High Idle Stumped

[Tcrate]

Well I am really stuck on this one now. So I have a high idle condition. It runs cold at 950 where it should be when warm. When hot it idles at 1100 which is where I expect it to be when cold. So I went through a number of items as follows:

1. Replaced all vacuum lines and ran smoke test. No issues found.
2. Pulled the intake and checked the function of the ICV. OK
3. Replaced the DME temp sensor which probably did not need it but while I was there I replaced it anyway. OK
4. I also replaced the throttle cable which was frayed and needed replaced.

So....... looking for any ideas now. I think I have expired all my options. I will add that before I pulled the intake I disconnected the ICV. The idle jumped up when I did so I think it is working as it should???

Anyone have any suggestions.


1987 944S 120,000 miles.

[rick13]

What about the tps. Is it supposed to "click" when throttle body is closed.

[Harriet03]

tps is well I like it

[Tcrate]

Oh yes. I checked that too. Clicks right off of its base position. I also checked resistance and it checked out in the various positions and within limits. Thanks for bringing that up.

[rick13]

After she warms up try applying a little pressure on butterfly flapper. Make sure its closing all of the way. Maybe its just a sticky throttle body.

[markivtruck]

check tps switch for oil getting in. is this a 2 valve car? check air flow meter for sticking flap. what is co. with o2 sensor disconected and before catalyst. fuel pressure regulator vacum diaphrams ok and dampner on fuel rail. bridge test socket by fuse box a&c and set idle speed without idle stabilizer in system. a few things to try. good luck

[Tcrate]

This is a 4 valve, 16V engine and the idle is not adjustable. I will check the throttle body. When I pulled it all apart it was very clean. The PO must have done some work to it. I will also say the PO changed the DME and I believe he changed out the AFM. It almost seems to me like there is something working in reverse here like there is a wire switched but I dont know.

[Tcrate]

To check CO I would need a tester to measure that correct?

[mallard ducks]

I have a 944S with the exact same problem and have spent literally months trying to track it down with no success. I've checked everything fuel and air related over and over again. New TPS seals, injector seals, vacuum hose, intake manifold gasket, confirmed no leaks before the TPS or after, new TPS sensor, everything's adjusted correctly, damper and FPR were good but I replaced them anyway, AFM/barndoor are good.

The one thing I am suspicious of is the ISV... in the closed position it's not closed. It's open quite a bit. There is a hex key adjuster for adjusting it (at least that's what I think it is), but it is "sealed" with a cover and my fingers aren't dextrous enough to get at it anyway without removing the intake, which on a 16V is a pain. I have also read about ISV seals breaking down over the years = vacuum leak.

[Tcrate]

That is interesting because I did pull the intake and I checked the ICV. It opened and closed quickly. However, I did not leak check it. I also disconnected it while it was running and the idle jumped up. This is really frustrating. So I am thinking about a new DME but that is an expensive experiment.

[Tcrate]

So I finally broke down and had this one into a shop. They diagnosed the issue as the throttle body sticking open when warm. They pulled it off, adjusted the TPS, adjusted the stop screw and cleaned it up. Idle is where it should be now.

[rick13]

Glad to hear.

[mytrplseven]

I had an issue similar to this and after replacing the throttle body shaft seals (leaking vacuum) and cleaning the bore and verifying complete movement, I also replaced the return spring with a new one, mine then worked smoothly.