safulop

After some extensive work on the intake area of my 88 S4 including new ISV, the idle characteristic is not what I would expect. It starts and idles at 700 when cold, then increases to 850 when warm (in Drive). It is 1000 in neutral when warm. This seems rather inverted. Any ideas what could cause that behavior of the idle control loop?
The idle is rock steady.

Speedtoys

Vacuum leak. Git back in there...

FredR

Brutal- but honest!

Check the more obvious external things before ripping everything out again. In particular take a close look at the brake venturi system connected to the servo- the internals of that thing can easily be displaced although I doubt such problem would show up as things get warmer but....? Also check the breather system/tank vent plumbing around the filler neck area.

Rgds

Fred

safulop

I knew that's what people would say, but I don't think so. I'm asking about other alternatives. In my experience, intake vacuum leak does not allow a solid idle below 700 rpm at all, especially not when cold. Also the idle is generally very high and erratic, when the intake is leaky.

FredR

Your symptoms are a bit strange thus you may be looking for something not quite so obvious -thus why I suggested looking externally first.

On your model the various connections in the vee are such that it is quite easy to miss tightening one of them although whether that would explain your symptoms is another matter.

There have been instances where even new ISV's have created problems. Maybe a few more of our friends can come up with some gems before you start ripping things apart again. I have had my inlet manifold off and back on again in a "good afternoon" so not a huge deal given you have recent experience of the task.

Rgds

Fred

marine928

Had a very similar problem, especially in reverse. Like you, I had refreshed everything including ISV/ICV. Ran two tanks of 93 with sea foam along with some plus 100 mph runs and problem corrected itself.
Now I have a clacking noise behind my middle console vent. Obviously some type of flap not actuating correctly. Will figure it out.
As said earlier, double check the easy vac tubes first with a mity vac and make sure they are connected properly.
Good luck. Vac leaks are annoying but very important to the overall health of our cars.

worf928

Did you powder coat, paint, or otherwise surface treat the water bridge?

safulop

Yes I'm not such a perfectionist, so I will probably just drive it around for a while and see what shakes out. Your problem, however, sounds like a comb flap issue that might be resolved by replacing the brake booster check valve -- as mine was.

I have great confidence in my auto shop and their mechanic, Klaus. They've been working on my cars since 2007 and have never been found to have done anything wrong -- despite suffering much indirect abuse in this forum. I think the chance they missed something under the intake is very remote, not worth ripping open again just on a lark.

I will keep the other vacuum possibilities in mind as I check around. In answer to another question above, the intake manifold was repainted, but not the water bridge. What about the water bridge? Affects the signal from Temp II? That sensor was also replaced.

ROG100

What Dave asked?

safulop

NO I did not do anything to the water bridge except change Temp II.

Today I tried it in the hot sun with the A/C blasting, which are ideal condition for low idle. In this circumstance, the idle was below 700 rpm while running warm. So it could simply be a condition where the brand new ISV is a little too aggressive at keeping the idle up. They have some weird little door riding on a bushing? Needs breaking in?

Tomkat80222

I don't know crap, but was the power disconnected while all this work was going on. It could be your electronics relearning the proper settings. My 94 does this sometimes as I disconnect the power due to a stereo issue that will drain the battery.

Speedtoys

That doesnt cause this much disruption...nowhere near it.

Short Term fuel map for idle comes in pretty fast, cruise as well.

marine928

Yeah, I will check the brake booster connection, if that's not it, will check the connections behind console per one of the many write-ups available through our esteemed members.

Thanks

worf928

Verify that the idle switch closed signal is reaching the LH and EZK...

... right after you verify that the ISV is connected to its harness.

safulop

^^^^^^^^^^
Will keep this in mind going forward, thanks. The ISV mount points were indeed changed, and Porsche does not manufacture this piece for the throttle body. A standard substitute was used instead, some sort of Mercedes part.