So I took my 07 STi for an MoT on Saturday and it failed on emissions. It does have a sports cat, but even after getting it really hot, it wouldn't pass.

So on Monday I had the engine Terracleaned to get rid of any deposits that might have built up, but it made no difference to the emissions test.

Yesterday the car was back in the garage to be checked over. The Lambda sensor was checked - okay, but the MAF was not. A new MAF was fitted and the car made it through the emissions test and I got my MoT.

When I came to collect the car when I started the engine I noticed the engine idle speed was all over the place for a couple of seconds, but it settled out so I thought everything was okay. However, the engine is definitely not idling as smoothly as it was before (drops as low was 400rpm). This afternoon whilst driving in traffic all felt fine, but the CEL came on. However, under boost the engine does not pull at all smoothly. On getting home I plugged my OBDII reader in and got one code. P0172 (fuel mixture too rich).

Is the replacement MAF at fault? The car drove fine with the old MAF.

I would note that the car is not standard. The engine was rebuilt a couple of years ago and remapped with a billet turbo and 3" exhaust. Could the problem be that the mapping was done with the old MAF and the map doesn't work well with the new MAF? Could the old MAF have been modded in any way to achieve a good setup?

Any advice welcome. I'm not sure what to do next other than check for air leaks in the vacuum lines.

 

http://www.clubwrx.net/forums/genera...0172-help.html

 

Okay, so I've checked the recirc dump valve connections and pipework - hard to see it all, but what I can see looks fine.

I've checked the vacuum lines that come off the intake pipe. All looks okay.

I've removed the MAF and inspected it. Looks brand new (erm, it is). The sensors are clean.

I did find the two halves of the filter housing weren't joined quite correctly at the lower end, which I've sorted, but don't imagine that's an issue since it's upstream of the MAF.

Is there any evidence of new MAFs being faulty?

I assume if one maps an engine with a specific MAF, then the MAF gets changed, that could cause poor idle as well as lean/rich issues under load?

 

Have you tried a ECU reset following the MAF replacement ?

Mick

 

Definately try an ECU reset. I had symptoms similar to yours a few years ago. ECU reset cured it.

 

Epic! Thank you. Looks like that might be the problem then. For the newage it seems I can't do that myself (was easy with the classics), so I'll have to speak to Surrey Subaru on Monday.

 

Turned out to be a Eurocarparts MAF. Fit for nothing but the bin. Proper Denso item now fitted and the car is running so sweet! Damned expensive though.

 

on the VAG Forums they slate Mafs from ECP, even branded ones seem crap,
glad you go to the bottom of it.