Spirit

My JDM STi v4 hasn't been running right for a little while now and it's been tough diagnosing it. MAF sensor was replaced initially as it looked like that, had her on the rolling road and still running lean. Whilst doing so found out the Lambda sensor wasn't working, replaced that which helped a bit, but still running lean...

After a bit of playing around it appears the standard recirc dump valve isn't sealing 100% - when ticking over it's still pushing air back in to the system. I changed it over for a friend's VTA dump valve off his Evo 2 and immediately I could feel my STi was running smoother and quicker.

Is it likely that the faulty dump valve is the sole cause of the car running lean? If so, I'll replace it with an uprated recirc dv as the vta is too noisy for me, but getting a bit fed up with replacing bits without really being sure what's wrong!!

Disclaimer: I know bugger all about engines so all the above might be completely wrong

jonny gav

iTrader: (1)

a leaking blow off valve on a maf based car will cause it to run rich not lean, is it running lean at WOT, high or low revs etc ?

the most common cause of lean running are fuel pump and maf sensor.

hope this helps

Spirit

It's running lean mainly in low revs, but a little bit at high revs too. MAF sensor has been tested and appears to be ok, but the fuel pump hasn't yet (another item I don't want to fork out to replace without knowing for sure that's the problem!).

Interesting though, that we tested the bov and it was pushing air out at idle, and when using my friends vta bov the car ran much better?

Thanks for the help

AVI-8

At idle and at low power settings it should cause it to run lean as there is a partial vaccum in the induction system, obviously because more air is going into the engine than is being metered.

But under boost where some of the air is leaking out through the dump valve, the engine should run rich surely because the maf thinks there is more air going into the engine than there actually is and the ecu is supplying fuel based on the maf input.

Spirit

Hmm, certainly sounds right for running lean at low revs at least. I would have guessed it was running rich at high revs as it pops back a lot when you let off the gas, but rolling road results disagreed.

I think I'll have to go with a new dump valve and hope that's the solution. Forge motorsport recirc looks like good value for money at about £100 - any other recommendations? Budget is tight so that's about my max spend.

Cheers.

[Davey]

How do you know its running lean?

Spirit

Had it on the 4wd rolling road at Clive Atthowes in Norwich, before and after replacing the lambda sensor

jonny gav

iTrader: (1)

it could be a fuel pressure problem form eather the regulator or the pump.

also check the vacuum pipe to the pressure reg of cracks or splits as a leak in this pipe will cause lean running at positive boost pressure too.

Spirit

Pump & regulator are the other items I'm considering replacing at the moment, but I'm lead to believe if it's a fuel pressure problem it should be leaner at high revs when more fuel is required, rather than at low revs which is where it's currently worst?

jonny gav

iTrader: (1)

i think it would be a good idea to get your fuel pressure checked.

you should have 2.8-3.0 bar with the vacuum pipe off, with the pipe on it should drop by the same as the vacuum pressure in the manifold and rise with the boost pressure over base pressure by the boost pressure so...

if you have 3.0bar with the vacuum pipe off and you are running abar of boost your fuel pressure should be 4.0 bar.

if you have -0.5 bar of vacuum it sould be 2.5 bar

hope this makes sense