The heater in my car blows out cold air when I'm on the freeway, or going fast. What's wrong?

Question

I have a 98 Subaru Legacy GT. When I bought it, I had the head gasket replaced and I thought that would fix the problem but it didn't. What would make my heater blow hot air when idling and cold air when at high speeds?

Answer 1

you need a hotter thermostat.

Answer 2

Well only about 100 possibilities. Assuming the head gasket job was done right, coolant level is what I'd check first. Then the thermostat. Then get trickier. Start the car cold with the rad cap off - rad cap, not reservoir cap. At first you should see no sign (or very little sign) of circulation. As the car warms up, you should easily be able to tell there is circulation, which tells you that at least the thermostat is open. Now it is warm, look for bubbles (kind of like the ones when you blow into a straw in a milkshake) which would indicate a problem like head gasket, cracked head, cracked block. Then, keeping your fingers out of the fan and belts, feel the rad hoses and heater hoses to see if they are getting hot. It is possible there is air in your cooling system, the heat valve (in line usually in a heater hose) is not working, or your heater core is plugged up. If you are still mystified get a garage to pressure test your cooling system. Also check to make sure the fan is not staying on all the time. Usually at highway speeds the fan does not run much, so if it stays on your temp gauge will tell you that you are running cold and you'll get little or no heat.

Answer 3

You probably don't have enough radiator fluid. The radiator fluid also travels through the heater core. Check the level and add some if needed. You might also have a leak somewhere. If this happens again, in less than a year, have it checked for a leak. I hope your Head wasn't cracked when you blew your head gasket. Did the car over-heat?

Also, the flap that changes the airflow direction from the heater core to the fresh air might be broken or loose. At high speeds, air would flow through easily and push the flap over if it's really loose. This could happen if you pushed the temperature slider over too fast. (I don't remember if the Subarus had sliders or electronic air temp control back then).

Another option is that the box that houses the heater core might have a fresh air leak in it.

Answer 4

You could have a coolant leak that would cause the heater to blow cold, if you have lost that much coolant then it needs to be looked at - If after say ten to fifteen minuets of the engine running is there white smoke from the exhaust? if so this could be Head Gasket another thing to check would be the inside of the oil filler cap is the oil clean or does it have a kind of white gunk to it this can also be the head gasket How much - I'm in the UK and would most likely cost about £120 approx $200 In the UK head gasket is quite cheap approx £30 ($45) hope that helps best regards Barry

Answer 5

check the coolant level, if good could have a air bubble in the system. you say they replaced the head gasket, maybe they didnt get all the air out and you had or still have a bubble

Answer 6

Could be thermostat. If you're a cheapskate like me, you could put a piece of cardboard on the front of the grill, on the front of the car. But just watch your heat gauge. You can adjust the cardboard like your own ghetto thermostat.

Answer 7

First check the quantity of liquid in the radiator, low fill, check for leaks, no leaks, no improvement, check the fan radiator to see if operating, no improvement,check the thermostat and replace if needed, no improvement, then take to your friendly independent repair shop.

Answer 8

What does the temperature gauge show in these conditions (moving and stationary)? If the temperature gauge also shows cold when stationary then the thermostat is probably stuck open.

Answer 9

A car's heater works on the water(coolant) in the radiator. It could be that your car is low on water. Better check it quick!

Answer 10

I had a similar problem and it was a bad heater core.

Answer 11

Call the manufacturer. It sounds as if they mixed some kind of electrical wires up.