idle problems

[carl74344]

1982 911sc us...i just got the car it was idling at 2000 warm or cold. the cruse control cable was holding throttle open.fixed that. now idle is at 975-1000 but surges. it jumps 100-500 rpms. up and down warm or cold. i looked around things look ok. while running i removed the oil cap no change.i thought when you removed the oil cap idle should change. not sure how to start the search.. new to Porsche but have worked on my cars before. not afraid to get in there and try. oh, the car runs fine starts cold or warm easy. thanks carl

[Grady Clay]

carl74334,
First, WELCOME to the Pelican Forum
You will find a lot of help here.


High and variable idle are commonly due to a vacuum leak.
There are various (well documented) techniques for finding vacuum leaks.
Don’t ignore the vacuum circuit to the front of your 911.

Again, WELCOME.

Best,
Grady

[carl74344]

thank you glad to be here. vacum crrcuit to the front? carl

[Grady Clay]

Try here:

Unstable idle - new fuel filter to blame?

The 'Search' Function is your friend.

Best,
Grady

[Paulporsche]

After you check vacuum leaks and ignition components, check your fuel and control pressures (WUR) and your CO setting.

Often a hunting idle is due to either a too low control pressure, resulting in a too rich mixture and/or a too rich mixture setting.

You might also want to check your O2 sensor and relay.

[Bob Kontak]

Hi Carl,

The search function can be a hassle for a new user initially. Click on search on the light blue bar above. Scroll to the first Advanced Search tab and select that one.

Enter something like "CIS vacuum leaks" in the keywords field. Then you can isolate to a particular forum if you wish. Play with it a bit. All of the below will be found repeated in search results.

This link is helpful for understanding a little about CIS. If you spend time with this you will start to understand the system is based on vacuum and pressure with just a little bit of electronics.

CIS Primer for the Porsche 911

The "front of the car" connection is referencing the brake booster hose that connects from the engine (driver's side) to the brake vacuum booster in the front trunk.

A big leak area is the airbox pop off valve. Take your air cleaner element out and see if you have a 2" plastic flapper on the driver's side of the airbox (on the flat area). Where that part is glued to the airbox can come loose and suck air. Also, make sure the flapper seats well and the o-ring is in place.

Advise if you have one. If you don't then that can surface other issues.

The post Grady linked is for an 87. Same concepts (leaks confuse the system) but different plumbing than your SC.

[Bob Kontak]

Originally Posted by Paulporsche

Often a hunting idle is due to either a too low control pressure, resulting in a too rich mixture and/or a too rich mixture setting.

Thanks for bringing this up. I remember when Lorenfb set me on fire for saying a hunting/surging idle was related to a vacuum leak.

Carl has no idle drop with the cap taken off so there is most likely a good chunk of false air getting in - bummer is there may have been adjustments to compensate which will have to be corrected.

[Paulporsche]

Yes. It's possible a PO has set the mixture richer to compensate. It might have been to compensate for a leak, but maybe, not realizing the linkage was wrong, to compensate for that.

It could also be simply set incorrectly.

Since the car seems to run well otherwise, a simple CO correction may do it, especially since he said that the throttle linkage adjustment stopped the high idle.

BTW I've read on this forum that apparently some CIS cars have the rev drop w/ the oil cap removed, and some don't, depending upon year. I think it was dependent upon the location of the connection of the tank breather hose into the airbox (or not). Was it that earlier cars exhibit this drop and later ones don't? Anyone have any hard data on this?

[Bob Kontak]

77 was the cut off for not having a drop. After that they plumbed one of the oil tank hoses into the bellows/boot bridging air sensor and throttle assy.

[carl74344]

thanks to everyone. i feel welcome... the guy i bought the car from said not to long ago he had brake work to include master cylinder. most likly the same guy who installed the cruse control cable wrong.i will check that tomarrow. i start the car and the idle at 975-1000 stays there even when warm. it just bounces up little kind of steady. car runs good once you get going. seems to have power.