1.8 8v GTI over-revving, possible inlet?

Hi guys,

My sis is having a few issues with her Mk1 1.8 8v GTI cab ('87 E plate) over-revving. I assume there's an air leak somewhere although can't see any obvious signs. She also mentioned she can smell fuel.

I have been told it's a common problem with the inlet manifold on these. Is that correct? I can't see it myself as an inlet manifold is a pretty solid thing, but over-revving and stinking of fuel does kind of sound like inlet.

If it is the IM, is it likely to be the same as a MK2 1.8 8v GTI one? Seen some of them available but none specifically for her car.

Cheers,
Rob.

Firstly, what do you mean by overrevving? Beyond the red line? High idle? etc

Porous inlet manifold is a very rare fault but there's plenty of gaskets and pipes off it which could leak, especially old rubber pipework which degrades etc.

Mk2 inlet manifold is different.

Yeah sorry I meant high idle.

Ahh I thought it might be different but just wanted to check, cheers!

squeese the AAV to see if the idle drops,if your not sure what it is have a searcharound the site and you should find it,

jaycab said

squeese the AAV to see if the idle drops,if your not sure what it is have a searcharound the site and you should find it,

Cheers, I'll give that a go. If the idle drops when squeezing it then I'm guessing it's stuck open or something letting air in for cold start? How much is a new AAV?

Also worth mentioning, it's smells quiet rich. I didn't know whether it would be sticking on the cold run causing it to overfuel and idle high all the time? Is this measured by a water temp sensor in the stat housing like on my old VR by any chance?

all cars run the cold start of water temp,phone vw for a price of a Auxiliary Air Valve then you can judge what you would pay second hand, :dontknow:

jaycab said

all cars run the cold start of water temp

The Mk1 Golf GTI alters the mixture during cold start and warm up by the warm up regulator. This gradually gets heated by thermal conductivity from the engine block and an electrical element (no coolant connection). It alters the control fuel pressure, which alters the mixture.

The only reliable way to measure mixture is with a gas analyser, so don't do anything too rash (like replacing the WUR!) without at least having a good idea if its wrong or not.

Rich mixture can (& does) cause a higher idle than a weak mixture, but check the air flow (eg aux air valve) first. If the AAV is stuck (either open or closed), you'd in fact see low idle speed at cold and higher idle speeds once warm, since its not varying the airflow (increasing for cold start and warmup).

paul_c said

jaycab said

all cars run the cold start of water temp

The Mk1 Golf GTI alters the mixture during cold start and warm up by the warm up regulator. This gradually gets heated by thermal conductivity from the engine block and an electrical element (no coolant connection). It alters the control fuel pressure, which alters the mixture.

The only reliable way to measure mixture is with a gas analyser, so don't do anything too rash (like replacing the WUR!) without at least having a good idea if its wrong or not.

Rich mixture can (& does) cause a higher idle than a weak mixture, but check the air flow (eg aux air valve) first. If the AAV is stuck (either open or closed), you'd in fact see low idle speed at cold and higher idle speeds once warm, since its not varying the airflow (increasing for cold start and warmup).

yes paul but all run cold start from water temp……….warm up is not cold start.

Yes but its irrelevant in this case, because its a running issue, not a starting issue.

herisites said

I didn't know whether it would be sticking on the cold run causing it to overfuel and idle high all the time?

You also suggest the aux air valve is faulty, which also operates in the same way as the WUR (senses temperature from conduction and has an electrical heating element in it).

The only component which senses coolant temperature and affects fuelling is the Thermo Time Switch but its clearly not at fault in this scenario. Even if it was faulty, the circuit with the TTS and 5th injector (cold start injector) is powered by terminal 50 on the ignition, which is only live when the starter motor is turning, so its isolated from running except during starting. And if there was an electrical fault or miswiring which meant the 5th injector was constantly firing, it would flood the engine so it didn't run. So its irrelevant to the original poster's running issue.

iirc the aav runs the same electric ciruit as the fuel pump and the wur, the cold start opens from the trminal 50 but also under hard throtle action,the op mentioned  about the the mk3 vr water housing,to which my answer was all cold starts run from water temp,i offered a simple test that would bring the idle down if at fault m8,you scared me with your co test,i dont want to be drawn in to a keyboard warrior thing :dontknow:

Mixture test is useful because the OP's sister can smell fuel; and that an air leak will affect the mixture. But more importantly, the basics (tuning the engine by checking and adjusting firstly mixture, then idle speed) should be done, since it might just be out-of-tune and no further fault. Unfortunately there is no way to test mixture without instruments.

Anyway, probably best to await any update from the OP on this one.

would it be worth checking the nut on the back of the throttle body and maybe the mixture setting in the metering head ?     also as for checking for air leak spray wd40  over the supected area and listen for any change ……