Chad!

What could be the cause of my car over fueling and/or running rich? Any ideas? And how can I fix it?

Thanks,

Chad.

Graz

Why do you think it's over fuelling? Have you fitted an AFR gauge, did it fail the MoT, getting an unusual amount of flames out of the exhaust Impreza's do tend to run quite rich on boost anyway.

Chad!

It hasn't failed an MOT but I thought I'd have it checked to see what it was doing with the decat pipes. I was told it was REALLY high. 7% I'm sure he said. But on sudden decelleration there is quite a loud bang/crack out of the exhaust. Add this to the hesitation problem I've got and I thought maybe this could be all part of the same problem??

New_scooby_04

iTrader: (4)

Is the hesitation at low revs/off boost? If so, maybe a shot lamda sensor? Get it to a specialist for a once over.

Ns04

vulnax999

iTrader: (7)

Vent to atmosphere dump valve perhaps ?

Chad!

It is at low revs that it's hesitant yeah. As soon as the turbo is spooled up it's fine.
And I do indeed have a Vent to atmosphere dump valve.

Graz

What's 7%? The important stuff is that on closed loop fuelling (i.e. turbo not spooled) you are getting a lambda reading of 1.0 or at least pretty close to it, if not the lambda and/or MAF sensor may not be functioning correctly. If the fuel mix is corrrect the the rest of the emissions measurements would generally fall within spec. However if you're running a de-cat that will almost certainly equate to an MoT fail, at least one cat is needed to remove the extra bit of pollutants needed to pass the MoT. Once in open loop the fuelling will become quite rich to cope with the boost from the turbo, if it's hesitant then maybe the MAF is getting a bit slow to react. The lambda sensor isn't used during open loop fuelling.

The bang/crack from the exhaust is almost certainly down to the VTA dump valve, its a known feature. As you lift off the dump valve vents, as it does so more air is drawn through the intake. The ECU interprets this as the engine taking more air so adds fuel accordingly, the overfuelling due to this will sometimes ignite in the hot exhaust causing the pop/bang. With the normal re-circulating dump valve the excess air is fed back into the inlet after the MAF so no additional airflow is seen.

Anyway as suggested if your worried about it best to get the fuelling checked out by an expert, usually done on a rolling road.