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2G Random idle issue coming to stops


Probationary Member
Nov 29, 2020
San Marcos, California
The last few weeks been having an issue with my idle. Vacuum will drop to around 5, car will nearly die. I'll rev it up and sometimes that helps and its right back to normal it seems. At frequent stops it will happen again but it's only random. Sometimes it will sometimes it won't. So I've looked at my compression, Timing, and leaks. During a boost leak a bunch of air was coming through the oil cap and dip stick (reasoning for checking compression) compression and timing seemed fine. Any ideas?


Proven Member
Sep 9, 2021
Broomfield, Colorado
I have a similar issue in my 1g that seems to be related to electrical load:
I haven't had a chance to get back out and log it since replacing my idle switch, but I've got a bunch of replacement intake gaskets and seals that I plan to install at some point. I'll be doing a throttle body cleaning too. Then a BLT to check for and fix any further intake leaks, and reset my idle per the FSM. Hopefully after all that my issue will be better or at least give me something else to investigate.


Probationary Member
Nov 29, 2020
San Marcos, California
Great input appreciate the responses! Been at work all day and haven't had any time to try them yet. To answer one of the questions, yes if I put the car and neutral and let it roll no matter the speed it'll still do the same thing. First thing I'll check is the isc since it seems to be the quickest and easiest!


Proven Member
Mar 2, 2011
fairfield, Virginia
Some thoughts:
1. If boost leak test showed air coming from dipstick & oil fill doesn't that mean a bad PCV valve? I thought I remembered that on turbo cars that valve has to block air going into the valve cover and if it doesn't that's what you get. Could even mean you've got a PCV that's stuck somewhat open all the time and that's causing a big enough air leak at idle to mess it up.

2. "Sometimes it will sometimes it won't." I would check first the closed throttle position switch -- it goes intermittent pretty often on high miles cars. You say you installed a new switch -- did you adjust it? 'Cause your symptoms would exactly fit a switch that's almost set right but not quite. The very tidy procedure in the shop manual may not always work because it does not allow much for wear on the TB on high-mileage cars like the ones we drive.

"I'll rev it up and sometimes that helps and its right back to normal it seems." This is most consistent with a 'slightly off' IPS setting.

I would carefully tweak the IPS very slightly in the direction the throttle shaft turns to open. See if that fixes things.

3. Then the ISC valve which can also go intermittent and not reliably control idle speed -- usually sticks rather than failing electrically on the previous gen cars that I know best; I don't know about cars with a stepper motor ISC.

4. The fact that the problem comes and goes excludes a BISS issue. That's always a good thing to check when the car's running decently -- who knows how it was set by some PO in the past -- but since it is not the current issue, not now.

I would not-not-not replace or even clean anything much before this is fixed. The more stuff you touch the more variables you've got -- like having to wonder if the new switch got adjusted just right considering wear on the TB.

Good luck with it!
Last edited:


DSM Wiseman
Feb 3, 2002
St. Charles, Illinois
#2 was first on my list of things to check. If the IPS isn't telling the ECU the throttle is closed it won't try to keep the engine from stalling or manage the idle. That goes had in hand with measure the ISC coils and checking if the ISC rehomes on ECU power up.
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