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Mike V.
Mike V., Auto Technician
Category: Chevy
Satisfied Customers: 20083
Experience:  25 years automotive experience, Certified Subaru Tech and Nissan factory trained.
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I have a 1992 Chevy pickup (C1500, 5.7L, V8, 150,000mi), ...

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I have a 1992 Chevy pickup (C1500, 5.7L, V8, 150,000mi), It started running rough and within 10 minutes worsened until it finnally died. It will restart but runs very rough and dies at idle or low rpms. When I increase the rpms it smooths out and stays running but dies again when the rpms come back down to idle. What is wrong with my truck? (I have checked the EGR valve and the EGR solenoid and neither are stuck open, I have replaced the fuel filter, and 8 months ago I replaced the distributor, distributor cap, and rotor. I have also checked for vacum leaks and have found none)
Submitted: 7 years ago.
Category: Chevy
Expert:  Mike V. replied 7 years ago.
Hi there! Is this fuel injected or carburated?
Customer: replied 7 years ago.
Fuel injected, also when it tries to idle it will almost die and then it will give itself a sudden increase in rpms back up to slightly above regular idle rpms (this lasts for about 4 seconds) and then the rpms will decrease back down to almost stall and then repeat this about 7 or 8 times and then it finally dies again. (Thanks for your intrest in this!)
Expert:  Mike V. replied 7 years ago.

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.

Click image to see an enlarged view
Fig. 1: The IAC valve can be on the throttle body, usually next to the throttle position sensor

Mike V., Auto Technician
Category: Chevy
Satisfied Customers: 20083
Experience: 25 years automotive experience, Certified Subaru Tech and Nissan factory trained.
Mike V. and 10 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
Reply to carguy1's Post:

 

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.)

Expert:  Mike V. replied 7 years ago.
The old Iac sahould not have come apart on you.If it did it was probably defactive.You may be in a situation where you have no choice but replacemant because it came apart. Make sure the throttle body is clean as well.After you install it turn the ignition on(don't start the truck)for about ten seconds.You should hear the iac motor working.Turn the key back off for 10 seconds.This should let the iac find it's range.If the iac does not help the idle,spray carb cleaner around the intake and any vaccum lines to check for a vaccum leak.If the idle picks up and stays up that is the next area to look,sometimes the engine will search if you have a vaccum leak.Let me know how it goes and if you need more help.I will be oand of all day.Thanks
Customer: replied 7 years ago.

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.

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