RPM jump with heat on

Hello,

 

I tried to search for the problem I'm having with my 2014 Legacy but can't seem to find anything on it.

Since it started getting cold and the need for the heat to be on, I've noticed that when I turn the heat on, my RPM will at times sit around 2-2.2k RPM even though I'm just cruising at 40 or 45mph. I mean same happens regardless of the speed as it goes over 35mph.

No codes are given, car doesn't feel funny, except for this. If I'm driving and I turn the heat off, the RPM will drop down to about 1.4-1.6k. If I turn the heat back on while driving, the RPM will not increase. It only happens if the heat is on and I'm taking off from a stand still. Then after a bit, RPM will drop on its own regardless of heat being on or off.

 

Any help on this would be appreciated.

same issue here, replaced t-stat, issue remains so I use paddle shifters to override it.

Could it be running in cold start part of the map?

Could be compensating for the added load of the AC compressor in some heat settings to dry the air.

Could be compensating for the added load of the AC compressor in some heat settings to dry the air.

 

^^ This. I know the front windshield defrost will turn on the AC in some modes.

Compressor is variable displacement it does not bug the engine down...

Compressors and other pumps, variable or fixed displacement, require horsepower to do work. If it's actively compressing, it'll put a load on the engine. Essentially no load when it's destroked, though.

And the ecu has compensation for the AC it show up on the Accessport as an adjustment in the tuning options. At least for the GT it does.

I remember Perscitus talking about that, although I can't remember if it was in regards to his 3.6R or his brother's CVT WRX.

I'm thinking the same thing as others here - that the RPM is based on the electrical load of the AC compressor. What fan speed are you using? High? That draws a lot of power and could, along with the AC compressor coming on, be the cause of the RPM jump.

Compressors and other pumps, variable or fixed displacement, require horsepower to do work. If it's actively compressing, it'll put a load on the engine. Essentially no load when it's destroked, though.

 

ECM compensates very well for increased A/C load I have never seen an idle surge as the ECM ramps up demand for A/C at idle.

 

 

OP, connect a data logger and look at your coolant temp. I am willing to bet that when you turn the heat on. it drops by a fair amount and this is causing the car to think that it is not up to full operating temp. Another test would to press the throttle down hard during acceleration and then tell me if you get an increase in heat output. It takes my car nearly 20 miles to warm up to the point where it operates normally.

On my GT there are two distinct idle speeds, one for AC on and one for AC off. All cars will turn on the AC in defrost mode, because the AC coil will condense moisture out of the air, then it'll push air through the heater core and then out to your vents.

 

With an Access Port you can even adjust these two different idles up or down 100 rpm at a time. There's no idle rpm for engine temp that I know of.

 

Even with the heat on full blast I have not seen my car's coolant temp be greatly affected. The biggest effect on car coolant temp is the airflow through the radiator. When you come to a stop light for 30 seconds you'll see the temp go right back up.

 

Try driving around with your heat on but shut off just your AC and see what your rpms do.