Poor MPG, potential causes?

[Marticus]

Hi all, for a while now I'm struggling with poor mpg. When I purchased the car I could comfortably get around 30mpg, but recently I'm averaging less that 25mpg.

Initially I had put it down to the fact that I was just doing short journeys, and 'enjoying' the car. But not long ago I did an airport run, and the best I could manage driving very carefully was 28mpg.

Within the last 5k miles the car has had a full service with complete timing set waterpump etc,
There are no errors showing on scan at the moment.
The only thing to change engine wise was fitting of ramair intake.
The only other possible symptom is a slightly rough idle on cold starts, which settles after a few minutes.

Based on the above, I'm thinking maybe it needs a carbon clean, I don't know if it's possible to effect the mileage that much.

Maybe MAF sensor, but I figure that would throw an error.

Any other suggestions?

[pudding]

25mpg was my ED30 life for 8 years    Best I ever got out of that car (stock intake and exhaust, Stage 1 revo) was 33mpg, and that was a solid month of 400 miles a week at really gentle throttle and sticking to every speed limit, which was incredibly dull.

What are the long term fuel trims like?  Have you got VCDS?

The MAF can be out of whack and not throw an error. The ECU just adapts around it unless it's shorted out, no signal or miles out of range.  It is the primary load sensor, so a new one wouldn't hurt if it's the ancient original one.

[Marticus]

25mpg was my ED30 life for 8 years    Best I ever got out of that car (stock intake and exhaust, Stage 1 revo) was 33mpg, and that was a solid month of 400 miles a week at really gentle throttle and sticking to every speed limit, which was incredibly dull.

What are the long term fuel trims like?  Have you got VCDS?

The MAF can be out of whack and not throw an error. The ECU just adapts around it unless it's shorted out, no signal or miles out of range.  It is the primary load sensor, so a new one wouldn't hurt if it's the ancient original one.

Yeah I'm probably being too picky  , I think it's the change that's bothering me more than the mpg.

No VCDS but I do have OBD11, and I'm pretty sure I can check there, do you know what the expected values should be?

And I'll probably swap out the MAF anyway and consider it a service item  , I've changes pretty much everything on the outside of the motor at this point anyway, so might as well complete the job.

Thanks for the help 👍

[pudding]

25mpg was my ED30 life for 8 years    Best I ever got out of that car (stock intake and exhaust, Stage 1 revo) was 33mpg, and that was a solid month of 400 miles a week at really gentle throttle and sticking to every speed limit, which was incredibly dull.

What are the long term fuel trims like?  Have you got VCDS?

The MAF can be out of whack and not throw an error. The ECU just adapts around it unless it's shorted out, no signal or miles out of range.  It is the primary load sensor, so a new one wouldn't hurt if it's the ancient original one.

Yeah I'm probably being too picky  , I think it's the change that's bothering me more than the mpg.

No VCDS but I do have OBD11, and I'm pretty sure I can check there, do you know what the expected values should be?

And I'll probably swap out the MAF anyway and consider it a service item  , I've changes pretty much everything on the outside of the motor at this point anyway, so might as well complete the job.

Thanks for the help 👍

No worries chap 

With a stock intake, long term fuel trims are usually in the 0 to 2% range.  With my aftermarket intakes, I saw as much as +12%, so basically the ECU was dumping 12% more fuel in than was necessary because the MAF tube calibration was so poor. I'm not sure what the RamAir is like for MAF accuracy, but if your long termers are up there in the 10% range, that might be your answer 

I would definitely pop a new one in. I think the genuine one from a dealer was around £140ish last time I got one, so not too horrendous.  Might also be worth replacing the O2 sensor in the downpipe as well if its really old as they can age, get clogged with soot and start reading leaner than actual, which again would cause the ECU to inject more fuel than is necessary.