The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

68c10airstream · 06-05-2015, 05:22 PM

I'm assuming your truck is a 1996 and newer 5.7. If your truck is stock and basically unaltered here I go, Ecm can't see the idle speed motor so it uses stepper motor counts to reference it's placement. As you drive 45 mph and higher the ecm fully extends the iac motor to fully seat it, then retracts it the prescribed amount based on application (a/c, no a/c at, or manual, etc, etc,) So the ecm constantly recalibrates the iac position. If you can get to a scan tool your iac position is usually around the teens for value, say 13 iac counts. As an example If by chance your minimum idle speed screw was backed out (throttle closed too much) the iac would compensate for this and iac counts would be higher say 20-25. If you had a vacuum leak iac would be zero or near zero and idle speed would be above target idle value. Now a little more in idle speed control info is this (just for the sake of info) iac motor steps (counts) are a rough adjustment and the ecm fine tunes it to get as close as it can by adjusting timing advance up or down as needed. Did you check base timing and go by the proper procedure for this?? If everything is normal i suspect a slow closing egr valve. Seen it a few times, remove it and clean it using carb cleaner and compressed air. If the egr is pointing up towards the front of the motor and has an electrical connection they seem to get dirty and be slow to respond closed, but rare. If the egr is at the back of the throttle body (throttle body injection-tbi) I have had more problems with those. If it's this style , remove it clean it with carb cleaner and compressed air and then record the gm 8 digit number and pay special attention to the letter after the 8 digits. Is it a "P" or an "N"?? 2 different operating egr valves based on the letter. When the egr is removed before you re-install it start the truck up (you may have to floor it) and rev it up pretty good to blow out the carbon in the intake manifold passage ways. I've probably flooded your brain, but now you have in print what to do. Good luck, Brian F.

06-05-2015, 05:22 PM	#2
68c10airstream Registered User Join Date: Mar 2013 Location: Marquette michigan Posts: 828	Re: Throttle Body Gasket? I'm assuming your truck is a 1996 and newer 5.7. If your truck is stock and basically unaltered here I go, Ecm can't see the idle speed motor so it uses stepper motor counts to reference it's placement. As you drive 45 mph and higher the ecm fully extends the iac motor to fully seat it, then retracts it the prescribed amount based on application (a/c, no a/c at, or manual, etc, etc,) So the ecm constantly recalibrates the iac position. If you can get to a scan tool your iac position is usually around the teens for value, say 13 iac counts. As an example If by chance your minimum idle speed screw was backed out (throttle closed too much) the iac would compensate for this and iac counts would be higher say 20-25. If you had a vacuum leak iac would be zero or near zero and idle speed would be above target idle value. Now a little more in idle speed control info is this (just for the sake of info) iac motor steps (counts) are a rough adjustment and the ecm fine tunes it to get as close as it can by adjusting timing advance up or down as needed. Did you check base timing and go by the proper procedure for this?? If everything is normal i suspect a slow closing egr valve. Seen it a few times, remove it and clean it using carb cleaner and compressed air. If the egr is pointing up towards the front of the motor and has an electrical connection they seem to get dirty and be slow to respond closed, but rare. If the egr is at the back of the throttle body (throttle body injection-tbi) I have had more problems with those. If it's this style , remove it clean it with carb cleaner and compressed air and then record the gm 8 digit number and pay special attention to the letter after the 8 digits. Is it a "P" or an "N"?? 2 different operating egr valves based on the letter. When the egr is removed before you re-install it start the truck up (you may have to floor it) and rev it up pretty good to blow out the carbon in the intake manifold passage ways. I've probably flooded your brain, but now you have in print what to do. Good luck, Brian F.