wondering idle on a 1982 911SC
 

Does anyone have a permenent solution to a wondering idle (from about 800-1400rpm) on the 3.0 with the CIS injection system?

Could be a number of things. It seems like the winter time allows it to show its face. Did you check for vacuum leaks? Does it only do it while it is cold?

I would do a search for CIS trouble shooting on here and you will probably see a few ideas.



Phillip

It's a wonder that some engines even run. :D

Wonder what it could be? :confused:

Doesn't sound like hunting with that much difference in RPM.

It's an '83 but more maintenance history is needed to give an opinion.

Tune-up? Distributor ever cleaned/oiled/serviced? Throttle linkage jammed by the clutch cable near the bell crank? When does the RPM change? During cold-start? At a red light? When doing what?

that would make for a serious rich condition. surprised it does not cut off if you give it gas and let the RPM's drop.

check fuse 18 for the dme relay
power to the WUR, 12v.
control pressure.

that order because of ease.

I unplugged my o2 sensor on mine, and its rock solid 800rpms.

It's an SC and SC's have no DME. :)

Carreras have DME.

'83 SC's have a Lambda ECU under the pass seat and when it doesn't work, the engine runs very rough in idle but no resulting increase in RPM and no surging or hunting.

I wonder why it wanders?

SC should idle at steady 950 RPM, unless it's just been started cold.

check your air/fuel mixture.

when your 02 sensor craps out, the ecm goes to a fixed duty rate, and could result in to rich a mix on a warm engine

Disconnect your O2 sensor. Reset mixture to 3-3.5% CO measured at the tailpipe. Problem solved. Idle hunts b/c frequency valve is trying to play with fuel mixture to meet emissions demands of the O2 sensor. Disconnecting pegs the Frequency valve at fixed position, 55-60% open I think. Vacuum leaks, even small ones, can contribute to the frequency valve idle hunting.

I had a very similar issue on a 76 turbo motor years ago. It turned out to be the WUR. I used a gauge set to test system pressures at various points in the CIS system and narrowed it down. Order one from our host if you don't have one. They are invaluable in trouble shooting CIS.