rpm jump

[ca1rangers]

when my truck is in park at idle I noticed that the RPM's will jump up about every 30 seconds. Its is a pretty big jump too. After messing around I did figured out that it only happens when the controls are set to vents or windshild or a combination of both. If its set to the floor it does not happen. My fan also has only one speed which is high. I know what that takes to repair. Will that stop my RPM jump or are they two diffent problems

 

[dburton07]

I know that when I had my 93 F150 and turned on the AC, the RPM's would fluxuate a little bit about every 30 seconds or so like you stated. I dont think theres a problem unless it's an obsurde amount of difference.

 

[Wicked_Sludge]

the a/c compressor cycles when the vents are set to windshield, so it sounds like the clutch cycling is causing your RPM change.

an improperly charged A/C system could be putting excess load on the engine, causing it to bog down, or (more likely) a sticky IAC solenoid might not be adjusting the idle fast enough to prevent the engine from accelerating after the A/C load is released.