1989 Chevrolet Suburban fuel system

  • 3 POSTS
  • 5.7L
  • V8
  • 4WD
  • 170,000 MILES

My truck won't idle. If on the accelerator, it runs fine. Let off the pedal, it dies. Replaced the throttle position sensor to no avail. Carbon building up in the throttle body blocking the free movement of plates shutting off the airflow preventing the TPS from doing it's job. I need help.

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Monday, February 17th, 2014 AT 6:21 PM

1 Reply

  • 29,775 POSTS

You misunderstand what the throttle position sensor does. It has almost nothing to do with maintaining idle speed. It simply tells the Engine Computer the throttle's position, direction of travel, rate of travel, when it's at idle, and when it's at wide-open throttle. The computer controls idle speed by moving the valve in the idle air control valve. That opens an air passage around the throttle blade. At the same time it increases the amount of time the injectors are held open to provide more fuel to go with that air.

Years ago it was somewhat common for that air passage to become blocked with carbon but with better additives in gasoline, we don't see that much anymore. Remove the IAC valve and look for carbon in there. You can also operate the valve with a scanner. By selecting various idle speeds, the computer will run the IAC valve back and forth to vary engine speed. You can also view live data to see what the computer is doing with that valve. If it is trying to increase idle speed but not having success, the IAC valve could be sticking. There could be a wiring problem too associated with that valve but that should be detected by the computer. It will set a diagnostic fault code related to that, but it may not turn on the Check Engine light. The light only must turn on if the code is related to something that could adversely affect emissions.

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Monday, February 17th, 2014 AT 7:43 PM

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