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Idle Speed Oscillates

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I have a 1999 C2 Cab with 33K miles on it that has an idle oscillation problem. Only happens when warm and happens most but not all the time. The idle speed oscillates up and down between about 500 and 1200 rpms (with the A/C off). I have replaced the throttle position switch, replaced the idle speed control, and have cleaned the throttle body and none of these fixed or seemed to change the problem. Is there another very likely culprit I could try or should I buy a code reader and get some codes out if there are any? Would appreciate some additional help on this one. Thanks much!! Dwight

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If you have replaced the idle control valve and the throttle position sensor and cleaned the throttle body then you likely have an intake leak somewhere.

You are only going to see that with an OBD II tool that can show air flow and TRA and FRA. Still it might not hurt to look for any pending codes.

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IF you have eliminated other things like Loren syggested, take a look at the temperature sensor. There have been many sensors that have gone bad and will give you the same symptoms. The sensor is located on bottom of engine just below A/C compressor. Pull off the connector and try cleaning contacts. Also, rule out a computer glitch by rebooting the computer.

Edited by 1999Porsche911
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I'm sure I'm missing something that is going to be embarrasingly obvious, but I'm still going to ask.

How does one reboot the computer? Isn't this done each time the ignition is switched off?

Disconnect the negative battery cable for a few minutes and then reconnect. You will need to reenter your radio code. The engine will hunt for a while until it relearns the f/a mixture, but should settle down if the reboot corrected the problem.

Have you cleaned the MAF?

Edited by 1999Porsche911
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  • 2 weeks later...

On re-booting the computer, is it true that turning the key to the on position for at least 1 minute and then off for 10 seconds will also re-boot the computer? The workshop manual calls out that procedure for something I think it calls re-setting but I had thought (maybe mistakenly) that it meant re-booting. Can someone please clarify for me.

Also, once re-booted, about how long does it take to relearn everything so I know whether it fixed the idle problem.



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