My 2006 Subaru Forester is overheating with no heat blowing through the vents. I have had a thermostat put in and a new heater core. Radiator has been pressure tested with no leaks. Unfortunately it doesn't overheat all the time. It will go a few weeks and then overheat and no heat blowing. Could be a bad thermostat. Can a head gasket be the cause but would a head gasket affect the heat?

  • When the car overheats are you at a stand still or are you driving? How's the cooling fan operation? If you turn on the A/C do both fans come on? How about external leaks? Subaru engines have known failure points on the back side of the head. If you shine a light down there can you see oil or coolant stains? How about the water pump? It's driven by the timing belt, a small leak might burn away before you know it.
    – Ben
    Feb 23, 2016 at 22:06
  • When it overheats - are you required to add a significant amount of coolant to bring down the temp? Feb 23, 2016 at 22:19
  • Every time the car overheats I have been sitting still and then when I drive away it will overheat. I do hear the fan running the whole time. One time the heat didn't work and then after a second replacement of a thermostat the heat did work but the engine overheated. It has never overheated while driving only after sitting still. I've had the heater core replaced, 2 thermostats, new radiator, hoses. No steam out exhaust at all. Engine runs great. Unfortunately with all these fixes I do have coolant stains, etc and coolant is still burning off so I am not sure about leaks.
    – TCrawford
    Feb 24, 2016 at 13:50
  • I have never had to add coolant at any time.
    – TCrawford
    Feb 24, 2016 at 14:03

4 Answers 4


Check the overflow tank on the radiator when the car is overheated. If that tank is full, you've probably got a blown headgasket - that's how I spotted mine.

For the thermostat; did you buy an OEM one, or after market? Foresters are very particular about the thermostat, and ONLY the OEM ones work reliably.

  • The overflow tank is full but does a blown headgasket affect the heat ?
    – TCrawford
    Feb 23, 2016 at 18:04
  • @TCrawford if all your coolant is going into the bottle yes. When you say it's full is it only full to the max mark or is it full to the overflow tube?
    – Ben
    Feb 23, 2016 at 22:02
  • I'm not sure how you make the connection between a full overflow tank, overheating and a blown head gasket? If the engine is overheating, it's lost enough coolant to not effectively cool the engine, so the OF tank should be low or empty. A blown head gasket will not always cause the engine to overheat, Feb 23, 2016 at 23:42
  • Why would an aftermarket thermostat not work reliably if it's of decent quality? What is there to be particular about, a thermostat is a pretty simple device, why would it work in one car and not work in another? Feb 24, 2016 at 8:39
  • It is full only to the max mark. I now have the second thermostat replaced and drive it after getting it back and it heats up again. I have never lost coolant.
    – TCrawford
    Feb 24, 2016 at 13:40

OK - I see from your comment above that the overflow tank is still full. In most cases, if you have a blown head gasket, you'll be losing coolant.

In can go into the main crankcase, in which case your engine oil will show the effects of coolant and oil mixing. The oil dipstick will have a mottled, milky appearance.

In can also go into the exhaust system - check the tailpipe for excessive steam and or coolant dripping. Also pop the actual radiator cap (while engine still cold, very dangerous if engine is hot) and rev the engine and see if exhaust fumes are coming out the top.

The no heat is a sign that hot coolant isn't flowing into the heater core, this could be the thermostat is stuck closed and not allowing the coolant to circulate through the engine. It could also be the water pump.

Hopefully it's not the head gasket, that's usually a sign your cylinder head is warped, and it's fairly expensive.

  • Usually the thermostat only stops coolant flow to the radiator. The flow to the heater core usually continues to flow when the thermostat is closed. Unless you know something specific to this vehicle.
    – HandyHowie
    Feb 24, 2016 at 8:16
  • my engine oil doesn't show any effects of coolant or oil mixing. no excessive steam out of the exhaust and I have never lost coolant or had to had coolant. The engine runs great.
    – TCrawford
    Feb 24, 2016 at 14:05
  • Handy Howie - yes you are probably correct however the net effect would still be the same as the source of heat (engine) is now out of the circulation loop, so you would still get cold air from the heater. Feb 29, 2016 at 17:08
  • Subarus are pretty notorious for head gasket issues, I wouldn't be surprised if that's the case here.
    – TMN
    May 11, 2016 at 13:50

I had a similar issue with one of my cars. The water pump was bad. The impeller was plastic and had cracked, so it wasn't spinning (at least not as much as it should be). Since the water pump is not moving enough coolant, you are not getting enough coolant passing though your heater core to heat the car. While you are sitting, the coolant is not moving and heating up, when you start moving again, it starts moving the coolant a little and moves the hot coolant reaches the sensor.

  • Did your car over heat or just not have heat? I am hopeful I may be getting closer to the problem with your reply! 5 times in the garage and it very expensive and still not fixed. THANKS
    – TCrawford
    Feb 24, 2016 at 14:39
  • I don't remember the heat as the issue was in the summer. I did have overheating problems related to sitting still for a bit. I had even rigged up a manual switch for the fan because I thought it was fan related. If your not loosing coolant and your fans are working, I would certainly be looking at the pump.
    – rpmerf
    Feb 24, 2016 at 15:12
  • Thank you so much. It has been frustrating to say the least.
    – TCrawford
    Feb 24, 2016 at 15:55
  • Yes, it is a difficult to troubleshoot issue, and it is not too common. I do all my own work, and spent months flushing the system, pressure testing the system, rigging the fans trying to figure out what was causing the issue. Not even sure how I found it.
    – rpmerf
    Feb 24, 2016 at 16:15
  • Just wanted to follow-up. My water pump was leaking so it was replaced and timing belt since I was already there and so far so good. The car so far has not overheated and doing great. Fingers crossed this has fixed it. THANK YOU!!
    – TCrawford
    Mar 7, 2016 at 17:00

No heat in the cabin and an overheating engine after servicing something in the cooling system... My bet is air trapped in the cooling system as it would account for both of the symptoms and matches the timeline of when the issue started.

Trapped air would also explain the overheating without coolant loss.

If needed I could elaborate on what to do to get the air out, but there's already a plethora of information on this.

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