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I need some help figuring out what's wrong with my 2007 Forester XT Manual.

Background:

  1. The headgasket popped in November of last year. I had it fixed by an RMI certified workshop. They skimmed the head, installed a new HG, timing belt, etc. I don't think they did the waterpump.
  2. Before that, the guy I bought it from had the downpipe decatted and the thermostat replaced (he even gave me the old one).

Symptoms:

So on Sunday we were driving around looking for houses on show and suddenly I notice the temp gauge going up to 3/4. I turn on Torque and it shows the temp is at 112 Centigrade (about 12% above boiling point of water, whatever that is in Fahrenheit). I checked the coolant level and while it wasn't full-full, it was enough to keep the car cool (I know because when the HG went, it only overheated when the coolant was almost completely empty). In an attempt to protect the engine, I turned on the heat on full but only cold air came out. I haven't checked for collapsed pipes. I also don't know the state of the heater core. All I can say is that it was working pretty well up until thursday or friday when I last had the heat on. I wasn't driving fast, I was cruising around looking for a house to buy. I didn't notice any bubbles in the coolant like last time, so I don't THINK the HG is leaking into the waterjacket again.

I suspect a bad thermostat or a blockage of some sort, but I have no solid proof yet. Anyone else have something similar?

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  • Is the thermostat electrical or mechanical?
    – Zaid
    Jun 30, 2015 at 12:11
  • Mechanical. It looks like a tweeter with a spring at the back :P
    – Captain Kenpachi
    Jun 30, 2015 at 12:32

3 Answers 3

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I think this points at the thermostat being bad. The reason I go in this direction is due to there being no heat coming from the heater vents. I'm not sure if Subaru has it this way, but many heating systems in cars only divert coolant from exterior of the motor to the heater core. If the thermostat is not allowing (much or) any coolant through, you'll only get cool coolant through the core and no heat transference. It also means you don't have any real cooling going on with your engine.

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  • This is my first thought too. I am just so careful because I KNOW these cars are fragile and I'm still paying back the previous HG repair bill.
    – Captain Kenpachi
    Jun 30, 2015 at 15:34
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Without knowing specifics about the WRX engine, it sounds like you have a decent plan of attack. Here are some general guidelines for how to test for possible culprits in a sudden overheating situation:

  • Cooling fan not running

    You'll want to check fuses, relays, continuity, and apply 12V to make sure that the fan hasn't mechanically seized.

  • Restricted coolant flow

    Visually inspect for collapsed pipes as you have already mentioned. I'm reluctant to say that a bad water pump would cause this since you mention the failure seems to be sudden and not gradual. Same comment applies to deposit build-up in the plumbing - possible but not probable.

  • Stuck-closed thermostat

    This should be easy to test. Remove the thermostat and immerse it in boiling water. The set-point temperature is usually stamped on the thermostat itself, so you want to see if the pintle "opens" when the temperature has reached.

  • Bad radiator cap

    The cap is responsible for regulating system pressure. A bad cap may explain loss of coolant from the overflow tank.

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2

It turned out to be perished radiator caps. I replaced both and the problem went away.

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