Brett San Diego

Car is a 1978 911SC.

I developed a fast idle problem that I noticed one day after a drive. I couldn't seem to associate it with any issues with the car or anything that I had done to it. Previously the idle was perfect at 950-1000 rpm. The car would cold start normally at 2000-2200 rpm. The idle would drop as it warmed to a low of about 1200 rpm, but then as I drove it, and it warmed up completely, the idle would slowly increase back to 2000 rpm. This behavior was reproducible.

I ruled out the auxiliary air regulator and deceleration valve. I tried spraying carb cleaner around to look for vacuum leaks. I checked fuel pressures, and all were good. Nothing indicated a problem.

This afternoon, my father-in-law and I decided to have a look. He's an old hand Mercedes mechanic and more of a "get in there and start playing around with stuff" kind of guy than me. After tightening the hose clamps in the rubber hoses between the airbox and intake pipes, we started the car and watched the idle drop and level off at about 1200 as usual. He then wiggled a couple of vacuum lines, then when he wiggled the cruise control cable, the idle noticeably dropped. A little more push, and the idle settled down normally to 950 rpm. I noticed there was an adjustment for the cruise control cable (see pic). A couple of turns of the adjustment, and the idle problem was completely solved. I couldn't believe it was that simple after turning this over and over in my mind for the last few months trying to reason which was the faulty part in the CIS system. If you think about what can affect the throttle position, there are two things (besides the butterfly just sticking): the throttle cable, and the cruise control cable. I really needed to take a step back from my deep thinking and get more basic. I still can't totally explain the slow idle increase upon warming except to say that something must have changed shape slightly when things got really warm to cause the cable to become a little tighter than when cold. Maybe expansion of the plastic adjustment nut, the plastic tip where the cable exits, or the plastic outside of the cable housingis to blame. Maybe the metal bracket that holds the cable adjustment bends as it warms causing the cable to tighten a bit. I don't know.

Here's the cruise control adjustment. We just turned adjustment nut a bit to slacken the cable ever so slightly.


In retrospect, I realized I may have caused the problem after all. I did a valve adjustment and changed plugs, wires, and distributor cap and rotor a while ago. When I was putting things back together, I couldn't remember if the cruise control cable should be routed on top of everything or under the heater blower tube. I first put it on top of everything. After a couple of months like that, I noticed that there were wear marks on the cable from contact with the AC condenser, so I re-routed the cable underneath the heater blower hose. My fast idle may have developed at the time that I re-routed the cable, but I can't remember if the two coincided. You can see in the pic above with the cable routed under the blower hose, that the cable doesn't exit the housing in a straight fashion. The exit tip of the cable housing is pulled a bit upward due to the angle that the cable comes from under the blower hose. So which is the correct routing?

Cruise control cable routing under the heater blower hose.


Brett

craiglwakefield

Bret,
My cruise contole cable routes over the top of the blower hose. I couldn't varify that that is stock routing though? The way the cable is connected to the bracket at the adjusting nut is loosy goosy on my 79 SC, it is not a very possative connection? It seems strange that a slight adjustment of the nut would effect much considering how loose the cable is in the bracket? Hey, but if a turn of the nut does the trick, good enough!

good lesson in your thread. Some times you just have to get your hands in there and do something instead of think it to death.
Tsunamiboy

Brett San Diego

Yeah the mounting to the bracket is fairly loose, but the threaded adjustment ultimately adjusts cable tension. And most importantly, IT WORKED. I am very happy.

Thanks for your comment on the cable routing. I think I'll switch it back to over the top. I have a feeling that is how it is supposed to be. Those marks from contact with the AC condenser may have been there before, and I just didn't notice them.

Brett

Jastx

Nice going Brett! It's a good feeling when you find the source of a problem. Even better when you fix it.

AaronR968

Been there done that with my old 951 took me forever to figure it out.

Got Boost?

Is it possible this would cause the high idle problem intermittantly and only with the engine very warm (think expansion)? I've been trying to chase down a similar problem for a while. . .

Brett San Diego

Got boost: I would say yes, it is possible. Maybe sometimes, the cruise control cable doesn't release well. Best way to test it, disconnect the cruise control cable from the throttle lever and see if you ever experience the fast idle problem. Or next time you've got the fast idle condition, fiddle with the cruise control cable and see if that fixes the idle.

Brett

Got Boost?

Thanks, I'll give that a whirl. I feel kinda' stupid for not simply just thinking "oh yea, I'll disconnect it and see if the symptoms persist".