[Closed] New Exhaust = reduced MPG?

Kwik Fit

There's your problem.

However being so big they'll replace anything if you aren't happy so you can go back to them and get a different brand or shape or something.

Ask on a Mondeo forum. The fact that it has two pipes out probably doesn't matter but it does suggest it's a different part.

Posh diesel should always improve MPG, although possibly not enough to notice. There's a chance it's dislodged some crud I suppose - pretend you're a BMW driver for a day (ie thrash it) and see if that helps. Is it still on that same tank of fuel?

EDIT: I did the maths, it seems not 🙂 Did you switch back to your original fuel?

new exhaust is wrong, strangling the engine and not letting it breath, had the same problem with an aftermarket CAT fitted to my AUDI, replaced the cat and everything was back to normal. That's what I reckon but I could be talking bollox

Been running the air con a bit more? Got a bit warmer about 4 weeks ago.

"Been running the air con a bit more? Got a bit warmer about 4 weeks ago."

Thats stopped working in the last 2 weeks so got to get that sorted now too!

Will have to stop recommending Mondeos soon !

If your Kwik Fit is anything like mine, they won't have put any gas sealant in the joints before doing up the clamps (TBH, this fairly standard practise). Until carbon deposits fill the inevitable gaps, the exhaust system will be blowing slightly, which does affect the mpg numbers until it seals itself..... though the drop you're seeing does sound large.

Your air con packing in may well be the problem. The air con is driven by an auxillary belt attached to all the other belts driven by the crank. Air con system is only lubricated by oil in the refrigerant, so if your refrigerant has gone then so has the lubrication and something my be dragging enough to put a little more load on your engine hence the drop in MPG, the motor tensions the belt to the main drive belt when on and it may be either too tight or just dragging, the other risk is if it's too tight it may overload the drive belt and cause serious engine damage. I've had a 4.0ltr Jeep engine wrecked by this in the past, at the same time as the petrol strike too, bad times!

More modern cars's air con systems aren't belt driven any longer as belt driven pumps greatly affect MPG when they're running.

Geordiemick - I was putting off the air con repair as I'm expecting a big bill. From what your saying though a regass may do sort it.

From limited reading I had done I assume either an electrical fault on the air con or air con clutch problem..

Thanks for the advice

Just spoken to Kwik Fit - They are going to do a re gas on the system tomorrow. If that doesnt work then its FOC. They are also going to check the exhaust.

More modern cars's air con systems aren't belt driven any longer

How do they work then? I know my Prius has a electric pump.

`tis over 30yrs since I stopped working on cars and after market exhaust systems with different internals/baffles which allow gases to flow a certain designed rate from the OM ones caused similar problems way back then ❗
Had a friend with an early Mk11 Golf Gti with exactly the same issue till he spat the dummy and had a VW fitted to prove to Kwick Fit there was an issue, got it all paid for 😆

i'm sure your car sounds awesome though and looks faster!

Hmm MPG problem solved... sort of!

4 weeks ago just before exhaust fell off filled up with Shell V Power diesel, just more than anything to clean the engine abit as I have not doen it since new. Avg mpg dropped to about 45mpg!

Since that tank filled up with Asda owns best which has roughly done 46/47mpg.

Last night filled up with Esso normal diesel. MPG now back up above what it used to be at 52mpg! Not bad for 80mph + on the motorway..

So morale V-Power not worth it (although not as much crap comming out of the exhaust now) Asda fuel gives less mpg, but not as bad as Sainsbury which before has reduced my mpg to low 40's.

Buy Esso diesel (same price as Asda)

Still need to get my air con sorted though...

So morale V-Power not worth it

That's really bizarre since V power or BP ultimate DOES have a higher cetane value and as such definitely burns better and more efficiently in your engine.

I did loads of reading about this - everyone reports smoother running and better starting; some report more power or more economy, or both.

I don't get a noticeable power improvement but I do get an economy boost of about 2-3mpg easily.

something else is up with your car - either it's the cleaning in V power dislodging crud which is/was affecting the injectors, or you had a bad batch or something.

molgrips - Member

That's really bizarre since V power or BP ultimate DOES have a higher cetane value and as such definitely burns better and more efficiently in your engine.

I did loads of reading about this - everyone reports smoother running and better starting; some report more power or more economy, or both.

Don't want to derail the thread too much but was that specific to diesel or does it apply to petrol too?

Specific to diesel.

Diesel is rated with cetane number. Cetane is a chemical that ignites really well under compression, so the cetane number of diesel is how well it ignites and burns compared to that. In any diesel engine fuel with a higher cetane number will simply burn more efficently. You can add this stuff called 2-ethylhexylnitrate to diesel and also veg oil/biodiesel to increase the cetane number. It's what they sell as power booster in the Halfords chav section (and it does work), and it's the same stuff they put in the fancy diesel. It's got a distinctive smell, and the fancy fuel reeks of it. It's actually cheaper to buy some of the power booster things than it is to buy fancy fuel. Cheaper still to order 2-EHN in bulk off ebay.

Fancy petrol has a higher octane number. This means that the fuel is less volatile and burns more slowly when ignited and is less likely to spontaneously cobust in the cylider. This means you can run a higher compression ratio and get more power. However the compression ratio is down to the engine design, so only powerful cars are designed this way. If they are designed to take advantage of higher octane fuel then if you put in normal fuel they have to I think retard ignition timing to keep it all running nicely (not sure about this, someone correct me).

So if you have a fancy car higher octane fuel will allow it to run to its potential, and it will have to knobble itself a bit to run on normal fuel. Normal fuel is what, 95 RON, super 97 and V power 99.

HOWEVER it is supposed to have other additives in it that help even in normal cars - I dunno if it does or not.