mike77

Hi, its a UK 1990 S4. The car will drive all day with no issues. If I turn the car off then start it back up again when the car is hot I get a low idle issue. The symptoms vary in severity sometimes. When I tried it just now the idle was at 333 rpm but was steady. Oil pressure was down half a bar to 1.5 and so was the volt reading to 12v. Other times the idle will drop out or hunt and I find I need to feather the accelerator to prevent stalling. Sometimes the issue will resolve itself after a few minutes driving other times it won't.

I would say the idle on the car is always a little bit low. With the car in D and not suffering from the hot start it is about 500-600 rpms. Voltage seems to drop as the car gets warmer. Cold it is about 13.5 and hot its 12.5.

Cheers,
Mike

mike77

Just recreated the issue just now. Let the car cool down after a long drive today. Took it out again and it was fine. Went back home for about 30 mins then took the car out again. Low struggling idle close to stalling, had to feather the accelerator at stop lights. Volts dropped right down to 11 at one point. After a while the idle settled out, remained low but not stalling low.

The symptoms are all a bit similar to TBF so I checked that and got 0.22 crank endplay, so not an issue I don't think. I have noticed that my battery positive cable is a bit warm. Possibly an alternator issue?

jtrygstad

I have been having similar problems, but generally only occurs during hot weather. Check out your ISV - idle stabilizer valve. It probably is frozen with rust and needs some lubrication with WD-40 or similar. Do a search on both ISV and ICV (or idle control). mrmerlin has a nice cut-away of a rusted ISV.

Mongo

How many miles are on it? It may be time to do an intake service to replace the ISV and clean the grounds on the back of the engine block all at the same time.

I had a low idle issue such as yours and replacing everything under the intake manifold that was vacuum related, including the ISV cured it. The volts are dropping because the RPM is dropping. While the gauge isn't accurate in the car for reading volts, a meter is touching the hot post in the front under the cover and using the negative probe to ground the voltmeter to one of the bolts holding the support bar across the engine.

IcemanG17

ISV is dying.........classic symptoms....what happens is the ISV ends up catching little bits of oil vapor from the crankcase breather.....over time it turns to gunk and it won't spin to the right spot a certain amount of "voltage" should spin it too....my 88S4 had a low-unstable idle.....new ISV fixed it....rock solid after that. The temporary fix is the WD40 trick Mongo described....it will work for a bit...but it takes a serious cleaning of the ISV or a new one to fix it....chances are you need an under intake refresh anyway...good excuse to pull intake, fix flappy, check knock sensors, check old vac lines etc

mike77

The car has 77k miles on it

Ok I need to do a bit more reading about the ISV it seems like a big job so would want to be sure before embarking on it. It was something I suspected in the past and I have tried the WD40 thing. Didn't make any difference. I'll try it again as if it worked it would confirm the issue. Re the ISV I'm not seeing many post about it stating that the issue was more pronounced on a hot start.

Someone else suggested Temp II sensor a while back but I never got around to checking yet so will do.

The other thing that pops up in some searches is the LH ECU. I can't find the post but I read somewhere that from cold to 70c the MAF plays a part in controlling idle and above 70c the LH ECU takes over. Is this true and does anyone have more info?

This temp seems to chime with what I am seeing in my car. I've been keeping an eye on temps when I am starting warm/hot. In my second post I said the car started fine after a fairly long cool down. I noticed the temp was 60 something. On the second start where the idle was bad it was around 80. Most of the times I recall having a hot start issue I think the temp was somewhere around the 80's.

Cheers
Mike

Alan

Are you sure the TPS works, the idle switch is what causes the ISV to actively manage idle - if the idle switch doesn't reliably engage the the idle will be unstable and much dependant on leakage, (of course as above it could also be that the ISV just doesn't work properly).

You can test the TPS idle switch directly at the ECU end, but you'd probably have to do it while the car is hot - since thats when you see the issue...

With a hleper you may even be able to tell the difference audibly (the click of the switch) between cold & hot cases... ear to the intake near the TPS.

At idle (no gas) should conduct, at just off idle should open.

I don't think this is the LH.

Alan

mike77

I checked this in the engine bay (cold) a while back. When I checked it I wasn't trying to correct this issue. I was trying to adjust the transmission cable and checked the others at the same time. The transmission didn't want to tighten up any further, perhaps some dirt in the thread, so I need to look again.

I could hear the click when the throttle cable was opened/closed. I'll do it again hot. Do I need to be checking at the ECU end?

Alan

Probably not if you can hear it click.

Alan

jtrygstad

I need a quick refresher. What does the acronym TPS mean? I too am having some idle problems.

Alan

Throttle Position Switch, it monitors the position of the throttle - on most years it just has an idle switch (engaged on when your foot is fully off the throttle => throttle plate closed) and a WOT switch for when the throttle is wide open (it engages well below 100% though).

The idle switch triggers air management via the ISV (Idle Stabilization Valve) - basically a managed volume air leak around the closed throttle plate that is varied to meet the target idle RPM.

The WOT switch usually selects a different map in the Fuelling ECU.

Alan

Dictys

This will sound a little odd - but have you checked your air filter is the correct way up? i.e the dome bit facing upwards.

Alan

It does sound a bit odd... I can't imagine that would impact idle - though I could see it choking off the top end perhaps...?

Alan

mike77

Alan, I read on another thread that you can test the ISV by driving down the motorway and lifting your foot off the pedal and the mpg display should say 90mpg. Tried this on my car and it worked and I got the 90mpg read out. If I have understood what you have said and the other post then this test would have checked both the ISV and the TPS as they are related?

It won't hurt to check it, so I will.


A couple of thinks I am thinking of trying

- Replace the ground strap
- Replace the 53B relays (is it 3 altogether in an S4?)

I read a post about someone with a similar problem resolving it with a relay change. I figure it doesn't hurt to change the above

Next I'm thinking about getting my ECU tested and rebuilt if necessary

Alan

Yes & no. This really only tests the TPS (not the ISV) It shows you that the TPS idle switch worked at that time* - you'd need to repeat to be sure it is fully reliable - and note that everything is much cooler at speed due to airflow so its not really an equivalent test at all...

If at speed you declutch with no throttle (obviously only on a manual) you'd be testing the ISV also. If the car didn't stall/struggle with low rpm you'd know both were working under those conditions. However this is an awkward test environment - you have other stuff to worry about - not recommended!

However this is a silly test for this case - you know things work OK except at hot temps after heat soak, so not only is it far easier to just test under those conditions - it's actually quite meaningless to test at speed under relatively much cooler conditions...

So - just heat soak the car and then test the TPS either audibly or much better with a DMM at the LH connector (or with a Bosch hammer).

* Now the reason you see 90MPG (which here is actually a proxy for infinity) is that the injectors are inactive under these conditions - and so you are using no fuel, (this is called overrun fuel cut-off). Given there is no fuel and the engine is just being rotated by the transmission what the ISV valve does is of no real consequence (except perhaps to the exact degree of engine braking).

Alan