Sanndgroper

Hi all

Looking for suggestions for the following problem.

I have an 88 S4 SE, fires up from cold absolutely fine, and idles perfectly for about 5 minutes, then shuts down as if you have turned the key to off, no rough idle, coughing or spluttering etc, however, if I set off driving after firing her up, there are no issues at all, until I pull up at a set of lights or a junction that is, at which point it can do one of three things, sometimes idles fine, sometimes the revs drop almost to the point where the car cuts out, but then recovers to idle normally, but more often than not, she just cuts out!

She can be started again straight away, at which point, normally idles ok after that, at least for a short period of time.

Oh, and to throw another symptom into the mix, sometimes idles high, all of these symptoms can be experienced in the one short trip.

Any suggestions on what to look at first would be gratefully received.

Cheers

John Speake

The idle CO needs to be set correctly so that the idle loop adaptation can cope with all conditions.

Set it to 1-1.5% tailpipe with engine at working temperature.

If the problem still persists then it may be the ISV is sticking, a search of here can describe the way of squirting WD40 into it.

Another thing to check is idle vaccum, measured at the fuel damper at the front of the engine, there may be a leak. It should measure around 21"Hg.

Avar928

Looks like you had this problem a year ago based on previous posts. What's been done to the car since then?

Could be a bad idle control valve, vacuum leaks, or a bad Mass Air Flow Sensor (MAF) or combination of them. Should probably check all of that. MAFs should for the most part get rebuilt already so do that or buy one with a core trade.

Gunmetal

Heya sandgroper, your obviously in WA? I can recommend a good mechanic if you need one over in welshpool, mines in there atm getting a TB WP.

1988 s4 Auto ROW black/black West Australia

Gunmetal

And is yours the s4 at autodelta?

1988 s4 Auto ROW black/black West Australia

Sanndgroper

Hi mate

Yes, in WA, I have been having my work done at Auto exclusive East Perth, they have done a fait bit of work on it now and I have been pleased with their work, the owner (Walter) races his Porsches, and does work for others involved in racing theirs, the premisies is pretty non-descript, but there is always some very impressive machinery in the workshop, he came highly recommended by another 928 owner in Perth, but always happy to recieve other leads to good mechanics closer to home.
Cheers

Sanndgroper

No that's not mine, but I was at Autodelta about three weeks ago (but not in my 928) and saw a black S4 while I was there, that was a LHD car.

Sanndgroper

Thanks for the info John, I'll check these out and let you know which one (if any of the above) is the culprit.
Cheers

Sanndgroper

Yes, the issue has been lurking for quite a while now, but shortly after I posted on this originally, the radiatior blew (resulting in a nice water / oil mix through the system) so the car was laid up for quite a while (waiting for the new rad and waiting for an age before someone I trusted could do the work) and shortly after having that work done, I decided to replace the timing belt, water pump and all front end seals, so in essence, hadn't really driven it since my original question. Thanks for your input though, I'll check all of the suggested possibilities until I find the fault.
Thanks again

Gunmetal

I use RPJ Motorsports in Briggs street Welshpool, hes been a porsche mechanic all his career, very smart guy on the 928's, we need to catch up and go cruising with Norm ( bogdann on these forums) 3 x 928 would look nice on the scarborough cafe strip.

1988 s4 Auto ROW black/black West Australia

Sanndgroper

I thought that was who you were talking about, I did give him a ring about my radiator job, he advised it would cost me approx 5k all up, but couldn't do the job for quite a few weeks, which was a god send as it happens, as I got the job done, including a Behr radiator from Roger, for just over $3k.

I'll let you know when the idle issue has been resolved, more than happy to meet up anytime.

fraggle

May be a dodgy cps or kick down relay.

Sanndgroper

Won't be a kick down relay as my car is a 5sp , but worth looking at CPS, I recently owned a Mercedes CL500 which is known to suffer from CPS issues, but according to the Mercedes mechanic, the cars can shut down at normal drving speeds, this has never happened with the 928, but worth checking al the same, cheers

fraggle

Make sure no one did troubleshooting and put a relay in the kickdown spot, it should be empty. My 87 acted wierd for years and it ends up I must have flipped pages or something because I has a standard #53 in there that was randomly shorting the tach signal to ground. It wouldn't happen until the car warmed up. Messed with the idle as well.

jcorenman

Does this car have an O2-sensor? Or was it Euro spec with no cats and no O2-sensor?

This sounds exactly like a messed-up (aging or dead) narrow-band O2-sensor. When the coolant temp reached 50-60C then the LH starts adjusting fuel based on the o2-sensor reading. If the O2-sensor is providing bad info then the LH takes that bad info and does stupid things with fueling, as soon as the coolant reaches the magic temperature.

So if you have an O2-sensor (on top of the crossover just ahead of the cats) then try a new one. They typically last around 100km's, plus/minus.

Cheers, Jim