Fast idle problem solved. It was a tight cruise control cable.

[Brett San diego]

Car is a 1978 911SC. Nowhere have I seen mention of a cruise control cable being the root cause of a fast idle, so this may be informative to some. I have read of others' fast idle problems with symptoms pretty much identical to mine (specifically Rocc's thread in which I just posted this same solution). If you have cruise control, check the cable tension!!

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 it comes from under the blower hose. So which is the correct routing?

Cruise control cable routing under the heater blower hose.


Brett

[1982911SCTarga]

Congrats on running your problem down. I noticed that wear on my cruise control cable from rubbing the condenser when I bought my SC. I used a zip-tie to keep the cable housing from rubbing the condenser (while not interfering with the throttle). As I recall, that cable is an expensive thing to replace.

Brian