Chevy Truck Goes 'Hunting'

courtesy of Bob, The Auto Answer Man

I have a 1991 Chevy truck with a 4.3 L engine which has 176,000 miles. The engine, while trying to maintain a constant speed will lope (jerk). The jerking resembles someone tapping the accelerator repeatedly. The computer returns no failure codes. I have replaced the following; fuel pump, fuel filter, ignition wires, spark plugs, distributor cap, rotor and throttle position sensor. The symptoms occur at any given time or temperature. I am at wits end and bank account is dwindling. Can you please help?

Look for a vacuum leak or a sticky IAC (Idle air control valve). The IAC is located at the throttle plates, and creates a bypass to control idle. First thing, disconnect the electrical connector to it. Does it still lope (or hunt as it is called)? If so, then slightly increase the throttle idle screw to about 50 rpm over spec. Does it still hunt? Do you hear a whistling? Check for broken vacuum hoses.

Also, when was the last time you checked the air filter? Is it clean and clear? If everything checks out, try disconnecting the battery for 30 minutes, reconnect and start the car. Do not touch the gas. The ECU will need to re-learn how to idle. Sometimes they "forget".
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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