frankvan

Usually when approaching an off ramp or a red light i gear down and let the engine do some of the braking effort - also gets the rpms into the right range for accelerating down the ramp or when the light turns green
But lately sometimes when i gear down it seems as if there is no engine braking at all and the car just stays at the same rpms. If I put car in neutral rpms fall back to idle normally, ie, it doesn't fall back slowly or hang at a certain rpm.
I've checked for vacuum leaks and all the connections to the idle switch, afm, etc... and all is fine. I think i have a current leak with either the coils or dizzy caps as i also get intermittent CEL's (for Hall Sensor) or ABS warning lights, but i dont think that would aggravate this problem.
I have not tried a DME reset or cleaning the ISV.

Rocket Rob

If you blip the throttle, does the engine RPM rise and fall normally? Could your throttle cable be hanging up?

frankvan

If i "blip" the rpms fall normally although sometimes they seem to hang up around 3K RPM. I was wondering if there could be a speedometer input into the DME that's causing the problem?

I did have a problem with a stuck throttle a while back where the short arm that controls the cruise control fell off and got stuck in the trhotle mechanism - i feel for those toyota drivers now (if it was indeed a real problem or just bad drivers?), scary s**t. But checked that and it's all good. Throttle closes normally when gas pedal released. Somehow a little air is getting past throttle body allowing rpms to stay up.

FWIW, my spoiler raises at 80 kmh vice the usual 70 kmh, but i think i also have a different final gear in this C4 as the rpm vs speed doesn't match up as per the graph in the owners manual or per my previous C4 (both are/were 89's).

medent

My spoiler rises at 80 kmh also. I think that is normal.

Vandit

I can't see how anything electronic (e.g. DME) that isn't mechanically connected to the throttle causing the revs to hang.

frankvan

Vandit: I was thinking that maybe the DME was sending a signal to the ISV to stay open a little causing the revs to hang. If I have the time this weekend i'll clean the ISV and see if that makes a difference.

medent: maybe it's a canadian car thing? my first C4 was a US model (spoiler definitely went up at around 70 kmh / 45 mph) but the current one is Canadian. Any other Canadians that can comment on spoiler lift off speed?

frankvan

Clean ISV and oil change don't make a difference
I am starting to think i have an intermittent vacuum leak and it seems more prevalent when the engine is warmed up. I sometimes getting a whistling noise from the engine area when driving (unable to duplicate when looking at the engine of course) that seems related to engine speed. Ahh, the search begins, again...

Gus

Ok, question now is ---What or how do you find a vacuum leak?. I have very small vacuum leak that I have been trying to find for months.

Rocket Rob

The traditional way for finding a vacuum leak is to spray a liquid around the likely leak spots on intake plenum while the engine is idling. I've done this using carb cleaner. When you hit a leak, the idle will drop since the carb cleaner is being sucked in and doesn't burn well. Its a pretty quick way to isolate the problem area.

Gus

Have kind of tried this method with starter fluid - which would cause engine to accel when fluid is pulled in at leak. But I think the leak is on the other side of some check valves so it has not been successful. Issue is that the vacuum canister does not hold pressure once engine stops- have installed vacuum gauge and can see bleed down once engine is shut off. So know I have leak just can't pin point.

FeralComprehension

In the past (turbo cars, mostly) I've contrived various ways of pressurizing the suspect circuit. 3-4 psi and even small leaks hiss like a snake.

Gus

Now that is an idea as I can block of certain hose lines and check specific lines.
Thanks

Steve Weiner-Rennsport Systems

Hi,

You've done the easy part here by cleaning the ISV with (hopefully) carburetor cleaner. If that didn't help, more detailed troubleshooting is now required.

Two other things can cause a high idle such as what you've described: intake system vacuum leak and a DME-A/C compressor wiring or connector fault.

1) Finding vacuum leaks can be done various ways and take a good deal of patience and some skill by using either carb cleaner (much better than starting fluid) or propane. The best and most accurate way is to use a smoke machine which is a piece of shop equipment made specifically for this task. Most good shops have one and this may be a good option for you.

2) The other issue is a possible short or loss of continuity in the wiring between the A/C compressor and the DME. When the compressor clutch is engaged, a signal is sent to the DME to raise the idle speed to compensate so the DME commands the ISV to open up a tad more. You'll need a good DVOM and the schematics to check this out.

Patience is Rewarded,....

frankvan

Thanks Steve.
the idle speed is actually fine and pretty stable, it just takes a long time to get back down to idle.
I ran an errand tonight and it seems the revs fall fine for the first few minutes of driving and then they start to act up again.
I also noticed that when i turned the CCU temp from about 24-27 down to a cold setting (ie, blue dot) the revs seemed to drop normally as well. Turning heat back up and the revs dropped slowly again - possible coincidence? I think i remember reading about a connection from the CCU to the DME but not sure of what it is.
FWIW, the AC compressor hasn't been engaged in about a month - last week was -20 and snowing! (it's warmer around the freezing mark now).

F

Steve Weiner-Rennsport Systems

You might also check the head temp sensor, its wiring and all connectors very thoroughly as thats another component that can generate similar symptoms.

Good luck!!!