autofil

My 86 Carrera 3.2 has been working like a dream, but yesterday the idle seemed to be very high. At hot engine the idle is almost 2000 RPM!
If I run on 5. gear and get the RPM down to lets say 1200 RPM and then stop driving, the idle speed will pick up slowly to 2000 again!

What could cause this?

Lorenfb

Think about anything that got changed/repaired since the car ran O.K,
e.g. was it at a repair shop recently? If no, then the troubleshooting
can get a little involved and depends on your background.

autofil

Nothing changed since the car ran OK.

I have good helpers around me, so any good suggestions on troubleshooting please!

Lorenfb

Check out this web site (www.systemsc.com) on the Diagnostics page.
Also check the following;

1. the idle switch to the DME pin 2
2. disconnect the idle valve and see if idle is normal
3. check for air leak around the intake boot
4. check the throttle linkage and make sure it's not sticking
5. check the voltage to the idle valve (center pin) for
overcharging alternator (S/B 14.0 - 14.75)

autofil

Hi!
I have adjusted my idle by the airflow bypass screw, as described in "project 35 in 101 Projects for your porsche 911, by Wayne R Dempsy". This seems to solve the problem.

Is this a correct way to solve the problem or have I done something stupid?

DGaunt

I'd be looking for a brand new vacuum leak somewhere....the proble is always where....

Good luck

autofil

Where should I look for leaks?
And how can I find such leaks?

autofil

When I was running on idle and then removed the oil filler cap to check my oil level, and then the idle stepped down.

The engine is effectively running as a vacuum sealed unit; when I remove the oil filler cap you are breaking the seal and the idle drops and the engine runs a little rough, this would mean that I don’t have any vacuum leaks, does that make sense, or am I wrong?

Lorenfb

Someone may have "opened" (counterclockwise turn) the air screw too
much. The idle system can "hang" if too much air bypasses the ISV.
If it's O.K., then assume that it's fixed.

autofil

Hi again!
The problem was not solved!
Idle was still unstable.
But have now determined that the micro switch detecting throttle of was not working (wiring into switch has gone bad). When connecting a jumper the idle is working just fine, stable and no problems

This switch seems to be very hard to get to and to replace, mounted on backside.
Can anyone please give me a description on how to replace this switch! ?

Lorenfb

You probably will have to remove the throttle body to replace the switch.

autofil

To much force on the switch was the problem, fixed!
Everythin OK, running nice, stable idle and less surging when cold!

Thanks, learned a lot from this!