GM Storm/Spectrum 1985-1993 Repair Guide

Idle Speed

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ADJUSTMENT



Carbureted Vehicles

See Figures 1 and 2

The cooling fan runs and stops automatically according to the coolant temperature. For this reason, make sure that the cooling fan is not running while performing the idle speed check and adjustment. If the fan comes on, wait until it stops before continuing.

  1. Confirm the following prior to checking or adjusting the idle speed.
    1. Check that the lead wires and hoses of the engine emission control system are connected correctly.
    2.  
    3. Check that the accelerator cable has 0.12-0.20 in. (3-5 mm) free-play when engine is at operating temperature.
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    5. Check that the ignition timing is within specification.
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    7. Check that the valve lash is adjusted to specification.
    8.  

  2.  
  3. Place transaxle gear shift lever in N or P position, block the wheels and set the parking brake.
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  5. Check the float level through the sight glass in the side of the carburetor, then start and warm the engine to normal operating temperature.
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  7. Ensure that the lights, heater fan, rear defogger, cooling fan and air conditioner are OFF . If equipped with power steering, place the wheels in the straight forward position.
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  9. Disconnect and plug the distributor vacuum line, canister purge line, EGR vacuum line and ITC valve vacuum line.
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  11. Connect a tachometer to the coil tachometer connector and a timing light to the No. 1 spark plug wire. Check the timing and idle speed.
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  13. If the idle speed needs adjusting, turn the idle speed adjusting screw. Adjust the speed with the choke valve fully opened and the air cleaner in position.
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  15. Disconnect and plug any vacuum advance hoses at the intake manifold (if not done already), then run the engine at 1900-2100 rpm and adjust the dash pot. Turn the dash pot adjusting screw until it just touches the end of the shaft.
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  17. If equipped with air conditioning, place the system in the MAX/COLD setting and place the blower on HIGH . Set the fast idle speed by turning the adjust bolt of the fast idle control diaphragm to 850 rpm on automatic transaxle or 980 rpm on manual transaxle.
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  19. When adjustment is completed, turn the engine off, remove the test equipment, install the air filter and vacuum lines. Check to ensure that accelerator cable has some play. If not, adjust cable.
  20.  



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Fig. Fig. 1: Carburetor idle speed adjustment screws



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Fig. Fig. 2: Check the float level through the sight glass

Fuel Injected Vehicles

Unlike carbureted vehicles which may occasionally need an idle speed adjustment, fuel injected vehicles use electrical devices (run by the ECM), in order to control idle speed. Although there is a throttle stop screw which may be adjusted, the manufacturer warns that it is EXTREMELY UNLIKELY that this adjustment will ever be necessary on a throttle body that has not been tampered with after production.

The throttle stop screw is sealed at the factory to prevent unnecessary tampering. Before removing the seal and adjusting the screw, eliminate all other possible causes of an incorrect or erratic idle. For more information, refer to Emission Controls of this repair guide for electronic engine controls.

If a scan tool is available, the throttle stop position may be checked by monitoring the IAC valve:

  1. Block the wheels and apply the parking brake.
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  3. Connect the scan tool the ALDL connector.
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  5. Start the engine and allow it to warm to normal operating temperature and enter Closed Loop operation. For more information regarding engine controls, refer to Emission Controls or 5 of this repair guide.
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  7. If equipped with an automatic transaxle, shift into D and then back to N .
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  9. Using the scan tool, select the power steering pressure switch input. With the A/C and all accessories OFF , the scan tool should rear OFF or NORMAL. If not, repair the power steering pressure switch circuit.
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  11. Allow the idle to stabilize, then select IAC valve display on the tool and read the valve counts. If counts are between 5-45, then the throttle valve position is acceptable.
  12.  

If the idle speed is not allowed to sufficiently stabilize, an incorrect count reading might result in unnecessary tampering or adjustment with the throttle stop screw.

  1. If counts are too low, check for intake vacuum leaks at hoses, the throttle body or intake. Check for a damaged throttle lever. If no other causes are found, remove the seal and adjust the screw to obtain the proper counts. Turn the screw in very small increments and allow the idle to stabilize each time.
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  3. If counts are too high, check for a damaged throttle lever or an airflow restriction by the throttle valve. If no problem is found, clean the air intake duct and remove residue from the throttle body bore and throttle valve using a carburetor cleaner (which DOES NOT contain methyl ethyl ketone) and check the count again. If counts are still incorrect, remove the seal and adjust the screw to obtain the proper counts. Turn the screw in very small increments and allow the idle to stabilize each time.
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