911 3.2 ‘87 - wont idle when warm

[PeterM]

Yes, as the title says, it wont idle when engine is up to temperature.

When cold it idles nicely, but when it starts to warm up, the engine also starts to hunt/surge in idle. And when complety warm then it wont idle at all.

When the engine is warm, and you open the throttle linkage a bit, so that the Idle position switch is open, then the engine keeps running fine.

I have checked the Idle speed control valve for resistance and voltage according to the service manual, and they are OK.
I am not sure if the ICV operates correctly. Is vibrates OK, but I am not sure how much the valve inside should rotate/open ?

Any suggetions on what can cause the “no idle when warm” “

[RedCoupe]

Did you try setting the base idle speed after jumping the proper pins in the electrical socket in the left of the engine compartment?

[PeterM]

Originally Posted by RedCoupe

Did you try setting the base idle speed after jumping the proper pins in the electrical socket in the left of the engine compartment?

Sorry, please explain what you mean by “jumping the proper pins.... etc”

Which pins on what plug ?

[76FJ55]

Idle adjustment procedure per page 24-9 of the Porsche WSM.

The biggest issue causing idle problems with these cars is generally vacuum leaks, so if following the above procedure doesn't correct your problem, I'd start my troubleshooting there.

[mysocal911]

Read the last post in this thread;Bench testing Carrera Idle Control Valve
When warm, jumper the temp sensor with a paperclip to eliminate the sensor as a problem source.

[76FJ55]

Sorry first post cut off half the image of the idle setting process.

[PeterM]

Originally Posted by 76FJ55

Idle adjustment procedure per page 24-9 of the Porsche WSM.

The biggest issue causing idle problems with these cars is generally vacuum leaks, so if following the above procedure doesn't correct your problem, I'd start my troubleshooting there.

I have just replaced all the gaskets on the intake manifold, and the problem started after that. So I think I will start checking for vacuum leaks.

[Nick Triesch]

If it started after that then go back and check your work. You could have left a gasket out! Or not Torqued correctly. I had this problem. Recheck the gaskets.

[Shorty62]

I had a similar issue and found it was a vacuum leak after I disturbed the lines when removing the inlet manifolds. Usually elbows and joiners tend to perish because of the heat and become fragile, look for cracks in the rubber as this is a sure sign.

[PeterM]

Now I have checked for vacuum leaks, and there are none. Also checked the inlet gaskets, there are all there. And checked the torque on the bolts on the inlet manifold - all good. So it still will not idle when warm (idles fine when cold).

When the engine is warm, and you open the throttle linkage a bit, so that the Idle position switch is deactivated, then the engine keeps running fine.

What happens when the Idle position switch is activated ?

Do you think that it is the Idle speed control valve that is faulty ?

[gomezoneill]

Possibly a disconnected vacuum line somewhere. If it started doing this after your work then it's something you did incorrectly. Recheck your work.

[911obgyn]

Another possibility is you fixed the vacuum leaks that someone has tuned around and now the idle, CO adjustment needs to be re done.
Bentley manual has procedure
Bypass the ICM as the previous post shows and see if you can get idle close when it’s warm

[PeterM]

Originally Posted by 911obgyn

Another possibility is you fixed the vacuum leaks that someone has tuned around and now the idle, CO adjustment needs to be re done.
Bentley manual has procedure
Bypass the ICM as the previous post shows and see if you can get idle close when it’s warm

Ref : “Bypass the ICM as the previous post shows and see if you can get idle close when it’s warm”

Do you mean the ICV Idle Control Valve ?

And how do I bypass it ?

Thank you for helping 😀

[brianlay]

Jumper pins B and C then adjust the idle speed. Jumpering sets the ICV to the middle of its range, doesn’t bypass it.

[ischmitz]

I'm surprised no one has mentioned the CHT sensor yet. A typical failure mode is when the original 1-wire sensor (relying on GND from the threaded portion into the head) starts failing due to corrosion. The engine control unit always sees a cold engine. At start-up this works out just fine. As the engine warms up the DME continues to enrich the mixture and as a result idle deteriorates.

Measure the resistance between pin 13 and pin 16 inside the 35 pin DME connector after unplugging it from the DME. With the engine cold you need to see around 2kOhm - 3.5kOhm. Then run the engine and get it up to operating temperatures where idle is bad. Again shut it down, unplug the 35-pin connector and repeat the measurement. Now you need to see around 100Ohm. If not the sensor is bad.

Ingo

[PeterM]

What happens when the Idle position switch is activated ?

When the engine is warm, and you open the throttle linkage a bit, so that the Idle position switch is deactivated, then the engine keeps running fine.

[brianlay]

According to the manual, the main function is to activate the idle control valve, as well as fuel cutoff during deceleration.

Have you jumpered B to C and set idle yet?

[ischmitz]

The idle control valve is always active (vibrating) but the DME controls it differently depending on the status of the idle micro switch.

Once the idle switch is activated different maps for fuel and spark are selected and the ICV is used to maintain 800 RPM. The later DME versions use 880 RPM. Engine speed is maintained by controlling the ICV opening angle. Think of it as metered air in addition to what passes though the small opening of the main throttle body.

When you jumper the pins in the diagnostic port the ICV is held at a certain opening angle and with that you set the throttle stop position to achieve 800RPM. Ideally the engine idles with the ICV around that opening angle after the jumper is removed.

The other important setting is the mixture at idle. That is done with the AFM air bypass screw. The screw effectively sets the AFM flapper position at idle. If more air is passed through the bypass (opening the screw) the flapper closes more and the mixture gets leaner and vice versa.

[PeterM]

Ref : “ When you jumper the pins in the diagnostic port the ICV” & “ Jumper pins B and C”

There are no letter markings on the diagnostic port. Can anyone post a picture, so that I can see which one are B and C

[ischmitz]