Mechanics

WH DOES MY TRUCK IDLE TOO FAST, THEN TO SLOW

1987 GMC Sierra • 188,000 miles

I have replaced all vacuum lines, replaced the following: EGR Valve, TPS, IAC, o2 sensor, rebuilt throttle the body, replaced the water pump, radiator hoses, temp sender, checked vacuum booster and flushed the cooling system. Truck is not posting any engine codes. This truck will idle about 2000 rpm then as low as 200 and fluctuates back and forth. It has stalled on occasion. Although sometime it will idle normally but that is rare. I have also replaced the exhaust manifold on the passenger side as it was cracked. Last year during winter season, I had trouble with engine at operating temperature but no heat. I had flushed the system, replaced the water pump and that resolved the problem. There is no oil in the cooling system.
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Jacob844
December 28, 2011.




Check ignition timing and compression. Check fuel pressure. Have it scanned for soft codes and live data. Soft codes won't set a CEL, so some shops won't try anything if they don't see it, so you need to tell them soft codes.

Rivermikerat
Dec 28, 2011.
Timing is correct for this make and model year 5.7L. Fuel pressure is within specifications on either tank. I have taken this to a shop and the consensus was to "smoke the motor" as they could not readily determine the problem that is causing the erratic idling. I am told that I most likely have an intake vacuum leak. I am almost resigned to just removing both heads and intake and replacing the gaskets.

Tiny
Jacob844
Dec 28, 2011.
Smoking the motor checks for vacuum leaks. You can come close to the same results with a can of carb cleaner, usually. When the leak is found, the idle will smooth out. It may not be an intake or head gasket. It could be a hose, plastic line, or fitting. How well does the throttle shaft fit in the throttle body? Is there any play in it?

Did the shop just check for hard codes (setting the CEL) or did they check for soft codes, as well? What about polling live data?

Rivermikerat
Dec 28, 2011.
When I was looking for the problem, I replaced all the plastic lines, hoses etc. I sprayed starting fluid all around the intake, TB, vacuum lines etc. With no difference. I also removed the plug to the IAC with no difference with regards to idling issue. I determined that spraying the TB around the vacuum lines, base gasket, linkage it was not the problem area. I must digress. I had the truck at a 2nd garage for repair. They could not resolve the issue and suggested I replaced the water pump. Replacing the water pump did not solve the problem. I had introduced into the cooling system which was to seal an internal leak (if it had one). I did notice an improvement with regards to cab heat but it did not last long; however that issue resolved itself, so maybe that material that was introduced into the cooling system did work, and in fact I do have an internal vacuum leak. I would lose heat from the temp gauge got over 200. Flushed out the heater core several times and that resolved that issue. The idling issue did not resolve itself. As far as codes, the mechanic just stated it did not throw any codes, did not elaborate hard or soft or mention about polling data.

Tiny
Jacob844
Dec 29, 2011.
Your statement about nothing happening when you unplugged the IAC made me sit up. I'm going to ask one of my colleagues to peek in here and give his opinion, but I believe that maybe the IAC is bad. Please have patience, as he may take an hour or two to receive the message and have the time to check in.

Rivermikerat
Dec 29, 2011.
My opinion is a manifold gasket leak or fualty injector. But this is the way to set the IAC up correctly.
THROTTLE BODY INJECTION
1. Remove air cleaner and gasket. Plug TBI unit vacuum port for THERMAC (Thermostatic Air
Cleaner). With Idle Air Control (IAC) valve connected, ground terminal "A" to "B" at ALDL
connector.
2. Turn ignition switch to "ON" position, DO NOT start engine. Wait approximately 30 seconds for
IAC valve pintle to extend and seat in throttle body.
3. With ignition on, disconnect IAC valve electrical connector. Remove ground at ALDL connector.
Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying
leverage.
4. Start engine. Adjust idle screw to specification. Turn ignition switch "OFF". Reconnect IAC valve
electrical connector. Install air cleaner and gasket.
SLOW IDLE SPEED (RPM)
NOTE: Adjustment is necessary only when throttle body assembly has been
replaced.

Merlin2021
Dec 29, 2011.
Thanks Merlin.

Rivermikerat
Dec 29, 2011.
Happy belated Merry Christmas and a Happy New Year 2012-Boston Mike and nice to see you again old timer-

My opinion sounds more like the IACV is defective or its wiring back to the computer-who knows could very well be the computer-also might be stuck partly open or operating on its default values

Unplugging the IACV should caused the idle speed to react its purpose is to target the idle speed at all times at curb idle speed or you can adjust for minimum air as suggested by merlin

Tiny
Rasmataz
Dec 29, 2011.
If its not set up right it may not work. That's why I included the instructions too set it up!
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Merlin2021
Dec 30, 2011.
Merlin. Thanks for that information. Is A and B the two top right hand openings that are side by side on the ALDL? I will have to look at the ALDL to see where it is grounded. Could you be more specific with regards to "Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying
leverage." Remove what plug? From where? At what point should I reconnect the ground to the ALDL? Thanks

Tiny
Jacob844
Dec 30, 2011.

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