What's wrong with the motor when it's idleing but continually try's to shut off but doesn't shut off ?

Asked by Oct 30, 2014 at 10:12 AM about the 1991 Chevrolet C/K 1500 Silverado Extended Cab SB

Question type: Maintenance & Repair

When it's idleing in any gear, it'll run
fine for a minute or 2 then the RPM's
will go all the way down and the
motor tries to shut off but it doesn't
shut off but instead idles back up to
800 RPM's. Please help.

5 Answers

425

The idle air control valve controls your idle. I would first look for a vacuum leak. There are only a couple vacuum line on that motor. Also i would pull the codes on the truck. Jump pins A and B with a jumper wire and count the check engine light flashes. If no codes or vacuum leaks, i would remove the IAC valve from the throttle body and clean any carbon build up from the IAC and the port it rides in. Also make sure the wires going to the IAC and the connector are in good shape.

Best Answer Mark helpful

It would help you get a more accurate answer to your question if you can provide more information on your truck - what engine, how many miles, fuel injected, throttle body, or carburetor, etc. There are multiple reasons for this. Without knowing which engine is in the truck, it makes it difficult to give an accurate diagnosis. I would start with the fuel system - carburetor or fuel injectors. When the gas tank is near empty, add a high quality fuel additive (carb or injector cleaner) and then fill the tank up with some 93 octane fuel. This is the easiest and cheapest first option. You can also take the truck to a Midas store. They will plug in their OBD tester to see if you have an stored codes. That would be my starting point. Best of luck.

It has a 5.7 v8, 2 wheel drive, 248,000 miles, and it's a fuel injected.

425

I'm going to assume (you know what the say about assuming) that your truck its throttle body injected. Check your fuel pressure at with the key on engine off and at idle. Im not sure what the pressure is suppose to be. I think its 15 psi or around there. It should be steady and not dip in any way.

http://jameshalderman.com/links/book_adv_eng_perf_5/s_and_c/Fuel_Pressure_Specifications.pdf

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