder why mercedes report a 15mpg differential between their e200-250 and 350 then ? Surely by your theory all should use the same amount of fuel.
So what’s the explanation for my two examples above?
Two long motorway trips with very similar average speed but with 15-20c difference in air temp. The journey in warmer air was actually 5mpg more efficient even though it included much more urban driving and for half the trip was carrying more weight.
You weren't doing 20-30mph seems to be the obvious one?
I was simply pointing out that my 3L flat 6 got considerably different MPG with an engine at operating temperature than below operating temperature and that the 3L diesel block takes a while to heat up at 20-30mph.
I understand the BMW "M" petrol engines of the same age were rev limited before operating temp is reached.. so I expect the diesel ones also have some change in mode ???
The drop in fuel efficiency is reflected across all my driving and can be tracked back to a specific event; the service. Pre-service it would reliably return 37-39 on a run without too much effort, now it's doing 32-34 and I'm consciously trying to improve that figure.
Next time I know I'm going to do a decent run I'll brim the tank before and after and see what the fuel consumption is that way.
The computer could be calculating the MPG incorrectly, and a service/ECU reset (or battery disconnect) could have affected that. I have previously noticed car MPG gauges aren’t consistent and the best way is to check on a full tank to empty.
That seems to be what happened to the van... I was actually in the same sort of space as doomanic as I was actually hoping for the inverse but whatever got reset seems to have recalibrated over time.
No, he said he used to get high 30’s and now it’s mid-low 30’s – so it’s about 15% which could be explained by a combination of factors mentioned above.
I have had to change my driving style significantly to get over 30 now, driving like I am currently would have seen low 40s.
Then either it's the fresh tires giving you an effectively larger diameter wheel, or they've left something like the MAF unplugged.
But as molgrips said, diesels are simple machines, they suck in a lot of air, add fuel in proportion to your right foot, and blow it out again. Which is why they'll happily run with no MAF, manifold pressure sensors, etc. But seeing as you've plugged a code reader in I'm guessing it's not that as they'd still flag up unless the reader wasn't compatible with the car's computer (they aren't universal, the manufacturers hide a lot of the detail in their own protocols).
Have a look for the obviously unclipped sensors under the bonnet, take it back and ask them to plug it into their Merc code reading machine, and if it's none of that then it's the tyres.
All the ‘it’s a 3.0l diesel’ comments make no sense to me.
You still have ~50% more pumping losses than a 2.0 i4, it's not as bad as a petrol, but it's still bad (which is why you get 3.5t vans with 1.5l diesels and 2x turbos).
+ if I absolutely floored my old Berlingo at 3500rpm (or wherever peak power was hiding, it was hide and seek champion), it's still only sipping fuel at whatever flowrate gave 90Hp. A 270HP engine has the potential to drink fuel at 3x the rate. Now bearing in mind the Berlingo couldn't even drag itself over the Ridgeway on the M40 at 70mph, there's going to be a lot of time when more powerful cars are burning more fuel even if they're just on cruise control. Plus every time you dab the noisy pedal to make progress, get upto speed on a slip road a bit quicker, beat a Berlingo away from the lights to the merging lanes etc, cumulatively that's all adding up.
I was simply pointing out that my 3L flat 6 got considerably different MPG with an engine at operating temperature than below operating temperature and that the 3L diesel block takes a while to heat up at 20-30mph.
My point was that air temperature makes a difference given similar trips.
I have had to change my driving style significantly to get over 30 now
OK..... you hadn't mentioned that before. Something might be up then! If it was petrol I'd consider the change from E5 to E10 but diesel is different.
The drop in fuel efficiency is reflected across all my driving
When I had 2 regular commutes my fuel efficiency was between 49.5 and 50.1 and because it was something interesting to do whilst driving I'd try and hit the 50mpg... and reset it in both directions. Winter temps aside that had no relation to my MPG across all driving. Another regular commute of a similar distance I'd get maybe 35 mpg.. depending on time of day.. I tried pushing that to hit 40mpg but other than very late at night I'd not even get close so it stopped being "interesting".
If I hadn't had a specific if inane interest in being a bit obsessive I doubt I'd think the driving was that much different between the 2 at 50mpg and the one at 35-38mpg..(I'd even stay a bit later at work sometimes). but when I was trying to get the 40mpg it became obvious that the on the face of it similar trip had much more stop/start or slow and accelerate and that was where I was losing the 12-15-ish mpg.
When I was doing the 50mpg commutes I'd also have some 35-40 mpg weekend journeys with bikes on and different places so my average across all my driving was probably more like 40mpg or something (tbh never really measured that I just let weekends be what they were)