1989 GMC S15 idling problem

  • 1989 GMC S15
  • 6 CYL
  • 4WD
  • 163,000 MILES
Hi, I have a idle problem. When I start up my truck it idles up to 1500 rpms then after a minute or 2 it goes down to about 900 rpms. I have change o2 senser, IAC, Dis, rotor, plug wires, spark plugs, fuel pump, fuel pump relay I've also change the manifold gasgets on because I thought that was the problem but then I seen that theTbi unit gasget was rotted out so I repleced that. I would thank you all if u can help me out.
Do you
have the same problem?
Saturday, August 30th, 2008 AT 4:03 PM

1 Reply

Could be fuel varnish deposits or carbon in the passages try cleaning it out

Disconnect the air intake ductwork from the throttle body.

Start the engine, then increase and hold the idle speed to 1,000 to 1,500 rpm.

Spray the throttle cleaner or engine cleaner into the throat of the throttle body, aiming for the idle air bypass port (usually located on the side or top of the throttle body opening). Give this area a good dose of cleaner (about 10 second's worth).

Turn the engine off to allow the cleaner to soak into the IAC passageway.

Wait about three minutes.

Restart the engine, rev and hold at 1,000 to 1,500 rpm, and repeat the cleaning process again.

Turn the engine off again, and reattach the air intake ductwork to the throttle body.

Start the engine and rev and hold to 1,500 to 2,000 rpm until no white smoke is coming out of the exhaust pipe.

If this fails to make any difference, you can remove the IAC valve from the throttle body and spray cleaner directly on the tip of the valve and/or into the ports in the throttle body. Let the cleaner soak awhile, repeat as needed, then reinstall the IAC valve, start the engine and run it at 1,500 to 2,000 rpm as before until no white smoke is seen in the exahust.

If the idle speed still surges after this, the IAC valve is defective and needs to be replaced.
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Saturday, August 30th, 2008 AT 4:10 PM

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