pete944

When I come to a stop and push in the clutch the revs will momentarily fall to around 400 or 500 rpm and then quickly back up to idle speed (900rpm). The car will shudder a bit when this happens.
My first guess is a dirty ISV. Would that cause the shudder? I know this was a DME issue in the early cars but I think this was sorted out by the time the S2 came out.
If it's the ISV, what's the best method to clean it out?
I also have a crack in th edistributor cap (which will be replaced) but I dont think it would cause the shudder. There's a high rpm 'roughness' that I believe the cap is causing.
Any other things I should check?

Skip

Could certainly be the ISV - gets built up with ozone/carbon. While some folks have luck simply cleaning it (spray in carb cleaner), others have to either take it apart for a more thorough cleaning (involves bending tabs, etc) or simply replacing the unit (best, ensures another XXX thousand miles or trouble-free running).

Check the operation of your TPS (throttle position sensor) while you're in there - these two units sometimes have similar lifespans. For more on the TPS, check here: http://www.tech-session.com/kb/index...x_v2&id=45&c=4

Good Luck!

pete944

Thanks Skip. I'll check out the TPS as well.

Perry 951

Pete... most likely the ISV.. shudder is from the low RPM's and it trying to recover. Worn motor mounts can make the sudder feel pretty bad.

To make you feel better, I went to the trouble of rebuilding mine.. and now it's locked closed. I have to feather the throttle until it warms up or I stall out. Buck up and get the new one and put in in when you store it for the winter.... that's my plan.

pete944

Brian, the motor mounts are new.
How much does one of these puppies cost?

Perry 951

I'm afraid to look.. IIRC.. over $100.

Danno

Unscrew the idle-adjustment screw about two turns while the car is idling. The drop in RPM is due to the time-delay in opening up the stabilizer to compensate for a too-closed throttle-bypass screw. The idea is you want 100% of idle air to come from the bypass and 0% from the stablizer. So unscrewing the throttle-bypass will let more idle air through and cause the DME to close up the stabilizer in order to maintain the same idle-speed.

pete944

Thanks Danno. I'll try that this weekend.

pete944

I just did a search on this issue and some have found that putting a little slack in the throttle cable solves the problem. I did adjust a lot of slack out of the cable when I bought the car. I'll try adjusting it a little and see what happens.

Perry 951

Danno - If you adjust the idle up when cold to keep it from stalling out, then you get a high idle when warm, or at least I did.

My idle is spot on 950 as soon as a little temp gets into the motor, but on initial start up for about 10 seconds, I have to give it a little throttle.

Pete - does your car want to stall out when warm? If so, you do need to asjust it as Danno said, but when cold, not sure what the solution would be outside replacing the valve.

pete944

My car was fully warm (actually kinda hot) when I noticed it. The car doesn't want to stall, just shudders.

Mike S

Sounds to me like your TPS is just out of adjustment. Listen for an audible "click" when the throttle is just barely opened(with the car off). You can adjust it by loosening both of the screws(two) and moving it ever so slightly. You want that thing to click the absolute second the throttle is barely opened. If you go too far your rpm's will hang up a bit before returning to idle....if you go too far the other way it will shudder like you stated.

Operator

seems like there is still no 'one' answer for this idle->rpm drop/shudder problem.. ive heard isv/tps/afm... so confused =/

awilson40

Theres never 'one answer' for anything...its a Porsche

AlexE

The best way to test this.... is short out B & C on the test connector. This bypassess the idle valve. You are supposed to set your idle by bypassing the valve.... Short it out and revs should stabilize and then pull the short out. You will see how the valve tries to find idle (it may shudder or lope up and down)...... On mine the valve must have been faulty. I swapped out for another and the shudder went away.

What I did find when setting my idle by bypassing the ISV that there is a fine line of about 3-4 full turns of the idle screw either way. One way if you go to far you get the car to stall out. If you go too far the other way you get the mysterious hunting idle syndrome 1100-1500 rpm loping.

On my car it was as close as a 1/4 turn before I found a good balance.

I would do what danno said first.... you may idle a little higher, but it may cure your problem for the time being.

P.S and also check your TPS as stated above.