Okay guys I'm new-ish to working and trouble shooting my vehicles but I'm getting stumped. I have a 2004 forester xs with 120,000 on motor, 250,000 on body. Only 1 check engine code which pertains to the EGR valve that's needs replaced. About a month ago the car randomly started overheating and my upper radiator hose failed and shot coolant out during my 1 hr drive home from work. I had assumed it was a bad hose and replaced it only to find I was wrong later after the same thing happened to the lower hose do to the pressure building in coolant system. I did some research and changed thermostat and radiator cap. Car started overheating again. Took it to a mechanic and they said radiator has blockage in it and he replaced thermostat. I then did the work myself and replaced radiator and burped the system when I refilled it.

Now--- the car runs for about 3-4 days (1 hr 2x a day) until finally the coolant has made it's way into the plastic overflow container filled it completely up. The coolant will NOT go back into the radiator after this point and I noticed after I drive it like this that there are small bubbles for 10 or so minutes in the overfilled overflow container. I've been back to same mechanic who then tested the coolant for gas to see if it was a head gasket problem. The coolant test came back fine with no gas in coolant?

Things I should mention: -Both heat and air work fine all the time. -The fans kick on at when car reaches temp. -The car only starts overheating while idling now and is fine when driving. - I smell burning coolant after it's at temp and i drive to or from work. I assume its pushing it's way out of overflow bottle and burning off on engine? -the car runs fine other than having very sluggish/slow acceleration when merging and climbing to speed on highway -My dad who has worked on vehicles for years suggested maybe the cat converter could be clogged and causing pressure in system?

Any tips or suggestions are welcomed, I appreciate it!!

2 Answers 2


A clogged catalytic converter will cause back-pressure in the engine, and cause the engine to run badly - you'd have trouble reaching 35mph with a bad clog, and it wouldn't just be on hills.

You've already tested for one head gasket issue - unfortunately, on older Subarus, that's always the first thing to check for! You have no exhaust gas in the coolant, but do you have any water in the oil?

Where would I go from here?

  1. Check your radiator cap. It should be rated at 1.3 bar.
  2. Check your receipt for the new thermostat. It NEEDS to be an OEM Subaru thermostat. One from the local parts company (where they say it will fit) won't work. The OEM thermostat is physically different from ones you buy in the store - it has a longer spring on it. If the mechanic says 'I've used these compatible ones for years without issues', find a new mechanic; he hasn't worked on Subarus much. A Subaru should ONLY have a Subaru thermostat.
  3. Test when hot. I'm not convinced (yet) that this is a head gasket issue, but some small issues only show up when the car is driven hot for a while, and over a few days. If the leak is small, you might not detect exhaust gas in the coolant after a few hours.

Maybe time for a dedicated Subaru mechanic to have a look, but on the evidence given, I'd be leaning towards the thermostat.


I would have immediately thought "head gaskets" as well. I have a 2005 Forester XS with 98+k miles, and have gone through 2 head gasket changes.

You mention "Only 1 check engine code which pertains to the EGR". Did you determine this via a scan tool? Do you know the DTC, and if so, what is it? That would be a good starting point IMO. Any DTC's detected by a scan tool can be cross-referrenced in the factory service manual for your specific MY, which will beter allow you to diagnose the problem. Personally, I am unaware of any connection (no pun intended) between the EGR and the problems you describe going on with your radiator/overflow tank. It's possible you have a blockage somewhere else in the coolant circuit.

I had an overheating issue with my '05 a couple of years ago. It took my mechanic (who is usually top-notch) three tries diagnosing the problem, before finally tracing it to the ECU. In other words, sometimes the problem lies in the last place you would expect.

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