Engine Surging w/ AC on

[98gmc2dr]

Hello all, I have come across an issue with my truck (1998 K2500 Vortec 5700 4L80E). When stopped in Drive with the AC or defrost (since both run the compressor) the engine will surge. Sometimes it is not by much, other times it will go up to around 850 and try to move a bit. Last week it dropped down to 400, so I decided it was time to try and fix it. I'm not sure what other information would be useful, CEL is not on and I replaced the compressor last May.

 

[PlayingWithTBI]

Could it be that your idle raises when the compressor comes on (which is normal) and if you're a little low on Freon, it's cycling more than it should?

 

[SAATR]

Remove the IAC motor and clean the throttle body thoroughly, paying special attention to the throttle blade and bore and the IAC passage. It's easiest if you remove it from the truck, but can certainly be done while installed with more difficulty. Pull the PCV and make sure it is clean and not sticking, and verify that all clamps and connections between the throttle body and the MAF are tight and there are no pinholes or tears in the tubing. Clean the MAF too, while you're e at it.

It is also possible to have a small leak in the vacuum booster (assuming you have vacuum booster brakes) that only is noticeable with the higher engine load caused by the AC compressor. But that is less likely than or leaking intake system.

 

[98gmc2dr]

I'll try those things when it's not raining later this week, thanks.

 

[Schurkey]

Connect a scan tool, look at the commanded idle speed vs. the actual idle speed.

There may be other clues in the data stream.