davecoombs

Hi,
I still have slight idle problems after checking everything> I connected the jumpers and no fault code is showing. I really do suspect the MAF but the code does not show on the flashing check engine light. Is there any way of checking that the MAF is f***d without replacing, also is it even possible that the MAF has failed but does not throw up a fault code ?
I also have another problem where the engine also appears to fluff a bit under hard acceleration. I checked all of the coil packs (1 was shafted) and replaced the faulty one. I also cleaned and resealed the breather on the left hand rocker cover (it was leaking v small amount of oil). All to no avail :-(
I'm kind of running out of ideas !
I think that the idle and fluffing is due to the MAF
All responses gratefully received

davecoombs

I hope its not det ????!!!

StephenDone

Watch the lambda sensor voltage and air fuel correction values.
On idle, the lambda voltage should oscillate between 0(ish) and 0.8(ish). The AFC should react inversely to this i.e. 0.8v (too rich) AFC negative value. Lambda V 0.1v too lean, AFC positive value.

If you see your lambda voltage stuck high, and your AFC at 0%, it means that the ECU has given up trying to get a good mixture, and is running open loop from the fuel map. It tries subtractin up to 20% from the fuel map before giving up. I had this on my car, and it is because a failing MAF sensor reads high at idle. (it then reads low at higher flow rates). If the MAF is reading high, the car is looking at too high a load on the fuel map, and puts in too much fuel. It then cannot compensate enough to bring the mixture back in line.

Hope this helps.

Steve

mutant_matt

And though Steve kind of implied it, the MAF can be duff (or on it's way out) without the ECU reporting a fault code about it.

The symptom of hunting at idle is often a MAF on it's way out....

Matt

boltona

Try cleaning out the throttle body. My Legacy GTB had idle surge/stalling problems and a mechanic relative of mine recommended (and helped) clean the throttle body. It was quite oily and mucky inside. Used brake cleaner. This can mean that where normally there is a very small air flow with a shut throttle, it can seal completely, and all idle control is via the idle bypass valve, which can't cope on it's own. It did fix the problem, although only temporarily (got slowly worse again over a couple of months). We suspect that oil is getting back in, as the oil seems slightly overfilled and a breather might be sending a lot through.

CRAIGFIN

Dave,

I was convinced that the MAF sensor on my MY99 STI Type-R was knackered. The symptoms were classic as described in all the posts I read. The idle was all over the place. I decided to fit a new Forge dump valve (just because I wanted one) and my car has never run better, the idle is now perfect. All this proves is to don't automatically blame the MAF sensor, I know I did. I have no idea why my car is now running better (air leak maybe!!!!!).

Good luck,

Craig.

davecoombs

Thanks for the advice guys

Matt Churchill

Dave, mine has just got recently. It was diagnosed by Power Engineering, so it may be worth letting them have a look at it. Might cost you a few quid, but better than a new engine I think.

My dealer had REAL trouble finding the problem, I guess because they can't simulate being on the move like a rolling road can.

Matt

MattRus

Dave,
symptoms sound just like a post turbo air leak.
My first check would be to clean the dump valve face and seat.
I have had a very similar problem and this cured it!
Otherwise check turbo to intercooler hose, and other hoses for splits, loose clips etc..
Air is metered via the MAF meter any leaks after this will cause an incorrect mixture to be burned. To weak off boost, to rich on boost (post turbo leak). The system is very sensitive to the slightest leak!

StephenDone

If you have a post turbo air leak, your mixture will be too rich on boost. If this is the case, then your lambda voltage will be abnormally high. The car meters the air pre-turbo, to decide how much fuel to use. If some of the air leaving the turbo isn't getting into the cylinders, but is leaking after the turbo, then the mixture will end up too rich. Conversely, if extra air is getting in, the mixture will end up too lean.

You need to get hold of a select monitor or DeltaDash, otherwise it's all just conjecture. A quick summary: If your lambda voltage is consistently high, it could be the MAF misreading the air flow, since they overread air flow at idle when they fail. If your lambda voltage is consistently low at idle, then it could be a post MAF air leak allowing extra air into the engine. Can't see how a dump valve would cause a problem _at_idle_, since whatever state it's stuck in, it recirculates the air post MAF, so the metering should still be correct.

Steve

[Edited by StephenDone - 4/26/2002 2:24:37 PM]

stavros

is there a website that we can find all these different things?
like if the voltage of the lamda sensor is..0.5...is it rich or lean...etc etc
you know stuff like that

StephenDone

There's some stuff on the EcuTek site. If anyone wants to compile a list of fault findings tips, then I'm happy to host it.

Cheers

Steve

MattRus

Stephen / Dave I understand the prevailing logic but with the symptoms described, which were very similar to a problem my car had, curing the dump valve leak / creep all but solved the problem.
T'was just a thought!

StephenDone

We shall see :-)

Oooh, the suspense !

Steve

davecoombs

I know exciting isnt it....
I am fitting a MAF this evening.
If the problem persists then I will look at the throttle body and also look for turbo hose leaks.
Tedious isnt it ?!!

davecoombs

And if none of that works I will cry (lots)

StephenDone

> And if none of that works I will cry (lots)
>
You might be down £300, but you'd be up one classy, high flow tea strainer.

Steve

Plantie

I am looking for a Dead Maf sensor.... If anyone has one... can I buy it off ya? plllllleeeeeaaaassssseeee!!!

Mail me

davecoombs

You can have mine if its dead for £500

Plantie

500, how about a tenner.

YHM reference post in my wanted thread

davecoombs

OK,
Changed the MAF sensor on Friday and the fluffing has gone away. However, it still idles a bit like a dog. It seems like it is about to cut out. I am now going to have a look inside the throttle body and clean it