My 92 LS400 idles at about 550-600rpm when A/C is off. But it jumps to about 1000-1100rpm when A/C is turned on. This seems a little higher than it supposed to be.

Is this normal? If not, how can I fix this?

Thanks!

 

shouldn't do that, sounds like it is not receiving enough juice or needs an adjustment

 

How can it be adjusted?

 

The engine ECU is programmed to raise the idle level when the A/C compressor clutch engages, or to raise the PS fluid volume if you attempt to turn the wheels.

Could it be that there is no freon left in the system and the A/C compressor no longer presents a full load?

 

I'm sure freon is still there because A/C blows cold.

Anybody else got other suggestions?

 

Check your idle control it might be failing.
I had to replace mine.

 

I see. where is idle control? and how can I check to see if its failing or not?

 

It may be something like a sensor. I am having a similar problem currently with my 1991 Honda Accord LX. It used to idle too fast, then I finally figured out how to lower it (car is FI) and now sometimes the idle fluctuates. Is your check engine light on? If not, probably not a sensor.

 

check engine light is not on, so I guess its not sensor related.

 

Your idle control doesn't light up the check engine lights.
My idle ran high on the A/C too and my mechanic replaced it and now it is steady.

 

does anybody know how to check idle control?

 

I have a 92 LS and it definitely increases the idle level when the A/C kicks in, just not by this much.

Your engine ECU is programmed to provide some nice "driviablity" features. As I said before, if you are setting parked and start to turn the steering wheel the engine RPM will automatically be raised in order to have the power steering pump provide enough flow and pressure to overcome the tires' friction.

Now I suppose that if the OAT sensor indicates the climate is extremely hot (1), or maybe even if the engine water jacket temperature is approaching the upper limit (2), the driveability programmming aspects of the engine ECU might come into play and raise the engine RPM above what one normally expects.

1. It is so hot that an inordinately high rate of freon supply is needed.

2. I think some cars are programmed to modulate engine RPM to provent control the emissions level.