1992 5.7l Idle Problem

[tealwood]

1992 Yukon 4wd 5.7l efi. By the way, simply the best vehicle I have ever owned. Recently developed an erratic / fluctuating idle. Starts great, cold idle OK, when warmed up idle moves from 900 rpm down to almost zero, seems to "catch" itself and speeds back up to about 900 rpm again. Does this over and over again while in park. If in gear, same thing except it will quickly stall. At road speeds seems to operate normally. Approx. 178,000 miles.

 

Any help greatly appreciated ............

[bill190]

Is the check engine light on? If yes, get the codes read.

 

Other than that, there is a gizmo called the IAC or Idle Air Control valve. This is an electric "stepper" motor which controls the idle.

 

As an experiment, you could try try shutting off the engine, wait about 5 seconds, then disconnect the IAC valve electrical connector. Then start it back up and see if it has a constant idle. (This might cause the check engine light to come on if it was not already on.)

 

If yes, then the engine computer is telling the IAC valve to reduce the idle speed. And the computer does this based on input from various electrical sensors.

 

One of these sensors could be giving the computer "false" information. The way to test these is with a GM factory service manual set and a multimeter. The service books will tell you how to test each gizmo and what the reading on your multimeter should be for each sensor and certain situations.

 

Then with the book in hand, you can test various sensors. Or you can look up the symptoms of the problem and it will have you test certain things. Or if there is a check engine light on and a diagnostic code, it will tell you what to check.

 

For example one sensor might measure the temperature of the coolant water. At 70 degree F. this should give you a certain reading on the multimeter. At 100 degrees F. another reading. 150 degrees F. yet another reading, etc. You need the books to tell you what wires to test, what to set the multimeter to for testing, and what results you should get with your testing (for your specific vehicle).