Idle drop
 

‘79 US. Start-up is normal, initial idle is normal until engine starts to warm up. Then for a short period of time the engine will briefly drop revs to about 400 RPM’s when going into neutral (e.g. approaching a stop sign), almost dying before recovering to about 800 RPM.

As engine continues to warm up, this issue fades away.

Any thoughts?

Thanks

Jason

adjust the fuel mix ..could be a little rich..what is your set up on idle when warm ..should be about 950....

Ivan

I have the same issue with my '89 964. It's not likely the same thing, but interesting.

This

CIS troubleshooting..........
 

Jason,

What was the idle speed after start up? What is NORMAL? Your CIS should initially idle at higher engine RPM when cold and gradually goes down to 950 after several mins. Several things things come to mind:

• Check for unmetered air.
• Check your fuel mixture and idle speed setting.
• Check your fuel pressures.

Tony

Vid of cold start this am

https://youtu.be/K6fi4J6u_dM

RPM’s settled at about 900 after a min or two.

Initial drop to about 600 rpm when engaging clutch to come out of gear at a STOP within first few mins of driving, then to around 700. Each time would then return to between 900-1000. This outing, it did not come close to stalling out.

Should I consider adjusting the decel valve?

I haven’t checked pressures yet, but recent smoke test showed no unmetered air.

Maybe I’ve got nothing to complain about?

Jason

Hi Jason,

I would go with Ivan - it might be a little bit too rich when the engine is cold.
1. Check the control pressure of the WUR and how it evolves during warming up the engine.
If necessary "tune" the WUR a bit to reach the factory values.
2. Check the sensor plate adjustment, correct it if necessary as given in the Porsche workshop manual
3. Check the CO value on an exhaust tester and make sure it complies with the given values from Porsche.
4. Therefore you have to tune the idle screw a bit as well at the end.

I personally would not expect that there's a vacuum leak, even if we cannot exclude it on a CIS engine

Thomas

I would suspect the Auxiliary Air Regulator shutting off too soon. The AAR supplies extra air during warm up. I had the same problem and solved it by fitting a replacement AAR. You can remove the AAR and apply a voltage across its terminals and watch it close. Needs to be cold to start with and then time how long it takes to close up.
Roy

F/up - car has been running great the past month with no further idle issue. I wish I could say that I discovered the source, but I didn’t. Seems it self-remedied, so to say.

Jason