overfuelling - where to look at for the cause?

[crazydriver81]

My car developed a problem which seems to be related to an overfuelling issue. Now I need some guide, where to look at for the cause(s).

The problem:
If engine is warm and you rev it, then go off pedal (without BOV opening-at least I cannot hear) it is huntig for idle RPMs and then stalls. If engine is restarted, you can recognize a bit of black smoke from the exhaust and it smells like unburnt fuel.
Additionally, when accelerating and going off the pedal and on the brake pedal, car stalls. Again it smells "fuelly",
Further, if accelerating a bit harder (not WOT) you can see black smoke behind the car.


Fuel consumtion is really bad and I constantly get black shades around the exhaust on rear bumper. The whole problem occured, when I restarted the car in spring this year. Not had the problem last year. Modifications = None which were not present alreay last year (255lph fuel pump on stock FPR, VTA BOV from Amber Performance).


Other than that, the car is running fine. Idle is stable and on cruising no problems. TPS settings are fine. No CEL flashng/lighting up. I changed the MAF sensor already and made a full ECU reset. It got a bit improved, which means the car stalls less often.

Checked boost pipework and found nothing irregular. Checked for hissy noise - nothing either. Boost buildup is fine at around 0.7 bar max.


Where to look at else?

[Kenneth]

Did you do a pressure test? Sometimes the pipes don't show any issues until pressurised.

The issues I would look for them and the order in which I would do it (mainly because I have a air compressor )
1) Check the pipe work under pressure
2) Check the BOV is re-seating on deceleration (you need to be under the bonnet, blip the accelerator and watch to see that BOV re-seats early enough, about 1000rpm is probably OK)
3) Clean out the idle stepper motor
4) check TPS voltages
5) Check spark plugs, leads and coils

[crazydriver81]

Thanks Kenneth, a lot of work to do for the coming weekend... Very good list of points to check.

[kiteman]

I had the same problem years ago, It turned out a rocker arm had just dropped off. It took a long time to discover what my problem was.
Refitted the rocker arm and everything was fine.

[crazydriver81]

so, i did a lot of checking. I did not manage to check TPS function, anything else is like it should be.

Anyhow, will check TPS function and refit the recirc BOV. Hopefully it is gone then.
@kiteman: How did you recognized the rocker arm being off? Any chance to recognize without taking the rocker covers off?

[kiteman]

so, i did a lot of checking. I did not manage to check TPS function, anything else is like it should be.

Anyhow, will check TPS function and refit the recirc BOV. Hopefully it is gone then.
@kiteman: How did you recognized the rocker arm being off? Any chance to recognize without taking the rocker covers off?

Iirc a mechanic friend suggested removing the plenum. this allowed him to shine a torch in and he spotted a wet spot. He then removed the rocker cover to investigate and found a rocker just lying there.

[fassi1]

How about O2 sensor, working fine?

[swinks]

Chris, funny enough, but in vr4 you barely will see difference between working o2 sensor or dead one. I was running with disconnected o2 sensor for almost 3 weeks, and... no difference, not noticable at least.

[Atik]

Chris, funny enough, but in vr4 you barely will see difference between working o2 sensor or dead one. I was running with disconnected o2 sensor for almost 3 weeks, and... no difference, not noticable at least.

To echo what Tomasz just said, I've been running my car with no O2 sensor for over a year. No noticeable difference from running with one, except the smell of unburnt fuel as you come to a stop after a hard acceleration!

[fassi1]

Yeah it makes sense cause they only use low fuel trim and switch to open loop very quickly.

[crazydriver81]

To echo what Tomasz just said, I've been running my car with no O2 sensor for over a year. No noticeable difference from running with one, except the smell of unburnt fuel as you come to a stop after a hard acceleration!

Hmm...upon reading this, I should check O2 sensor as well. Smell of unburnt fuel sounds familiar plus on O2 voltag reading from the turbo timer barely something happens...

[crazydriver81]

finally found the cause - the VTA BOV. I changed back to std. recirc BOV and no more issues... STRANGE what a bit of lacking air can cause