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GM-Frank, ASE Master Certified
Category: Car
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Experience:  37 Years Automotive Diagnosis & Repair. Experience with all makes and models..
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1989 GMC truck: idles..the throttle body and sensor, egr valve, and it

Customer Question

1989 GMC truck idles too fast, have changed the throttle body and sensor, egr valve, and it has been in the shop.
Submitted: 8 years ago.
Category: Car
Expert:  GM-Frank replied 8 years ago.




There are really only 2 possible causes for any fuel injected engines to idle too high.


One would be that the ignition timing is advanced too far...and the other would be that unwanted air is entering the combustion chamber.


Naturally, the throttle plate must be fully closing at idle.


When ignition timing is being set on your vehicle, there is an important step that many younger technicians may be unaware of. This is the "Set Timing Connector".

This must be unplugged in order to set the ignition timing correctly.


The Timing Connector breaks out of the engine wiring harness conduit adjacent to the distributer. It is a single wire sealed connector that has a tan with black stripe lead.


Many times this connector is very difficult to locate. It can even be taped up in behind an electrical plastic cover or even hidden under the the engine harness where it is not readily visible.


Base Timing, with Manual and Automatic Transmissions: 0°(TDC)


NOTE: Timing specifications are listed on the Vehicle Emissions Control Information label under the hood. Always follow the Vehicle Emissions Control Information label procedures first before using the following procedure.


Set timing under the following conditions:

  • Normal operating temperature.
  • Air cleaner installed.
  • A/C "OFF".
  • Transmission in neutral.
  • Disconnect SET TIMING connector (tan/black wire) located in harness close to distributor. DO NOT disconnect 4 wire connector at distributor.
  • Connect timing light and adjust as necessary by loosening hold down bolt and rotating distributor.


  • Tighten hold down and recheck the timing.
  • Stop the engine and connect SET TIMING connector.
  • Clear ECM trouble code by disconnecting ECM power source.


If a qualified technician is unable to find any external vacuum leaks on your vehicle, then the next step after being sure the ignition timing is set correctly is to check the operation of the Idle Air Control Valve.


Any type of Vacuum leak will cause a high idle RPM. It may be coming from The Valve that is designed to open up and allow additional air to flow into the engine. Your engine should be equipped with a Idle Air Control Valve. This Valve may or may not have come with the new Throttle body you have installed. This valve will normally have a square 4 terminal/wire connection and is a motor that will move a pintle in and out to adjust the amount of air for the ECM requested idle RPM.


The IAC valve must be functioning both electrically and mechanically in order for the engine to idle at the correct speed or RPM.


Below is a diagram of the IAC circuit as it is connected to the ECM.





Testing the IAC may require a scan tool in order to observe the IAC counts during testing.


Circuit Description :

The ECM sends voltage pulses to the IAC motor winding causing the motor shaft and valve to move "in" and "out" a given distance for each pulse (called counts) received. This movement controls air flow around the throttle plate, which, in turn, controls engine idle speed.

Test Description : Numbers below refer to circled numbers on the diagnostic chart

"Scan" tool must be in open mode during test. Keep A/C "OFF" during entire check.

  1. Test with engine in drive and continue with test, even if idle is erratic. If idle is too low, "Scan" will display 80, or more, counts or steps. Engine speed may vary 200 rpm, or more, up and down. Disconnect IAC. If the condition is unchanged, the IAC is not at fault. There is a system problem. Proceed to step "3" below.
  2. When the engine was stopped with ignition "OFF", the IAC valve retracted (more air) to a fixed "Park" position, for increased air now and idle speed during the next engine start. A "Scan" will display 100 or more counts.
  3. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light, on all circuits.
  4. There is a remote possibility that one of the circuits is shorted to voltage, which would have been indicated by a steady light. Disconnect ECM and turn the ignition "ON" and probe terminals to check for this condition.

Diagnostic Aids :

An unstable idle may be a system problem that cannot be overcome by the IAC. "Scan" counts will be above 80 counts, if too low, and 0 counts, if too high.

  • If IAC valve pintle position counts are low or zero, check with vacuum leaks at vacuum fitting, tees and hoses, the throttle body and the intake manifold. A bottomed (zero count) IAC valve pintle may result in an idle speed above specification. Refer to minimum air rate check.
  • If IAC valve pintle position counts are high, look for carbon build-up in the IAC valve air inlet passage or evidence of tampering with stop screw. Also check for low engine power or excessive accessory loads.
  • System Too Lean (High Tailpipe Air/Fuel Ratio - Engine speed may vary up and down and disconnecting the LAC may not stabilize engine speed. If "Scan" and/or Voltmeter reads an oxygen sensor output less than 300 mv (.3v), check for low regulated fuel pressure or water in fuel. A code 44 (lean O2 sensor) may be set. A lean tailpipe exhaust with an oxygen sensor-output fixed above 800 mv (.8v) could be a contaminated sensor, usually Silicon. This may set a Code 45 (rich O2 sensor) even with lean tailpipe exhaust.
  • System Too Rich (Low Tailpipe Air/Fuel Ratio - System obviously rich and may exhibit black smoke exhaust. "Scan" toot and/or Voltmeter will read an oxygen sensor signal fixed above 800mv (.8v).

Check for:

  • Injector leaking or sticking
  • High fuel pressure
  • Air leak in MAP transducer line
  • If O2 is normal or low, inspect for air being pumped into exhaust manifold in front of O2 sensor (or exhaust leak)
  • Throttle body - Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore. (Repair as required)
  • A/C Compressor or relay failure - See A/C diagnosis if circuit is shorted to ground. If the relay is faulty, idle problem may exist.
  • Refer to Rough, Unstable, Incorrect Idle or Stalling. See Diagnosis By Symptoms - No Trouble Codes.

Your technician must keep in mind that the ECM looks at many parameters such as "A/C request", to determine where it moves the IAC pintle for optimim idle RPM adjustments.



I hope this information is helpful


Take Care



Customer: replied 8 years ago.
I have tried all of the things that you said to do, no vac leaks. did ignition timing and it is a new iac, still running fast. Had it coded again and it said egr valve and map sensor and changed them also still no results running way to fast. Is there any thing else.
Expert:  GM-Frank replied 8 years ago.




The IAC can be brand new, but if the signal it receives from the ECM is incorrect, then the IAC pintle may be out too far when it should be shutting down the air to the engine.


There are testers used to check to be sure the ECM is sending out a signal to reduce air flow by moving the IAC pintle in.


IF you are absolutely sure there are no external vacuum leaks allowing extra air into the a bad power brake booster leaking air...or a gasket under the throttle body leaking air...then you should try plugging off the inlet port on the top of the throttle body when the air enters for IAC control. If you plug the IAC air inlet port on top, and the RPM's drop way down, then your IAC is not moving the pintle inward and closing off the air flow as it should.


Let me know how that goes.