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2002 GMC Sierra 1500 - fluctuating idle

I am experiencing some issues with my truck when i’m driving down the road. When I have accelerated and then start to maintain the speed I can hear my truck sound like its speeding up/slowing down, and I noticed on the RPMs that it fluctuates slightly when it does this, not a major increase in RPMs just enough to notice that it is moving. It doesn’t seem to be doing this after a while of driving as well.

It almost sounds like a vacuum leak but I don’t think there is one.

I had just replaced the catalytic converter on the driver side and that code went away, and Catalyst Efficency Below Threshold I believe Bank 2 popped up. There is also no misfire codes I have changed plug wires/plugs. I used a infrared thermometer to see if there was a difference in temperature before and after cat, but I didn’t see much of a difference in the temperatures.

Truck had a new spider injector put in, and I don’t know if fuel filter has been changed or not.

Can anyone provide me with a reason why it could be doing this, and how to fix it?

The problem might be with the Idle Air Control valve.


If the IAC valve opens and closes slightly as you drive, the extra air allowed into the engine is detected by the O2 sensor,
The computer then adds more fuel to the engine which causes the RPM’s to increase.

Then when the IAC valve closes,and the air is cut off, the RPM’s drop back down.


Hm, makes sense. Would that still be the case even if I don’t receive that code? Also, would that have any effect on the cataltic converter code?

You usually don’t get a code for a malfunctioning IAC valve.

The IAC valve is an engine control device. Not an emission control device.