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Overheating when the a/c is on

Have a 2000 subaru outback, automatic, with 231,000 miles, whenever I turn the a/c on and drive for a about 10 miles the car start overheating, if I turn the a/c off the temperature gauge goes down. Changed the thermostat, and both fans are working. Maybe this may help, the a/c compressor does not turn off when the a/c is on and if I am going down the hill, I shift the gear to neutral or if on a traffic stop and shift the gear to neutral the temp gauge most of the time goes down. Help, too hot in Kansas, and 2 kids on the back.

Have you ever had the radiator flushed? I would also check all the radiator hoses, looking for soft spots.

This is usually a sign of a cooling system problem, not an AC problem, if it happens while you’re driving. Like JEM said, check the radiator and hoses, at 231k you may need a new radiator or a flush or new hoses.

I flashed the radiator, maybe I need a new one. Thanks for the quick response.

Clearly the added load from the AC compressor is causing the engine to create more heat than the cooling system can dissipate. The possible cause for an inability of the cooling system to keep the engine cool include

  1. a radiator with plugged or partailly plugged core tubes. Thes can be diagnosed by “mapping” the heat dissipation using a pyrometer. Flow through the radiator can also be checked.

  2. a weak water pump. Im[ellars can erode over time. The flow of the pump can be tested.

  3. a restriction in the system. Generally this’ll be a sticking thermostat or a collapsed inner liner on a radiator hose. T-stats and hoses are inexpensive toe replace, and at this age it’d be best to just do so.

  4. fans, but you’ve already checked those.

  5. a blown headgasket. Have you been losing any coolant lately? Has you coolant looked muddy? If in doubt, this can be easily checked for in a number of different ways. If you remove the radiator cap, run the engine, and see bubbles coming up out the fill hole that’s a singn that you have a headgasket problem. Verification can be done with a pressure leakdown test.

My coolant does not looked muddy and not loosing any coolant, may have to change the water pump and put in a new radiator, maybe there lies my luck.

This happened on my car. It NEVER overheated until I turned on the a/c ( this was late in the cars life ). I did notice all the fins on the bottom of the condenser were closed shut from highway debris. My guess was with the extra load and limited air flow it heated.

You need a new radiator.

When the AC is off, there’s enough cool air passing thru the radiator to prevent the engine from overheating. When the AC is turned on, the heat released from the condenser located in front of the radiator reduces the radiator’s abililty to remove the heat. And the engine starts to overheat.

The radiator is partially restricted. And that’s what’s causing the engine to overheat when the AC is turned on.


Check to see your system is holding pressure, doing a squeeze test on the upper radiator hose is a simple way. Maybe if your lucky a new radiator cap is all you need.

You might try taking a garden hose or an air hose and from the engine side of the radiator direct the water or air from the hose through the radiator and out the grille. If the fins of the radiator are clogged with bugs and debris, the radiator can’t get rid of as much of the heat. This may not solve your problem, but it won’t cost anything and is worth a try.