I have a 2007 Hawkeye Impreza STI Spec D that's throwing a P0172 ("bank 1 too rich") code most times it's driven.

The problem only seems to appear once it's warmed up. When starting from cold, it runs great for 15 minutes or so - fuel trims seem normal.

Once the problem occurs, if I idle the car it stinks of fuel suggesting it really is running rich. Using an OBD reader I can see the short term trim sticks firmly at -25%, as if it's trying to pull all the fuel out it can. The car also seems to misfire - it runs 'lumpy', sometimes very noticeable but sometimes quite subtle. There's a "wandering idle" - if I jab the throttle and release it, the revs drop down too low and the car wants to stall.

The bad fuel trims are worse at idle. If I give it some revs the correction eases off, sometimes briefly back to zero.

With the fuel trims stuck at -25%, a pending P0172 code will usually get raised after about 5 minutes, and a CEL shortly after that. Once the problem is occurring, it always seems to persist after restarting the car. The only exception is if I leave it to cool down completely - e.g. overnight - then it's usually OK for a while the next time it's run - until it warms up again.

One strange aspect is the problem sometimes goes away for a while. Almost every time it's been with a mechanic, it'll fail to reproduce the issue, or it comes back running fine and will give me ~40 miles with no issues.

Relevant history:

1. Head gaskets done 18 months, 4k miles ago at Subaru main dealer (and heads skimmed)
2. P0172 codes started about 6 months ago. Would initially appear every 5 journeys / 30 miles, but have since become more frequent
3. New MAF sensor fitted ~5 months ago - a like-for-like replacement for the Denso unit already fitted. Made no difference.
4. New spark plugs fitted 3 weeks ago at an independent specialist (ex-Subaru main dealer mechanic). The old plugs were reported to be in good physical condition, all "biscuit brown" as expected with no deposits on them. After the work was done the car ran great for 30 miles or so, but the problem reoccurred.
5. 2 weeks ago, car went back to the independent specialist. He suspected a bad coil but couldn't reproduce the issue after hours of trying, so had to give up on this process.
6. New front A/F ratio sensor fitted 1 week ago at Subaru main dealer. It came back from the dealership and was running great, suggesting the issue was fixed. But around 4 journeys / 20 miles later the issue reoccurred. The dealership suggested fitting the new sensor because they said they'd found a P0030 "heater circuit" code, although I never saw this code myself.
7. 5 days ago, started the car again from cold (after disconnecting the battery - not sure that's relevant though) and it ran great again for another
8. Tonight - started the car, got a pending P0420 ("catalyst system below efficiency bank 1") code, but the car seemed to be running well, with fuel trims normal. Went for a longer run and ~15 miles later the fuel problem reoccured. -25% fuel trim, pending P0172, lumpy wandering idle. Brimmed the tank with fuel (first time I've used Tesco Momentum 99 instead of V-Power) and suddenly it's running MUCH worse than ever before. Really lumpy, idle jumping up and down, and keeps stalling at traffic lights.

Would really appreciate any insights as this has become a real mystery!

 

this sounds very like a vacuum leak or air getting in somewhere. check all hoses etc for a split pipe or one has popped off. you may have to get a garage to do a smoke test ti find it. p0172 is an air leak and is the reason why it runs ok from cold start until the engine is warm then it is sucking in hot air which messes with your AFR,S which then sends your fuel trims all over the place.its also the reason why it goes back to zero almost when you rev or hold constant revs as the throttle plate is open and the leak is not so evident.. my turbo inlet pipe was torn and letting in air which caused the same conditions as you are having so check them all.

 

Front O2 sensor again maybe?

 

P0171/0172 Diagnostics pdf for MY06/07

Coolant Temp sensor??
Fuel pressure regulator?

 

This.

 

Thanks everyone for the feedback. I wanted to give an update with what seems like the resolution on this.

I happened to notice that when the car was misbehaving there was quite a noticeable whistling sound coming from the fuel filler cap. If I would then start to unscrew the cap I'd hear quite a lot of air rushing through - not just the small 'whoosh' you'd normally get but more like a constant draw of air through the filler cap.

As an experiment I did some (gentle) driving with the filler cap off and sure enough after that I was unable to reproduce the issue! On a whim I jabbed the little valve in the filler cap with a screwdriver, put the filler cap back on... and I've now been unable to reproduce the issue in several months and 450 miles of driving.

I realise it sounds bizarre and unscientific, but this does seem to have resolved the issue. Previously I was getting this problem most of the time I used the car. Now it seems I can't reproduce the issue at all.

I'm assuming an excessive vacuum was building up in the tank and throwing things off but I really don't know. I'm just glad it's working well again!

 

3 gallons per 20 miles, all burned below 6k rev band
impressed with 2.5l

 

Just to give a final update on this issue, it was a bad evap purge valve solenoid behaving erratically.

Replaced that and the issue never occurred again.

 

I am having the same issues as this, my subaru impreza non turbo bugeye 2002 is stinking and making me feel ill from the fumes. I have In engine management light on for a p0420 code.
My bank 1 sensor has 2 metal prongs sticking out could this be the issue? Does anyone know what they are or where they plug into? I'll attach a photo.
I'm new to this and have no idea how to start my own thread on this issue I need to sort out.
Do I just buy a new cat and get it replaced or is there more of a problem here?


Does anyone know where this plugs into? This is my o2 sensor on the cat.