Idle fluctuates

[DZGunner]

When my engine is warmed up or just at standard operating temp, the idle fluctuates when I'm at a stop or just parked. This applies to when I'm in gear or in park. The remedy thus far has been to just barely keep my foot on the gas and the idle sits very smooth, but as soon as i take my foot off the gas it will fluctuate between low (400 rpm) and high (1100 rpm). I've already changed the IAC valve about a year ago and i don't imagine it should go bad that fast.

anyone have idea of what could cause this?

I will try to make a video of it later to better detail what I'm experiencing.

[Mrmerlin]

check the coil wires for rubbing on their runs if they rub on any metal then they can short out and cause a low idle also inspect the ends where they plug into the coils and caps

[stepson]

If the Throttle Position Sensor (928.606.157.00) is not adjusted correctly, IIRC the switch will not activate the ISV properly and idle "hunting " can occur.

Fortunately, you can check for proper positioning of the sensor/switch with the engine off and the hood open. Move the throttle cable very slowly by hand towards an opening position as you listen very closely for a faint click right off what would be the idle position. Then let the throttle cable move back towards a idle position while listening again for the faint click just before it gets to the idle position. Chances are that you won't hear that click on a consistent basis. If you do hear the click consistently, look elsewhere for the problem.

Unfortunately, you can't adjust the position of the sensor without basically removing the intake.

[Outbackgeorgia]

Mine would stall at a stop sign, no fun. I replaced ALL vacuum hoses, two check valves and all is well. Rock steady idle (computer managed, no adjustment on the LH cars)
The rest of the system must be in good health, fuel, ignition, grounds.
Dave

[MainePorsche]

Originally Posted by DZGunner

When my engine is warmed up or just at standard operating temp, the idle fluctuates when I'm at a stop or just parked. This applies to when I'm in gear or in park. The remedy thus far has been to just barely keep my foot on the gas and the idle sits very smooth, but as soon as i take my foot off the gas it will fluctuate between low (400 rpm) and high (1100 rpm). I've already changed the IAC valve about a year ago and i don't imagine it should go bad that fast.

anyone have idea of what could cause this?

I will try to make a video of it later to better detail what I'm experiencing.

As this is occurring with a warm engine, I am thinking away from your Temp II Sensor. But as compulsive as I am, if it were my car I would evaluate it anyway. If you have the WSM's you can check the particular resistances vs. temps. If you don't I'll attach a link for the WSM's (Workshop Manuals) at the bottom. Also, make sure your electrical connections to the Temp II are in good shape and the contacts clean. If you need a new connector, Roger has exactly what you need.
As to the ISV (I'm not sure what IAC valve is), these can get gunked up in a hurry from crankshaft oil ventilation that runs into the air intake. Aside from taking the manifold off and removing the ISV to clean and bench test, there are two ways to partially clean it. The abbreviated way and the more labor intensive 'getting the cleaner directly into the valve' way. Abbreviated way - spraying your cleaner down the hose by the cable quad and having half go to the valve and the other half into the air intake when this hose forks. More labor intensive way - pull the MAS - spray through a long tube (ie vac line) into the port on the Left side of the MAS housing. This will direct all your cleaner directly to the ISV. I use a non residue solvent like Brakeleen. WD 40 leaves to much residue to which this valve is exposed to.

Originally Posted by stepson

If the Throttle Position Sensor (928.606.157.00) is not adjusted correctly, IIRC the switch will not activate the ISV properly and idle "hunting " can occur.

Fortunately, you can check for proper positioning of the sensor/switch with the engine off and the hood open. Move the throttle cable very slowly by hand towards an opening position as you listen very closely for a faint click right off what would be the idle position. Then let the throttle cable move back towards a idle position while listening again for the faint click just before it gets to the idle position. Chances are that you won't hear that click on a consistent basis. If you do hear the click consistently, look elsewhere for the problem.

Unfortunately, you can't adjust the position of the sensor without basically removing the intake.

Excellent point regarding the TPS.
You CAN set the TPS by adjusting the throttle cable at the cable quad. See my post in someones thread about an S4 'running heavy'

928 Workshop Manual link:

Manuals - Porsche

[Niles]

Here's what I went through to fix mine. I initially did an intake refresh including new IAC, TPS and fuel injectors. At the time, I did not adjust the TPS properly, but fixed it later. I repaired vacuum leaks in the ventilation controls. None of these fixed my fluctuating idle. I checked the temp 2 switch and O2 sensor. They checked out okay. I even tried a different brand of fuel. What finally repaired my issue was installing a spare computer. Test and check everything else first, but it may turn out to be the LH computer.

[Pstallo]

I have the exact same problem except that I've got an 86 911 Carrera 3.2. The idle pulses from about 800-1100 every second when it's warmed up (doesn't occur when cold). Pressing the accelerator straightens everything out, but when I idle it starts again. I replaced the idle valve control and the air filter, but no change.

Did you have any luck o rule anything else out DZGunner?