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Tim's Auto Repair
Tim's Auto Repair, Mechanic
Category: Car
Satisfied Customers: 15966
Experience:  Have owned a repair shop for 25 yrs.
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1994 gmc truck starts fine but wont idle or stay running

Customer Question

1994 gmc truck starts fine but wont idle or stay running
Submitted: 7 years ago.
Category: Car
Expert:  Tim's Auto Repair replied 7 years ago.


first,check for vacuum leaks..but the problem may be the iac valve


Description and Operation


IAC Valve Operation And Fuel Pressure Regulation

Engine idle speeds are controlled by the control module through the Idle Air Control (IAC) valve mounted on the throttle body. The control module sends voltage pulses to the IAC motor windings causing the IAC motor shaft to move in and out a given distance for each pulse, (referred to as IAC counts on a scan tool). This movement controls airflow around the throttle plates, which in turn, controls engine idle speed, either cold or hot. IAC valve pintle position counts can be seen by using a Scan tool. 0 counts corresponds to fully closed passage, while 140 counts or more (depending on application) corresponds to full flow
Testing and Inspection


Chart C-2 - Idle Air Control (IAC) System Check
Wiring Diagram For Chart C-2 - Idle Air Control (IAC) System Check



Circuit Description :

The ECM controls idle rpm with the IAC valve. To increase idle rpm, the ECM moves the IAC valve away from it's seat, allowing more air to pass by the throttle plate. To decrease rpm, it moves the IAC valve toward it's seat, reducing air flow by the throttle plate. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).

Test Description : Numbers below refer to circled numbers on the diagnostic chart.
  1. The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
  2. This step checks the quality of the IAC movement in step 1. Between 700 rpm and about 1500 rpm, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 rpm), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
  3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.


  • Ignition "OFF" for 10 seconds
  • Start and run engine for 5 seconds
  • Ignition "OFF" for 10 seconds
Diagnostic Aids :

A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the IAC valve. Out of control range IAC "Scan" tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC system problem.

  • Vacuum Leak (High Idle) If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 rpm, locate and correct vacuum leak including PC V system. Also check for binding of throttle blade or linkage.
  • System Too Lean (High Air/Fuel Ratio) The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting the IAC valve does not help. Code 44 may be set "Scan" O2 voltage will be less than 300 mV (.3 volts). Check for low regulated fuel pressure water in the fuel or a restricted injector.
  • System Too Rich (Low Air/Fuel Ratio) The idle speed will be too low. "Scan" tool IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. "Scan" tool O2 voltage will be fixed above 800 mV (.8 volts).

Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensors "Scan" voltage will be slow to respond.

  • Throttle body - Remove IAC and inspect bore for foreign material.
  • Refer to "Rough, Unstable, Incorrect Idle or Stalling". See Diagnosis By Symptoms - No Trouble Codes Stored.
  • If intermittent, poor driveability in idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance, or replace IAC.
  • 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.
Service and Repair


IAC Thread Mounted 10mm Pintle



  1. The Idle Air Control (IAC) valve is an electrical component and must not be soaked in any liquid cleaner or solvent. Otherwise damage could result.
  2. The IAC valve for this application is thread-mounted and has a dual taper, 10 mm diameter, pintle. Any replacement of the IAC valve must have the correct part number, with the appropriate pintle taper and diameter for proper seating of the valve in the throttle body.


  1. Disconnect electrical connector.
  2. Remove IAC valve, use a 32 mm (1 1/4") wrench.
  3. Remove IAC gasket and discard.
  4. Clean old gasket material from surface of throttle body assembly to insure seal of new gasket.


Idle Air Control Valve Pintle Retraction (Manually)



  1. If the IAC valve was removed during service, its operation may be tested electrically with the IAC/ISC Motor Tester (J-37027 or BT-8256K) or equivalent. Before installing a IAC valve, measure the distance between the tip of the valve pintle and the mounting surface. If the dimension is greater than 28.0 mm (1.10 inches), the valve pintle must be retracted to prevent damage to the valve. This may be done electrically using an IAC/ISC Motor Tester or manually by exerting firm pressure, as shown in the illustration, with a slight side to side movement on valve pintle to retract it.
  2. No physical adjustment of the IAC valve assembly is required after installation. The IAC valve pintle is reset by turning the ignition ON for ten seconds and then OFF. The ECMthen resets the pintle to the correct position. Proper idle regulation should result.


  1. Install the IAC valve into the throttle body with new gasket. NOTE: New IAC valves have been reset at the factory and should be installed in the throttle body in an "as is" condition, without any adjustment.
  2. Tighten IAC valve assembly to 18.0 Nm (13.0 lb-ft).
  3. Reconnect electrical connector to IAC valve.
  4. START engine and allow it to reach operating temperature.


RH Side Of Engine.
Customer: replied 7 years ago.
Is this a dealer part or can I buy it at Auto Zone,also is it something I can do myself, I have just average mechanical abilities.
Expert:  Tim's Auto Repair replied 7 years ago.


yes,you can buy this at auto zone..may be around $40+/ should have no problem doing thisa ..just follow the instructions


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Customer: replied 7 years ago.
I just replaced the IAC vavle and their is no change
Expert:  Tim's Auto Repair replied 7 years ago.


remove and clean the egr valve with carb cleaner....also if you have a vacuum egr valve ,,the egr solinoid may be bad[this is what the vacuum line from the egr valve connects to,]


Customer: replied 7 years ago.
Can you provide a picture showing the location of these parts
Expert:  Tim's Auto Repair replied 7 years ago.


i dont realy have any good pics of these

the egr valve is located on the intake near the throttle body

the solinoid is on a bracket on top of the engine on the passenger side


Service and Repair


EVRV Filter Replacment

The EVRV filter should be replaced every 30,000 miles.


  1. Grasp and pull filter "OFF" with a rocking motion.
  2. Push new filter "ON", making sure cut-out for wires is properly aligned.