1997 GMC Safari Rough Idle under slight load

RHETT UNGER

MEMBER
1997 GMC SAFARI
6 CYL
2WD
AUTOMATIC
13,300 MILES

I just replaced the Mass Flow Air sensor and the throttle Position sensor and a new air filter and checked air intake ducting for cracks and replaced the hose from the intake ductiing that connects to the valve cover. Plus made sure all clamps were tight. What is still happening is a erratic idle under a slight load. With the car in gear, foot on the brake pedal and I turn the AC on, idle becomes erratic. In park idles fine. In gear with foot on brake pedal and AC off once in a while will have real slight erratic idle. The only other thing that will cause erratic power is under slight acceleration on a slight up hill grade will cause engine to run rough. I am not getting a check engine light. I have 3000 miles on a rebuilt motor with new spark plugs and wires. It was not until after I put on about a 1000 miles that problem began to happen. Just before I had the motor rebuilt the fuel pump quit. So I replaced it and the inline fuel filter next to the gas tank.

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RHETT UNGER

MEMBER
4 POSTS

Last night I replaced both Oxygen Sensors and PCV valve and PCV valve hoses and problem still persists. When vehicle is cold problem does not exist until it starts to warm up then progessively gets worse. I also noticed that at idle just when it appears that engine will die throttle kicks up then it repeats itself.

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Wednesday, June 17th, 2009 AT 7:24 PM

RASMATAZ

MECHANIC
75,992 POSTS

Let me try to help here.

Try cleaning out the idle air control valve and the EGR valve and also check the MAP sensor and come back with some results

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Wednesday, June 17th, 2009 AT 8:08 PM

RHETT UNGER

MEMBER
4 POSTS

I just removed the idle air control valve and it has soot built up on it. I tryed pushing and pulling by hand the plunger and it would not move. Is this a sign that it is stuck and needs to be replaced? Do I spray carb cleaner into the throttle body housing where the valve mounts to remove as much black soot as I can? I ran the test to see if the IAC is working. When I turned the key to the on position the plunger moved out slightly. When I turned the key off the plunger extended out much farther than retracted then stoped and a second later retracted just alittle bit more. Is that how it is supposed to work?

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Thursday, June 18th, 2009 AT 5:38 PM

RASMATAZ

MECHANIC
75,992 POSTS

An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. The car computer or ECM (electronic control module) receives information from various sensors and will output signals to adjust the IAC motor in or out to adjust engine idle speed by controlling engine idle air. An IAC motor can fail one of two ways, either the motor short circuits and stops working or the motor will develop high resistance and cause the IAC control motor to react slowly, either failure will cause the engine to stall at idle. When a trouble code scan is performed it sometimes won't always detect a failed or weak IAC motor. To check the IAC motor remove the unit, with the wires connected turn the key to the "on" position without starting the engine, the IAC should move in or out. If the IAC motor does nothing it has probably failed, replace it with a new unit and recheck system. Note: while the IAC motor is removed clean (use aerosol carburetor cleaner) the passages the IAC uses to control idle air speed.

Here's how to clean the IAC valve:

Disconnect the air intake ductwork from the throttle body.

Start the engine, then increase and hold the idle speed to 1,000 to 1,500 rpm.

Spray the throttle cleaner or engine cleaner into the throat of the throttle body, aiming for the idle air bypass port (usually located on the side or top of the throttle body opening). Give this area a good dose of cleaner (about 10 second's worth).

Turn the engine off to allow the cleaner to soak into the IAC passageway.

Wait about three minutes.

Restart the engine, rev and hold at 1,000 to 1,500 rpm, and repeat the cleaning process again.

Turn the engine off again, and reattach the air intake ductwork to the throttle body.

Start the engine and rev and hold to 1,500 to 2,000 rpm until no white smoke is coming out of the exhaust pipe.

If this fails to make any difference, you can remove the IAC valve from the throttle body and spray cleaner directly on the tip of the valve and/or into the ports in the throttle body. Let the cleaner soak awhile, repeat as needed, then reinstall the IAC valve, start the engine and run it at 1,500 to 2,000 rpm as before until no white smoke is seen in the exahust.

If the idle speed still surges after this, the IAC valve is defective and needs to be replaced.

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Thursday, June 18th, 2009 AT 7:25 PM

RHETT UNGER