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I fill up once a month when visiting family.
It's 65 miles home on A roads and motorways. I always average 60 mpg on this trip . 2 fillups ago I got 50 mpg.
I thought maybe it was a headwind but check car rolled ok ,which it did.
Filled up again and once more 50 mpg.
Previously the only time it dipped to 50 mpg was on the commute with stop start traffic.
Is there anything obvious to cause to decline?
Car runs perfect and gets the 130 mile blast every 2 weeks.
It's a Ford 1.5 tdci.
Check the tyre pressures? Not likely to make that much difference though
Doing multiple dpf regens?
I think i got a noticable (although not 20%) improvement with a fresh oil change.
I take it you are calculating it manually, per tank, so in your case this has been a couple of months?
colder temps might do something but I'd be surprised if it was that much. have you had the car and been checking mpg for multiple years?
Biggest difference for mine (on my repeatable commute) was always school holidays, only took a minute or two more but was 5-10% better economy. Doesnt seem relevent to you though.
Tyre pressures or different tyres?
Extra weight gain at Christmas.
Walk next time fatty.
Cold weather?
Tyre pressure drop - caused by cold weather?
I certainly notice a wee increase in diesel consumption over the winter.
Could be weather and I believe winter diesel is used in the UK from mid November to stop ‘waxing’ at lower temperatures.
I always reset the mpg on the trip computer when I fill up.
Pressures are bang on.
First time with low mpg was in October so not too cold.
Now I think about it, it hasn't done a Regen when turning off for a long time. Assumed it didn't need it due to getting good runs.
When was the airfilter changed?
And is it getting up to temp?
it hasn’t done a Regen when turning off
I doesn’t wait until you stop the engine to regen. It’s just the only time you’ll notice a regen is if it happens to be the case a regen is in progress when you stop the engine.
the regen also stops when you stop the engine - so what you’re noticing is an interrupted regen. What your hearing is the fan running on to cool things down because of the extra heat involved
Air filter changed last March.
From Wikipedia:
The United Kingdom differentiates diesel fuel into Summer and Winter Fuel according to BS EN 590[36] and BS EN14214 (biodiesel).[37][38] The same numbers are used in the Republic of Ireland. Distribution of winter biodiesel in the United Kingdom starts in November and ends in March.[36]
Have you got a leak when the engine's running? Injector leakoffs or return lines? That wouldn't neccessarily leave a pool on your drive as it only leaks while you're driving.
I’d not be worried. Suspect any or a combo of: DPF regen; cold weather; low tyre pressures; increased electrical load from cold weather ie heated seats etc.; weather (wind); lower efficiency levels due to wet road surfaces.
Winter diesel? It contains additives to prevent freezing, I believe. My diesel MPG has dropped a wee bit recently.
Radiator thermostat stuck open perhaps. Is the temp gauge lower than normal when engine would normally be up to normal temperature?
My dips in consumption were always regen related, and about that over a similar journey, so if this persists I'd be looking for a regen-related issue.
Any sticky brake calipers? Warm wheels after a long run are the telltale
My diesel currently doing the same. Happens every winter.
Dropped from mid sixties to mid fifties mpg
Cold, windy weather. Lights on more often, heating cranked up, probably a drop in tyre pressure, winter diesel, engine running while de-icing car etc....
Yep work van (Transit Connect Diesel) struggled to reach 45mpg on a 200 mile (ish) run today. It’s in good health (5500 miles). Sounds extra crap and rattley in the lower temps I’m sure too.
The tyres look the correct pressure, I’m not checking them at close to zero and predictably the TPMS is broken.
I'm in the camp that suspects a few elements that add up to make the difference. Higher electrical load from lights, blower, + heater rear and front screen (if you have that) and heated seats too if you have that. Means the alternator are always on load (modern cars unload the alternator to wring more efficiency in the official mpg / co2.
Failing injector is another possible cause (and a classic failure for older tdcis though I don't know if that still applies). They can start out basically just a little bit baggy and leaky, meaning one cylinder gets too much fuel, not enough to cause noticable problems or big clouds of smoke but enough to just be wasteful. It does usually get worse, I always had clouds of smoke and one time had it fully hydrolock.
There’s no smoke. It’s service is due early March.
Will any of the potential faults mentioned do any harm or should I book it in now to get it looked at?
As I mentioned the first bad mpg was October so not too cold.
It's normal. My XC 60 diesel used to drop 5-8mpg during winter. Added use of heater/air con/ heated seats etc all added up. It returned to normal in the spring.
I've had this mystery in the past and turned out to be break pads being bare and rubbing on the disks. This was to the extent that eventually the disk was red hot and it knackered the disk and caliper. Worth checking