Now that it is getting cooler (heater weather) I noticed that my idle goes up by about 250rpm when I turn the heater on similar to what is expected to happen when the A/C is used. Is this normal?

 

Did it do it before?

 

Thats part of my problem, i don't remember. I don't drive the D everyday but it has done it for several weeks that i have been paying attention.

 

Maybe the compressor is kicking on.

 

Assuming the A/C compressor is indeed off... There vacuum lines/systems associated with the heat controls. I'd suspect a leak somewhere there, which would cause the computer add fuel to make the mixture right, but the more fuel will make it idle higher.

 

If the selector SW is in defrost mode then the a/c compressor comes on.

 

dolittle beat me to it, check the position selector

also a 2001 had electronically actuated heater controls, not vacuum

 

So the defrost (both positions) kicks in the compressor? Is this to keep the moisture at bay? I will check to see if the position makes a difference. Thanks

 

In any defrost position the AC compressor comes on, and yes, it is to remove moisture from the air being blown so it dries off any moisture (fog) on the windshield.

 

Thought I'd add that if the evaporator core is below a certain temperature (37° is sticking out in my mind) the compressor won't engage, otherwise moisture would freeze and build up, blocking airflow, and causing the AC's low side pressure to drop and the high side to go up.