Yet another issue with my 98 5.4L. For some reason it has started to run rough while at idle at stops, but only with the A/C on. When I turn the AC off the engine runs fine. I did notice several weeks ago while on a trip to Greenville that the AC blower would randomly switch from the dash vents to the defrost vents. I do not know if this is related. Does anyone have any idea what is going on with the truck? I can do some mechanic work but this is way above my pay grade.

Matt

 

Check the vacuum lines behind the battery. Any OBD2 codes? I would clean the throttle body and IAC.

 

I do not have any codes, and yesterday it ran with no issues. I have no idea what is going on with the truck. I will take a look at the vacuum lines and get back. Thank you for the input.