So my car's idle repeatedly surges between 800 to 750 rpms at a stop and on a warmed up engine. Kind of sounds like it's struggling. Interestingly it gets worse when the A/C is off and is less noticeable when the A/C is on but still present. It's 2000 C4 with 58k. Daily driven and drives normally when in motion. Surge only happens when at a complete stop. Recent work in the last couple of months includes: oil/filter change, fuel filter replaced, new engine mounts, MAF sensor inspected, tb inspected and cleaned, and fisters installed. Bouncing idle was there before all the work was done. Anyone with a similar problem have any tips or general ideas? Could it be a pulley or something on the accessory belt?

Also on another note I notice that the engine feels like it shudders and shakes a bit harder than it did before the new engine mounts. Shifting also feels notchier at times. I thought having a failing mount replaced with brand new Reins (OEM stiffness) would make the car feel better, not worse. Could this be a sign the transmission mount is going out now or could it be something else? Thanks.

 

Good question. i have noticed mine does about the same. Look forward to the responses

 

Remove the throttle body and then remove the idle control valve (ICV). Clean the ICV. Cycle it with a 9V battery. The butterfly valve should operate smoothly and not stick. You will find that it looks black inside. It really is supposed to be shiny clean with a bright metal finish. I cleaned mine starting with carb cleaner, then Goo-Be-Gone, then alcohol with Q-tips. Reinstall the TB with a new O-ring.

 

Also possibly a vacuum leak

 

Throttle body has been inspected and cleaned and is very shiny now. I had not thought about replacing the o-ring.


I've done the oil filler tube test and when I removed the cap the whooshing sound was very loud and the idle responded when opened so I don't think there is a vacuum leak.

 

I was not talking about the TB except it is good that it is clean and shiny. I am referring to the Idle Control Valve which is on the TB. Since your problem is idle related, it probably needs to be cleaned out. The ICV needs to be removed from the TB housing, and then cleaned out.

 

Correct me if wrong, but only cable throttle cars have a IVC. What year is OP's car?

I have a '99 C2 cab. I had a hunting idle for 6 months. In this time I cleaned the ICV, MAF, TB, etc. Searched for vacuum leaks by smoke testing many times.

In the end it was my AOS going bad.

If OP could give more details I could be more helpful. Does the idle hunt every time you idle when warm. If you indeed have an ICV does the idle change when you wiggle the wires connected to it? Does the idle ever rev up to about 1200 rpm and stay there?

 

See if it goes away with the MAF disconnected, although a vac leak is always a suspect for idle issues that get better as RPM increases.

 

Pelican describes a test-hack for the AOS that involves temporarily plugging the AOS hose. But a digital manometer on the oil filler is a much better diagnostic tool.

 

He said it was a 2000 C4. Simple enough to check if the TB has a cable, then it has an ICV.

 

Car does not have a cable throttle.

 

I would suggest first reset the ECU. Undo the battery negative terminal and let it sit for 15 minutes. Reconnect. Then do an E-gas calibration...turn the car on (not start), keep foot off gas pedal, wait one minute and turn car off, remove key, then start up as normal. Do a drive so the ECU can set its values. Following that, you will probably have to start checking for vacuum leaks and intake leaks.

 

sent you a PM

 

might send me the same. I want to hear the end to this. My car only goes up and down about 500 rpms. but it didn't used too.

 

The CEL is dark? Have you checked for any pending error codes?

Symptoms/behavior similar to what I experienced with my 2002 Boxster at its first AOS failure. Started out with a varying idle.

If it is the AOS it may not have gone far enough the old try to remove the oil filler tube cap with the engine idling "test" will be of any value.

There is a more reliable test that measures the amount of pressure in the crankcase. I don't know the details but it might be worth researching this and doing it.

The symptoms/behavior is also similar to when my 2002 Boxster experienced a VarioCam solenoid/actuator failure. Although in this case there was a pending error code which pointed to the solenoid/actuator. I turned off the engine and upon restart the behavior was gone and it stayed away during the 30 mile drive home but was present the next day when I drove the car to the dealer.

'course it could be something else too. My AOS experience is kind of varied. The 1st one went from a mild sort of varying idle to smoking like a Chinese coal power plant in really no time as I futzed around trying various things.

The 2nd failure all I got was a CEL once in a while on a 2K mile drive. But after I reached my destination the next morning on the hotel parking lot the oil smoke just billowed from the exhaust and I knew.

The 3rd time the engine was fine then when taking off from a stop I spotted a cloud of smoke in the mirror. I had no prioer warning. Tried to nurse the car 10 miles to the dealer but in a mile or so the smoking was constant and the engine running rough.

My point is it is hard to believe the AOS could be a little bad and remain a little bad all this time.

The VarioCam system might linger in some kind of twlight zone a while though. It depends upon how much or little the car is used.

If no CEL, if no pending code, there's not much to go on. You might want to seek out a highly regarded Porsche shop and have the car in for a check out.