Idle low until AC is on

When my truck is warmed up it will idle low and vibrate. When I turn on the AC it idles normal. What could be the cause? Other threads I see with idle problems say their AC lowers the idle. Also, what RPM is the notch before 1,000? My normal idle is usually above that notch, but drops a little below it when the idle is low.

 

Thinking it through, the AC is putting more load on the engine when it's off, right? First guess would be the AC compressor or the less expensive AC belt idler bearing. The clutch may be engaged when it shouldn't be, bearings, could be the idler bearing is going bad, who knows, that's another diagnosis route.

Try removing your AC belt just as a test. It will be a PITA because it's in back and the other belts also have to come off. If it's time to change out your belts you can cheat by cutting the AC belt.

If the idle lowers when the AC is on it means the clutch is engaged when the AC is on and putting a load on the engine. As I recall this is semi-normal operation but too much and it would need to be addressed. . . mine has been down for a while and not on my list of priorities. :-D

 

Typically the engine will rev higher with the AC on, to compensate for the increased load it requires from the engine. Usually this is a pretty minor increase of ~100rpm (from the normal ~750 - 850 rpm). So your AC is correcting your low idle, and AC isn't the root cause of your problem.

I'd check your IAC valve. Should be a resistance test you can perform on it. I can get you a picture of that page from the FSM later after work.

 

Idle doesn't lower when AC is on, only increases. I'll have to open the hood and check to see what it's doing when turned on. Thanks. The compressor and belts are all a couple years old.

 

Thanks. IAC valve is something I plan on trying, since the low idle has been an issue lately. It just doesn't throw that code, only PO420.

 

My 95 is controlled by the VSV, idle up and down when the vacuum switch is triggered off and on. Maybe yours doesn't have the VSV since it's newer but mine works good. Maybe you need to clean the intake and TB to improve the low idle.

 

I have the same problem. I have to turn it on to keep it from bogging down. What causes slow idle Im going with cleaning throttle body. Can AIC be cleaned?

 

From what I’ve read, the IAC valve can be cleaned.

 

IAC Valve Testing and Cleaning (youtube.com)

 

Oh man
Mine was filthy
In fact, took a lot of advice from OSilver when I did clean and RR’d IAC gasket