Fast Idle Problem on 79 SC 3.0

[JJG911]

This is my first post as I am a total newbie to the Porsche community after being a VW fanatic for the past 14 years. I bought a 79 911 SC yesterday with 97K original miles and am having a heck of a time with the engine idle. While driving today I noticed that the idle was as high as 3000 RPM when stopping for a traffic light. I could tap the accelerator and it would drop to 2200 or so, then very erratic as sometimes it would drop to a normal 800-900RPM. I have read some posts that indicate the cause is probably an air leak somewhere.

Please help me get started with checking the most likely places to check and the proper procedure. I am pretty savvy mechanically so don't mind "digging in" to check possible causes. Thanks in advance for any much needed guidance.

[timmy2]

CIS problems, Vacuum leak or cracked airbox are possible causes for your '79 idle woes.
There are numerous posts here describing all.
Use the advanced search and select titles with those key words and you will find tons of information.
Ask specific questions about the methods and repairs after doing some troubleshooting with the information you find.

Good luck, there are many here that are willing to help once you list your troubles.

[JJG911]

Thanks. I'll start checking for leaks tomorrow. Also have ordered the Bentley manual. Will follow up with results.

[snbush67]

Jim Williams CIS primer will get you headed in the right direction...fast, bookmark and enjoy;

CIS Primer for the Porsche 911

[timmy2]

Pick up Wayne's 101 projects book while you're at it.
It is great for everything you need and possibly will want to do with your car.

[Tsunamiboy]

Timmy2 is giving sound advice. My 79SC had a busted air box that you couldn't hardly tell it was cracked and it messed with my idle. The pop off valve was installed to late, the box was already cracked by the time I installed it

[timmy2]

Originally Posted by Tsunamiboy

Timmy2 is giving sound advice. My 79SC had a busted air box that you couldn't hardly tell it was cracked and it messed with my idle. The pop off valve was installed to late, the box was already cracked by the time I installed it

Mine too, and I even had it out on the shelf while I was doing my head studs...
Put it back in and set my air filter on fire when the pop off valve blew out!
Found a used stainless one for less than a new plastic one, fixed a split weld and it's been working well since. (Knock on wooden floorboards)

[Joe Bob]

Air box....

[Vin-barrett]

the erratic RPM changes would lead me to the decel valve first but any old CIS system should be inspected for air leaks as best you can

+1 on Jim Williams CIS site, great resource for learning about your system, it has parts the CIS (watercooled) VW's did not

[kodioneill]

Check the throttle lever fulcrum on top of the engine sometimes the shaft gets hung up due to wear. Check for a sticking vacuum advance also very common.

[JJG911]

Thanks guys for aiming me in the right direction! I can see already that this list is going to be a great help. And yes I'm going to be learning a totally new language (Porsche) that didn't exist in my air cooled and waterboxer VW world. Things like air box and decel valve are new to me, but I consider myself a fast learner who just turned 72 yo this week.

Now to find the problem....

[millarg]

FYI - If it's the airbox, I have a beautiful Webb Stainless Steel airbox with built in pop-off - has the larger intake runners that you need.

[JJG911]

Well, I haven't solved the idle problem but found some problems that seem to help a little. First, the clip that holds the accelerator cable in place above the air box is missing causing the cable to move in the bracket. I fixed this temporarily until I can figure out what's missing. It appears to be similar to a fuel injector mounting clip?

I found one plug wire lose at the distributor cap. Also, the hose that runs from the canister forward of the oil filler to the air box was an extremely lost fit. At the air box end, the hose is only about 2/3 the size of the hole. Should there be a reducer fitting at the air box? Is an air tight fit necessary for this hose?

When I cranked the car it idled at around 1800 RPM then slowly came down to 850, but after driving for a few minutes the idle speed of choice is now around 1800 RPM. Better than yesterday's 3000 RPM, but still way fast. I haven't opened the distributor yet to check the lash, but any suggestions at this point?

[timmy2]

Yes the hose you describe needs to be a tight fit if it is the one that runs to the back of the CIS rubber boot. The oil tank is subject to vacuum as well so both hoses going to it need to be a good seal. There should be an elbow for the hose to attach to the airbox rubber boot.
Top hose in middle of this picture is what I'm guessing you are referring to: