Guys - this is embarassing - My '94 GTS is acting all wimpy on me and I don't like it...

My idle is running fast @ about 900-950 and hovers about a bit, the car does not want to pull cleanly from a standing start, just wants to stall out - but once underway (with a bit of clutch feathering) pulls as good as ever. There is no overal power problem just this low RPM (standstill) issue. It does coast at low RPM (~1K) a bit unevenly but picks up again quickly with gas.

Starts easily every time, occasionally stalls out after starting when it's still hot.

What kind of issue could seriously affect power @idle but not higher RMP/full power... Where should I start to look on this one??

Alan

 

Alan, how many miles are on the car? Random guess could be vacuum leaks. I had similar issues on my S4 around 100k and the big "plenum removal and valley service" was due.

K

 

K,
Nowhere near that much .. only about 43K... still tell me more.. what exactly did you do (/have done).

Alan

 

Well before I ever bought the car with 69k on the odometer Susan at Devek alerted me to this upcoming service. Also, German Auto Art did the service not me. The intake manifold comes off and there are lots of little rubber hoses and such in there that dry out due to all the heat. In my car the idle would hunt, or cut out, but always refire. I don't remember a hot start problem, but these gremlins first would occur when it was damp out (rare in LA) then increasingly all the time regardless of weather conditions.

The hoses can also get brittle due to age. A friend inherited her fathers 928S4 which was never driven much but was in absolutley pristine condition. They started the drive back to Maryland from LA(at high speed) and from what I understand, around Utah, had a problem with the car not restarting when hot. This happened at refuelings, first, difficulty in starting up then it just would have to sit to cool down. Finally they stopped shutting the car down altogether and basically let it run each day until they stopped at a motel for the night. I don't know anything about the idle symptoms the car displayed or power just offline. I do recall her husband telling me that the intake manifold had to be removed and a lot of the hoses underneath were worn out etc.

This was over 2 years ago so please forgive me for not knowing all the details but that's what I remember.


K

 

K,
OK - thanks, living in Phoenix, AZ this could certainly be the case. If they can dry out & crack anywhere - it would certainly do it here. This seems to have got progressively worse over time and I think I've been compensating for it - but its way worse now - time for action.

Alan

 

If you have the intake manifold removed for any serve or repair, consider replacing the idle regulator valve, the knock sensors, and the throttle position switch at the same time. It is so labor intensive to get in there, that you might as well do the preventive maintenance at the samae time. In case the paint starts peeling off of the intake, this is also the time to have these parts powder coated.

Now, I'm not saying this will resolve your idle problem!

 

My '87 S4 has a high idle, too. It idles at a rock-steady 800 rpm, and nothing I do seems to make any difference. Last weekend, I removed the intake manifold FOUR TIMES <img src="graemlins/crying.gif" border="0" alt="[crying]" /> and still no change. I replaced all gaskets. I replaced any hose that wasn't perfect. I put ty-raps on all vaccuum lines. I tested the idle-speed regulator, the throttle switch, the air-mass sensor, temperature sensors, and the flappy valve and actuator. I replaced all the vaccuum connectors and check-valve at the brake servo. And I meticulously reassembled the lot. No change. I've been spraying water at every hose, every gasket, and at the injectors, but no change in idle speed, no "slurping" sound. I unplugged the injection and ignition controllers, left them disconnected for ten minutes, then reconnected them. No change. The car starts and runs perfectly apart from this one problem. If I had any hair left, I'd pull it out! I'm beginning to suspect an LH problem.
<img src="graemlins/soapbox.gif" border="0" alt="[soapbox]" />

 

The idle on the late cars is fully computer-controlled, and is non-adjustable. As long as any problem (such as an air leak) is within the range that can be corrected by the ECU, the idle will be held at the specified level. If the leak is too large, the idle will be uncontrolled, and probably would not be a steady 800 rpm.

My first suspicion would be that the tach is wrong. Have you checked it with an accurate external tach?

And does it really matter that much? I certainly wouldn't pull the intake just because the idle was 100 rpm too high!

Have you checked the control unit coding per page 28-69 in the Workshop Manual?

 

Thanks, Wally. I have checked it with a digital tach, and it's a steady 800 rpm. Ordinarily, I wouldn't bother, but the car is likely to fail the California smog-check because of it. The placard under the hood says 675 rpm, +- 25 rpm.

I have assumed (up until now!) that the car is completely stock, but I'll have a look at the control unit code later.

I probably will also pull the cover off the LH unit. Maybe someone "chipped" it.

 

[quote]Originally posted by Alan Moore:
<strong>...the car does not want to pull cleanly from a standing start, just wants to stall out - but once underway (with a bit of clutch feathering) pulls as good as ever.</strong><hr></blockquote>

Silly, stupid, easy check that falls under the "Why didn't I check that first?" category of troubleshooting: Make sure you have a clean air filter in place. Remove your air filter completely and see if your pick-up returns to being crisp. My S4 had terrible initial acceleration charcteristics and I checked everything and couldn't find a problem. Just for the heck of it I pulled the air filter and the car went right back to vrooom. A new filter was the "fix". Probably not what's wrong with your GTS, but it's a simple matter to rule this out!

 

Michael,
Unfortunately - brand new air filter less than a month ago... but agree worth adding to the list.

Alan

 

I've heard that some of the aftermarket chips can raise the idle speed on some cars.

 

What's a "plenum removal and valley service" ???? <img src="confused.gif" border="0">

 

The "plenum" on the 928 is the intake manifold casting. The "valley" is the space in the "V" between the two banks of cylinders. Once the "plenum" is removed, you have unrestricted access to the "valley", so you can replace parts in there, such as the knock sensors, hoses, idle-speed regulator, switches etc, which are impossible to reach with the plenum in place.

 

Alan,

I’m pretty new in the 928 business and I am starting to grasp the numerous things which can be wrong with the 928. So I can’t give you any professional advise, but know this:

My ’89 928S4 had a high idle speed as well. With a warm engine it sometimes even started revving in a pulsating way up to about 1500 rpm. When the engine was hot it did nothing but stall, when trying to head off. Greg Nichols tipped me about checking the idle speed regulator. <a href="http://ourworld.compuserve.com/homepages/nichols/tip443.htm" target="_blank">http://ourworld.compuserve.com/homepages/nichols/tip443.htm</a> I had it checked by in the Porsche garage and they said they put some juice on it “externally” I did the WD40 procedure as Greg described and it runs great for about 4 weeks now.