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Tim's Auto Repair
Tim's Auto Repair, mechanic
Category: Chevy
Satisfied Customers: 15969
Experience:  Have owned a repair shop for 25 yrs.
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1989 chevy truck: up it idles fast for about 2 min

Resolved Question:

1989 chev truck S10 4.3 eng. when I start it up it idles fast for about 2 min. then it idles back to normal stays that way until I start it again then it does the same. what is my problem and how do I fix it?.....Ray
Submitted: 6 years ago.
Category: Chevy
Expert:  Tim's Auto Repair replied 6 years ago.


does the engine light stay on?if so,can you give me the computer codes?

Customer: replied 6 years ago.

the engine light comes on for a while (stays on about 15 min. & goes off it will come back on again after about 15 to 20 min. and stay for a while) The shop here said that they can't pull a code on it. I put a new throttle position sensor on it, but may have messed it up I did'nt take the battery cables off, should I try another TPS?



Expert:  Tim's Auto Repair replied 6 years ago.

no..i dont think its the tps...first..remove and clean the egr valve with carb may be sticking open a little causing a vacuum leak from the exhaust...also,remove the iac[idle air control]valve and clean it also,these will get black soot and carbon on them and not work right sometimes...but..the problem may be the egr solenoid,,[this is what the vacuum line from the egr valve connects to]it may be opening the egr valve when you first start the engine..i will give you a way to get the codes without a scanner,,please print


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Reading Diagnostic Trouble Codes


ALDL Connector

The ECM incorporates a diagnostic program which, when activated, will flash codes stored in the trouble code memory through the "CHECK ENGINE" lamp. A diagnostic connector, located under the instrument panel, is used to activate the self-diagnosis system. The connector,allows access to system circuits and contains a test terminal which, when grounded with the ignition on and engine stopped, causes stored trouble codes to be flashed on the "CHECK ENGINE" lamp.

When the diagnostic terminal is grounded (jumper terminal "A" to "B") with the ignition on and engine stopped, codes are displayed in numerical order, always starting with Code 12 which indicates that the self-diagnosis system is operating properly. Trouble codes, are displayed in the following manner: Code 12 is displayed by one flash, a short pause, then two flashes and a longer pause. Code 12 will be displayed 3 times in this manner, then any additional codes stored will be displayed 3 times each in a similar manner until all codes have been displayed. Once all codes have been displayed. The process will be repeated as long as the test terminal remains grounded. DO NOT ground the diagnostic terminal until the ignition is "ON" and engine is "OFF" (not running). After trouble codes have been noted, the ground wire should be removed from the test lead and the engine should be started. When the engine is started and there are no other codes except Code 12, the "CHECK ENGINE" light should go out after approximately 5.0 seconds, indicating that the ECM has not detected a malfunction. However, if the ECM detects a current malfunction, the "CHECK ENGINE" lamp will remain illuminated while the engine is running. A trouble code indicates a possible system malfunction. If a trouble code can be obtained, even if the "CHECK ENGINE" lamp is "OFF" when the engine is running, a "SYSTEM PERFORMANCE CHECK" should be performed and the indicated circuit should be inspected.

Use the appropriate diagnostic charts if trouble codes can be obtained with the "CHECK ENGINE" lamp "OFF." A System Performance Check and a thorough physical inspection of the circuit involved should be carried out if any of these codes are evident. Repair as necessary.

Removing battery voltage to ECM terminal "R" for a period of 10 seconds will clear all stored trouble codes. This usually can be accomplished by removing the ECM fuse, if equipped, from the fuse block. In some cases it may be easier to disconnect the negative battery terminal. However, doing so will result in deprogrammed ETR radios, electronic clocks and Trip Computers.

Clearing Diagnostic Trouble Codes

Stored trouble codes in the ECM memory can be cleared by removing battery voltage from the ECM. To do this on S/T 10-15 models, turn ignition "Off," then remove battery positive cable or remove ECM fuse for ten seconds. On all other models, turn ignition "Off," then disconnect ECM harness from positive battery pigtail for 30 seconds. Trouble codes should be cleared after any repairs have been made performed. To prevent damaging the ECM, the ignition switch must be in the "Off" position when disconnecting or reconnecting power to the ECM.

After the ECM memory has been cleared, a change may be noted in vehicle performance. To restore normal vehicle performance, drive with engine at normal operating temperature at part throttle, moderate acceleration and idle conditions until normal performance is restored.

egr valve

On Intake Manifold, Near Throttle Body

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 ECM then 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.
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