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Tigger937 · Posted: Sat Dec 15, 2007 2:00 pm Post subject: Wild Idle

Lately, my car has been having the infamous runaway idle problem. This is after the car is warmed up. I would unplug the intake air temperature sensor, the idle would settle down and then reconnect. Soon thereafter, it would run away again, usually >2000 rpm.

My next move was going to be to disassemble the WUR as I was thinking maybe it was corroded and sticking. But, I first decided to try and locate the offending part by banging with a big screwdriver. When I banged on the TPS several times, walaa, idle returned to normal. Sounds like intermittent contacts inside the switch. Anyone else have this problem?
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1981 931 (Concours)
1982 931 (Daily Driver)

"Think outside the box"

Paul · Posted: Sun Dec 16, 2007 12:27 am Post subject:

OK Fonzie, how about testing your TPS with an ohm meter or continuity tester for better results.
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Smoothie · Posted: Sun Dec 16, 2007 1:55 am Post subject:

I think this is more of an S2 problem, and ambient temperature plays a role since it will happen when it's cold to extreme cold, but not when warm. I had it a couple years ago and noticed that unplugging the air temp sensor brings the idle down immediately, and that sensor is attached to the DITC box, so apparently DITC is raising the idle by advancing the spark.
While troubleshooting the problem on my car, I made adjustments to the AAV that seemed to fix it, but then the problem returned later. Then eventually the problem went away completely after making A:F mix and idle adjustments, so I concluded that it was an incorrect settings problem that somehow resulted in the DITC advancing spark too far in extreme cold. I also had the TPS apart for cleaning and testing, but that had no affect - though given that smacking it with the screwdriver did something in your case, obviously the tps might have something to do with it (if the timing of your smack wasn't just a coincidence). Just maybe, cold intake air is affecting the operation of the TPS (the TPS is connected to the lambda box which affects the A:F mix, so there's a tie-in to the mixture again). -And in extreme cold, with the idle already high, more extra cold air is pulled in, keeping the TPS cold due to it's location (I'm thinking the TPS might get warmed by the warmed-up engine if less cold air were being drawn in, but the large quantity of extra-cold air that's drawn in while it's idling high, keeps the TPS cold).. -Or maybe it's a sticking throttle, again due to extreme cold, and it also stays cold due to the high idle and large quantity of extra-cold air being drawn in. Actually, that seems more likely than the cold-malfunctioning TPS scenario. If you've seen my past post on it, I installed a momentary, normally-closed push-button switch to take the air temp sensor out of circuit whenever it's pressed. Though I haven't had to use it in a while, it may have been solving the problem because by bringing the idle speed down, engine temp was allowed to heat the throttle and unstick it? -Then again, I'm pretty sure I manually operated the throttle while troubleshooting it, and didn't find it sticking...
In any case, try the switch trick. It can be installed in a temporary way that facilitates easy removal-replacement as desired.

If it is a TPS problem, you'll have to be carefull to keep it cold and unaffected by hand temperature while testing.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Tigger937 · Posted: Sun Dec 16, 2007 12:12 pm Post subject:

Hmmm.........never thought of it as being temperature related. Maybe the cold is causing the oil on the oil soaked contacts to thicken to the point where contact pressure isn't enough to overcome the contact resistance? Or maybe the cold is deforming the metal contacts in such a way as to reduce contact pressure? I also tried blipping the throttle to see if it was a sticking throttle plate to no avail. Either way, I won't be able to track this down for another month as I just don't have the time right now.
_________________
1981 931 (Concours)
1982 931 (Daily Driver)

"Think outside the box"