Any GM techs here, HELP ..fast idle problem nobody here can solve
Feb 17, 2015 | 10:54 PM
#21
I would think this problem has to be post TB in order to cause some sort of idle run away unless it is simply a sensor. I would also think if its a problem with a sensor there would be other symptoms. The intake air temp should not have an effect on the commanded idle speed. Can you log commanded idle speed?
I may be stating the obvious here but I would guess the car is commanding high idle, rather than having something cause a higher idle than commanded. In my mind that rules out a faulty intake or even a vacuum leak. It was my understanding that commanded idle speed is controlled purely by engine coolant temp... i guess thats what we need to get to the bottom of. Are there multiple engine coolant temp sensors? Another interesting point is that at a lower ECT the tune should also add a small amount of timing... I wonder if that is happening as well.
I may be stating the obvious here but I would guess the car is commanding high idle, rather than having something cause a higher idle than commanded. In my mind that rules out a faulty intake or even a vacuum leak. It was my understanding that commanded idle speed is controlled purely by engine coolant temp... i guess thats what we need to get to the bottom of. Are there multiple engine coolant temp sensors? Another interesting point is that at a lower ECT the tune should also add a small amount of timing... I wonder if that is happening as well.
Unfortunately I am not that skilled not sure if I know how to log commanded idle speed .........but will try ........ but yes I can see where that would be good to know ...
FWIW ........ there is a recall on the TPS sensor on this model BUT only if you throw a code and I have no codes ........ plus the symptoms had nothing to do will cold idle speed
Feb 18, 2015 | 07:20 AM
#22
On The Tree
Joined: Feb 2004
Posts: 166
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From: North Georgia
Do you have access to an emissions evap smoke tester? You can inject the smoke into the intake duct work and possibly isolate source of vacuum leak, you can also unplug maf sensor to rule it out, I have seen some maf's cause crazy issues.
Feb 18, 2015 | 06:26 PM
#24
OK thanks .......... Dumb question can I log those PIDs with my Autocal ??
FWIW .... timing does not change when idling about 700rpm when warm from normal 525 rpm warm idle
Very cold day today ........ after about hour driving in city truck finally idling back to normal 525 temp at 195 ............ then left on 150 mile hwy drive ect around 192-195* f when I arrived and in town truck idling up at 700rpm !!!! .......ect never below 192 for 2 + hours .......... once truck off hwy and in town idled back to 525!!!!
FYI .. was in Arizona for month of Jan with truck ........ no issues , cold start fast idle came down to 525 once ect at 143 f as the stock tune shows it should be
FWIW .... timing does not change when idling about 700rpm when warm from normal 525 rpm warm idle
Very cold day today ........ after about hour driving in city truck finally idling back to normal 525 temp at 195 ............ then left on 150 mile hwy drive ect around 192-195* f when I arrived and in town truck idling up at 700rpm !!!! .......ect never below 192 for 2 + hours .......... once truck off hwy and in town idled back to 525!!!!
FYI .. was in Arizona for month of Jan with truck ........ no issues , cold start fast idle came down to 525 once ect at 143 f as the stock tune shows it should be
Last edited by CDN SS; Feb 18, 2015 at 06:32 PM.
Feb 18, 2015 | 06:58 PM
#25
Is it possible that the truck is not actually reaching a very high temp? How cold of an outside temp are we talkin? Have you tried lowering the coolant temp that it requires to hit the lower idle speed?
Feb 18, 2015 | 07:57 PM
#26
-25*C (-13*F) right now in Winnipeg.
Never used an Autocal as I have a V2, but I think you can use the V8 software to load PIDs into the Autocal for Black Box logging....click on BBX. Or use the Autocal as a pass through for logging to a laptop using either the V8 software(click on Scan) or the V7.5 Scantool software. Have to select the correct ECM(E38) to view the PID selection list.
Never used an Autocal as I have a V2, but I think you can use the V8 software to load PIDs into the Autocal for Black Box logging....click on BBX. Or use the Autocal as a pass through for logging to a laptop using either the V8 software(click on Scan) or the V7.5 Scantool software. Have to select the correct ECM(E38) to view the PID selection list.
I have seen that E38 PIDs list in V8
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