So, I did a full intake refresh this summer on my S4, replacing all rubber pipes and hoses, cleaning fuel injectors, etc.. Besides renewing all of those items, i hoped to track down the source of my erratic idle. After putting the intake back together, the 928 operated much the same, with no change in the erratic idle.

So, last month I took the intake apart again, and replaced the idle control valve. Ta-da! No erratic idle. However, my idle is now much higher, and stays high. I have put about 200 miles on the car since then, so I think the computer has had plenty of time to learn with the new idle control valve. There is no change at all though. When I start the car, it jumps immediately to a 1500 RPM idle while in Park or Neutral. As a result, a shift into Reverse or Drive is met with a pretty loud "thunk" as the engine is revving pretty high. When the car is in D, it is closer to 1100 RPM steady. After warming up, the idle will drop to a little under 1000 RPM while in a drive gear, but putting it in neutral will kick it back up to around 1200 RPM.

Now, I don't believe I have a vacuum leak - I checked and double checked while putting things back together. And, from what I understand, there is no idle adjustment on the 928. What do I do? Is it possible the Idle Control Valve is faulty?

Secondarily, and I don't know if this is related, but the car shifts pretty damn hard now. Not all the time, and usuall at lower speeds, but the shift is hard enough to worry me about the stress on the tranny. It is a very obvious "thunk" into higher gears, lower gears don't seem quite as bad. It has never been a real smooth shifter, but this is pretty bad. I have double checked the tranny fluid and it is full up. Any ideas on this one either?

Thanks!

Bill

 

Bill,

The LH controls the idle speed. With the AT, there is a feature where the idle speed drops a bit more when in a drive gear (to reduce crawl). You either have a vacuum leak that the idle speed regulator cannot over come, the regulator is defective, have a wiring problem, or your LH is going bad.

I see that you have two 928s. Remove the LH from the car that runs properly and plug it into the fast idling 928 [no need to remove the LHs from both cars, just fasten the fast idling 928's LH connector to the "loose" LH, it doesn't have to be bolted in place). If the idle problem goes away, you know your LH is not managing idle speed regulation properly. If it makes no difference - you know the drill.

Good luck,
Rich

 

This sounds like a vacuum leak - the shift pressure is regulated by a vacuum modulator that is fed by intake vacuum. The vacuum line to the tranny's modulator is under the air filter housing. Pull the housing, find the line that goes down over the bellhousing and back to the tranny. Make sure it is connected to a vacuum source. Pull the line out of the source and test the line to the tranny with a vacuum tester. It should hold steady vacuum. If not, the line may have broken or pulled off the modulator at the other end. Or the modulator could be bad. You can try to adjust the modulator. Come back if you think that is it. Really give all the vacuum lines another look. Test the blue/black check valve at the brake booster (although a bad one should not raise the idle). I don't think one of the small lines being broken or detached will affect the idle that much, but a problem with the vacuum to the tranny modulator can make the shifting too harsh.

 

Bill, I just did a refurb on my intake on my 1985. After I was done, I had the same issue, high idle. I found that with all the moving of the harnesses, my connector from the throttle position switch to the harness had worked loose at the harness, but not completely disconnected. Connected it back, and idle is back to normal. Check that the throttle position switch is working and hooked up

 

Thanks guys - much appreciated! I will check all of the above. I hope i don't have to dive back under the intake, although I feel I am getting pretty good at it now

Bill

 

A vacuum leak will default the modulator to its harshest setting giving clunky changes - I'm with Bill. Let us know.

 

Third vote for vacuum leak!

 

Thanks for the suggestions guys. I swapped in another LH a few months ago with no changes, so i felt pretty confident that wasn't the issue. So, today I swapped out the original rebuilt MAF that was previously in the car - I had purchased another MAF from a List member, and that had been in the car for a few months. Once I put the original MAF in and went for a spin the idle seemed to calm down a bit, and the shifts weren't quite as harsh. The shifting is still not as smooth as I like, so I am betting there is a vacuum leak somewhere - more investigation to continue!

I did check the throttle harness per Glen's suggestion and things look good there...good idea though...

Bill