Drop in MPG - is a problem coming?
 

Guys

I think I have a real dog of a Friday afternoon WRX - 1 yr old , 10k miles.
Just had to have the clutch/flywheel replaced (not done under warranty :mad: ) and 3rd to 4th sycromesh. Just prior to this I had noticed the MPG dropped from 24 to 20, yes I'm sad and monitor it properly.

It appears to have stayed at this 20MPG mark post repairs, so could there be another problem?

Cheers Nige

It's worth checking all the sensors around the gearbox area, could be that when the gearbox was removed/replaced something was disturbed or broken, WORTH A TRY.

How often are you using the car? Engines use a bit more fuel in winter??

Just my 2p worth :)

It's used every day and same journey to work, etc.. The drop off happened suddenly and stayed at the lower point.

"Dealer" says there is nothing wrong ??

Nige

Find out what it is and let us know. I keep a close eye on my consumption. If it drops I will make a point of determining if the MAF is on the way out. If this leans out the mixture it could lead to engine failure.

Sounds more like you are going rich. Could be MAF, or Lambda.

"Lean" doesn't mean more mpg!

Lean = less fuel = more MPG

Both lean and rich give less mpg because neither are efficient.

Yes, for a car that aims to run at ideal Stoich mixture all the time that would be true, but Scoobies run very rich under boost to reduce temperatures and avoid detonation. If the MAF goes and the mixture starts to lean out it is common to see an improvement in MPG, as well as better performance (the efficiency issue you mentioned) for a while before it all goes bang and burns a hole in the No.3 piston.

I understand only the very later (possibly only from MY00) cars ran rich for the protection you talk about. The "bang" you mention only happens after running too lean.

Nigeboy says he has a drop in mpg not an increase.

No, they all run that rich, the later cars have better mapping that helps to protect the engine, but that can be done with changes to timing etc. All scoobies run VERY rich on WOT.

Yes, a bang of the sort I mentioned would only happen if it was running lean, and Nigeboy says his MPG has decreased, which points to it running rich not lean, but if you remember it was actually you who suggested it could be running lean and claimed that it could run lean and see worse MPG:

I was purely making the point that lean often does mean more MPG with a scooby, not that Nigeboy is running lean, his decrease in MPG suggests the opposite, and although I believe it is not totally unknown for a failing MAF to over-read and cause the mixture to run too rich, it is perhaps more likely that the lambda is going or that he has an air leak somewhere.

Yes, I did mention he could be running lean and that could cause lower MPG. After a fashion you agreed.

I can see the point you make. I disagreed that running lean gives you more mpg as my understanding of lean is "less fuel than optimal" whilst yours appears to be "lean relative to normally running rich".

O.K, fair enough, terminology difference. I would consider lean to be less fuel than is required for the engine to run happily and safely. In a scooby that could still be pretty rich as compared to stoich ratios at WOT on full boost. I have a Lambda link in my '97 car and it runs very rich on full throttle. I expect to see the highest light on the Lambda link lit at WOT, which indicates 9% CO, 11:1 AFR. If it was only lighting the next one down (7%CO, 12:1 AFR) I would consider it to be running lean since it would certainly be detting heavily and would start to be in serious danger of going pop in short order. In an absolute sense though, it would still be running rich, just not rich enough...

I suspect the battery was disconnected when they changed the clutch, this would have re-set the ECU and with the colder weather may have allowed more engine advance, therefor more power = more fuel used.

Mine has dropped 2 mpg in the past few weeks with no resets or anything. You gotta love the cold weather. More power which you can't use due to slippy conditions :D

Check your battery terminals and earths are nice and tight. My 03STi PPP was misbehaving at low engine speeds. Tightening terminals (positive on battery was a bit slack) cured slight misfire and moved town (30&40 limits and mobile cameras everywhere) MPG from 22 to 24/25. Checking fuel consumption isn't sad, its a good indicator of all being well under the bonnet, although you can't own a Scoob and moan about the fuel cost.

Brun, hasn't rained for a fortnight round Hull, roads are so so greasy and unpredictable, I'm praying for some rain to give it all a good wash.

Cheers, Diccy.

Lol tell me about it...nearly lost it and ploughed into a barrier while pulling away down Boothferry road :(

How come the clutch wasn't a warranty job since the car is just 1 year old?
Doesn't GB cars carry 3 year warranty?

Well apparently the cars aren't designed for enthusiatic driving and it had just worn out. As a "wear and tear" item the clutch wasn't replaced under warranty

N