My friend has a 2001 GMC with a 6.0L and a couple minor mods like air intake and a wester's tune. He said it just started this past winter and it surges between 1900-2200 rpm on the highway. It doesnt do it around town, punching on it, or just accelerating. Only at a constant speed on the highway. I said it could possibly be a vacuum leak but it would always do it, not just cruising. Can someone help.

 

Sounds like the converter locking/unlocking

 

could be a funky throttle position sensor. how does it behave just idling?

 

sounds like the lean cruise in the tune is active. Probably was too noticeable until winter blend gas.imo

 

Do you have the ability to log or check your AFR? It could be running lean, and that can cause a surge. It could also magnify this if the surge moves the timing into a different area of the table.

 

Could be a missfire?
when i first put MSD plug wires on my truck i had a problem like that, surging/missing. but only while cruising.
Put some cheap plug wires on it and no more problems.


Id check it for codes to begin with.

 

Westers tune

Can you data-log it?