Idle problem
Thread Starter
4th Gear
Joined: Mar 2019
Posts: 4
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Idle problem
1991 . When I first start it, it will idle fine for about 2-3 minutes. After that it will jump up about 200-250 rpm and stay there. If I shut it off and crank it back up, it immediately goes straight back to the high idle. I have replaced numerous parts including the ECM, but was wondering if the prom chip could cause such a problem.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Not likely a PROM issue.
Sounds like soon as it enters closed loop the ECM is bumping up the idle.
Time to check: manifold vacuum, fuel pressure and such.
You may need someone with a monitor device who can see just what the ECM is doing.
Sounds like soon as it enters closed loop the ECM is bumping up the idle.
Time to check: manifold vacuum, fuel pressure and such.
You may need someone with a monitor device who can see just what the ECM is doing.
Team Owner
Joined: Jul 2004
Posts: 28,502
Likes: 3,302
From: Hartford WI
1991 . When I first start it, it will idle fine for about 2-3 minutes. After that it will jump up about 200-250 rpm and stay there. If I shut it off and crank it back up, it immediately goes straight back to the high idle. I have replaced numerous parts including the ECM, but was wondering if the prom chip could cause such a problem.
Next, scan the vehicle with a scanner and NOT a code reader. See what the Coolant Temp Sensor says according to the ECM and hit the sensor with an infra red thermometer to verify. Now, check the IAC counts with everything but the motor off once it reaches stable operating temp. Is your timing spot on? Is the fuel pressure at Key On Engine Off at 43 psi? Does it hold? What is the WOT fuel pressure like? Before we throw any more parts, verify.
Thread Starter
4th Gear
Joined: Mar 2019
Posts: 4
Likes: 0
Sure it could but first we need to know what the EPROM is about? Is it stock? Is it reprogrammed? What mods? If you are 100% stock, have you cleaned the TB lately? By clean I mean take it off, take the cover plate off, take the IAC off and the IAC housing. Clean all passages with brake cleaner. Spray a gentle mist of cleaner on the IAC Pintle. Reassemble with fresh gaskets.
Next, scan the vehicle with a scanner and NOT a code reader. See what the Coolant Temp Sensor says according to the ECM and hit the sensor with an infra red thermometer to verify. Now, check the IAC counts with everything but the motor off once it reaches stable operating temp. Is your timing spot on? Is the fuel pressure at Key On Engine Off at 43 psi? Does it hold? What is the WOT fuel pressure like? Before we throw any more parts, verify.
Next, scan the vehicle with a scanner and NOT a code reader. See what the Coolant Temp Sensor says according to the ECM and hit the sensor with an infra red thermometer to verify. Now, check the IAC counts with everything but the motor off once it reaches stable operating temp. Is your timing spot on? Is the fuel pressure at Key On Engine Off at 43 psi? Does it hold? What is the WOT fuel pressure like? Before we throw any more parts, verify.
I had the same exact problem with my 08 Silverado just 2 months ago, did an idle relearn and it's worked fine since.
Team Owner
Joined: Jul 2004
Posts: 28,502
Likes: 3,302
From: Hartford WI
The TB and IAC have new gaskets and have been thoroughly cleaned. The only code I can pull is H 41 through a paperclip. Have tried a obd1, it will start to read, but then backs out saying something about a bad connection (didn't know about this until yesterday). I'm not sure if a scanner would work due to this. Manual gauge cluster hasn't worked since I got it 2 years ago. Just the digital part works.
I had the same exact problem with my 08 Silverado just 2 months ago, did an idle relearn and it's worked fine since.
I had the same exact problem with my 08 Silverado just 2 months ago, did an idle relearn and it's worked fine since.
