Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
OK.The first thing to look at is the idle air control valve.Here is some info on it.Sometimes you can remove it and clean it with carb cleaner.Make sure to clean the throttle body as well.They tend to get a carbon buildup on them and can't function properly.After you clean it if the truck runs better then you have found the problem.If not let me know and we can go from there.Thanks
The engine idle speed is controlled by the ECM through the Idle Air Control (IAC) valve mounted on the throttle body. The ECM sends voltage pulses to the IAC motor causing the IAC motor shaft and pintle to move in or out a given distance (number of steps) for each pulse, (called counts).
This movement controls air flow around the throttle plate, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen using a scan tool. Zero counts corresponds to a fully closed passage, while 140 or more counts (depending on the application) corresponds to full flow.
Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor
I went and bought an IAC valve ($50.00) but the clerk told me that if I install it I can't return it, so I thought I'd better check with you one more time before I install it. I took off the old one and cleaned it with carb cleaner, and while I had it out I tried to move the plunger by pushing and pulling but it was very stiff. At one point I pulled the plunger completely out separated from the rest of the unit, I put I back in but I got the impression that it shouldn't have come lose like that. Additionally, when I disconnect the wires from the IAC it does not change the performance. Once I started the truck with the IAC unit completely removed and the pick up ran smooth but at a very high RPM. Do you still think it is the IAC Valve? If so I'm ready to install the IAC Valve part.
Oh one more thing, with the IAC valves out I plugged the wires in to them one at a time and had my wife turn the ignition on, and both the old and new IAC valves reacted the same (I felt a slight click movement as I held the unit in my hand but neither unit moved the plunger at all, although the plunger in the old unit is currently sticking out more than that of the new unit.)
The new IAC valve did not make a difference, but I agree it probably needed a new one since the old one came apart. So I'm going to look for vacum leaks and see what I can find.