Elevation-related idling problem

[JHanson]

Hello all,

Yesterday my wife and I drove from Tucson (elevation 2,400 feet) to the top of nearby Mt. Lemmon (elevation 9,000 feet) in our 1982 911 SC. The car has been running perfectly around town and on the highway, and ran perfectly up the mountain. However, when we stopped in the little village near the summit, I immediately noticed the idle speed hunting up and down, between about 700 rpm and 1200 rpm. After a bit it settled down. But after driving around up there it wouldn't idle at all, but died if I didn't keep my foot on the accelerator. Once under way it ran fine, and ran fine back down the mountain. However, it died once at a stoplight in town too, after which it seemed to run okay again, although I now notice that when I disengage the clutch coming up to a stop the tach dips quickly below 500 rpm before returning to 800-1,000. (Not sure if it's been doing that all along and I just noticed it.)

The car has a recent full tune, so I suspect this has something to do with the CIS system itself.

Any thoughts?

Thanks,

Jonathan

[longhornchris04]

Car is running rich... not really a surprise. CIS doesn't really compensate for the altitude, even with lambda (which may not be working).

I had a similar issue with my '79 motor when I went from L.A. (on the ocean) to Big Bear Lake (~7000ft). My car was already slightly rich (by choice), but on the mountain I ran into 3 big issues that sound like what you describe:
1: Hunting for idle - assuming it idled at all
2: Extremely rapid drop in RPM whenever the clutch was depressed - i.e. when trying to shift, especially when trying to shift agressivly
3: Inability to 'capture' idle if the clutch was depressed at anything over 2000rpm, and even then it would fall to ~500 before it started hunting. If I release at say 3000rpm, the car would stall.

My solution was to tweak the fuel mixture a little bit, then adjust it back when I dropped back in altitude... and I only did that because it was being particularly difficult around town and I had the tools with me.

So, if you decide to drive the car to Colorado for vacation... bring along the tools. Going to the mountains for a day, probably not too big a deal. Just my $0.02

[JHanson]

Thanks, Chris - that certainly matches my experience.

Given that my 30-year-old carbureted Land Rover and Land Cruiser both run great around town, yet can negotiate 9,000-foot elevation without forcing me to do a three-pedal dance to keep the engine running, it seems like there ought to be a way to get the Porsche to do the same, even with the "primitive" CIS. Am I asking too much of it? Will leaning it out enough to keep it from stalling at high elevation make it too lean lower down, and also make it run too hot in summer?

I believe the oxygen sensor (under passenger seat) was replaced before I bought the car. Is that the Lambda you referred to, and shouldn't it at least make some difference?

Thanks for helping me understand the system.