2000 silverado high /rough idle when cold - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 11-30-2014, 12:02 PM   #1
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Default 2000 silverado high /rough idle when cold

ok more troubles again
2000 silverado 4x4 4.8l. when its cold out and only when the truck sits over night , when you first start it up it revs to about 2-2500 rpm then drops and stalls. does this 2 ir three times then runs fine. no codes were showing. its been doing this for a few weeks. then the other day I had a
p0306 code. i replaced the coil plug and wire and the IAC valve. started it up and it ran fine. drove about 30 miles and all good. went out this morning and it revs back up and dies again. got me stumped. had it to a shop and they thought it was the iac valve to. he says my intake gasket is fine but i am sure he only checked it in his shop
after it was warm. possible the intake gasket???
any help would be appreciated. thanks
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Old 12-01-2014, 12:04 AM   #2
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Does it idle high when warmed up. First thing comes to mind is intake gaskets or excessive carbon on the throttle body plate.
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Old 12-01-2014, 12:20 AM   #3
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hi. no idles good and runs good. takes about 5 tries when starting first thing in the morning. the first five or so start are all the same. it fires right up but revs right up to between 2000-3000 rpm then drops right down to below 500 after about 5 secs then sputters and dies. some times it will keep reving up and down... really high to really low for a minute then will run good... this is usually about the 3rd or fourth start up after the
first few stalls.
all new o2 sensors. new maf. new iac. cleaned throttle body. new plugs a while ago. and not throwing any codes. other than the p0306 the other day but ifixed that one.
got me stumped and obviously my mechanic to.
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Old 12-01-2014, 08:58 AM   #4
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does sound like a vacuum problem but i dont know how you would check since its so brief. how is the battery and terminals?
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Old 12-01-2014, 10:01 AM   #5
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battery is good,no problems at all turning over, and it has been -10 to -20 the last few days here
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Old 12-01-2014, 10:19 AM   #6
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sometimes when the battery is old or the connections are bad, the voltage will go down while cranking cold enough to reset the learned values, but normally you can kind of hear it cranking slow.

normally you would test a vac leak by letting the engine stabilize and some carb cleaner and listen to the engine but may not work if it seals after the engine warms a little. i might try it any way, do you have a scan tool with live data?
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Old 12-01-2014, 10:48 AM   #7
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no I don't have one with live data I don't think? it gives me freeze frame data but I don't really understand it, as long as it gives me the codes that'* what I can handle lol.
it isn't throwing any codes yet anyways, maybe I should just try and change the intake gasket?
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Old 12-01-2014, 02:32 PM   #8
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Does your Freeze frame data show fuel trims %? If so what'* your short and long term numbers at for both banks 1 and 2?
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Old 12-01-2014, 02:47 PM   #9
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I am not sure what data it shows for sure but I will check tonight, I did delete all the codes on it Friday when I fixed the p0306 I had, do I just hook up the scanner and start the engine then to show the feul trim data? or will it be stored in there already
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Old 12-01-2014, 03:02 PM   #10
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it wont be stored, you have to record it while its running bad, then depends on he scanner how you view it
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