Air Fuel Correction +25% at Idle
 

Hi all,

I was hoping someone maybe able to shed some additional light on this particular problem currently on my 2011 STI Hatch

The background is:

Recently rebuild 2.5 engine upgraded with forged items (typical spec), uprated RCM Fuel Pump and H&S Sport Cat downpipe. The engine was rebuilt due to head gasket failure.

The car was booked for a remap with Bob R, during the initial set-up it was noted that the AF Correction was +25% at idle although far less at higher rpms. At the time it was suspected a faulty fuel pump installation or pump itself. The MAF was cleaned and an initial inspection of the hoses all seemed fine.

So, the fuel pump installation was checked and found to be correct (including the additional O-ring), so then a suspect fuel pump, new one ordered and replaced upon checking the AF correction was styill +25%, eliminating the fuel pump.

Checking the AF correction after installing the new fuel pump the AF correction reduces to zero at ~2500 rpm. thus not giving a definitive conclusion.

After resetting the fault code the car seemed fine for a week but when left to idle it stuttered then check engine back on - assumed AF +25% required!

Other scenarios are

- Faulty MAF
- air leak in the vacuum piping

Basically are their any other things that we are not considering or has anyone else experienced something the same?

Thanks

pban

Fuel pressure gauge would have shown whether fuel pump or regulator was actually bad or not.

Air leak on the vacuum hose to the fuel pressure regulator could cause a static fuel pressure that doesn't get lower on vacuum or higher on boost, which could cause you to run rich when idling, and lean on boost. Fuel pressure gauge would easily show whether this is the case or not.

Other air leaks etc can also cause similar as could MAF failure.

have you performed a secondary air pump delete? If not could be it's on the way out/leaking pipe somewhere. Even if it's been deleted could be the plates have come undone a little when putting it back together.

I believe the secondary air pump has been removed so thanks will check the blanking plate

Check the Purge Canister control Valve. these stick open and cause it to draw air into the system at idle

Is it only under closed loop that it occurs?

Umm not sure?

The +25% appears only at idle it seems (ie watching the value for "fuel correction" when reading the ecu and increasing/decreasing the rpm), in fact when the car is left to idle the rpm is very up and down, then occasionally you hear the car almost stall then the ecu obviously corrects and idle rpm picks up. Under normal driving above 1500-2000 rpm the car feel absolutely fine no hesitation at all in the throttle response