I havent yet really started chasing this down but it bothers me to no end.

 

2005 Outback Ltd 4eat, 199,000 miles (6000 ish on engine)

 

This started after the engine was replaced by Subaru. The engine rpm has a wide swing when your sitting in D, stopped, foot on the brake and the AC is on. I would say maybe a 300-400 Rpm swing as the AC cycles on and off. Its enough of an rpm swing you can feel the car surge. 

 

I havent had time to start physically chasing it down as life has been cruel to me last few months, but warm weather is coming.

Id start by checking for any vacuum leaks and cleaning the IAC valve, if it has it. Also check throttle cable adjustment.

 

If it's the engine with electric throttle, the TB may need to be cleaned.

No help here, but I had an '86 Olds Delta 88 that did that all the time, from brand new.  The engine certainly had the power to overcome it, but you could always feel when the AC kicked in while idling.  In fact, I'm not sure it's not sort of "normal-ish" considering the extra load on the engine.

does the a/c relay tell the ECU to boost rpm or...how does that whole system work with the cycling load from the comp.  ?

Yes, the ECU receives a signal when the AC is turned on. It then raises idle speed to correct for compressor drag, turns the cooling fans on, and grounds the compressor coil relay to engage the compressor. On newer cars this is done via the communication network that all of the modules in the car use. On the older stuff (pre 05 ish for Subaru) the ECU gets a 12v signal from the thermal sensor on the evaporator core.

 

 

On many newer vehicles it also sends signal to the TCU to alter auto transmission shift points slightly to account for the extra throttle needed during acceleration.

nipper, does it seem the rpm go from normal to higher than they should or lower?

 

that is, is the ECU boosting for a load that isn't there (compressor clutch is slipping or ???)

 

OR, are RPMs dropping because the compressor clutch engages but the ECU isn't boosting the RPMs?

 

05 isn't DBW so, is there a solenoid somewhere that boosts rpms?

 

 

(man I'm ignorant of this part of a car's system!)

The issue goes away with the AC off (car has dual zone auto climate control).

AC blows cold

It feels like it is over compensating when the AC comes on, and under compensating when it kics out. The rpm does drop lower then it should when the AC kicks out, but it is no where as dramatic when the AC kicks in.

any work done recently? replaced the TPS ?

 

if it seems to do it only at idle, perhaps there's a throttlebody or IACV issue?

 

just throwing out wild ideas.

I'm going to try cleaning the throttle body once (and if) I feel better.