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11 Posts
Discussion Starter · #21 · (Edited)
OEM radiator or parts catalog? A lot of aftermarket replacements have lower cooling capacity which can cause high pressure. This is an odd problem for sure. Time for long shots.

Can you rig up a compressed air line to your exhaust & intake? If the coolant seals in turbo are starting to wear, either turbine or compressor side, then it can pressurize cooling circuit.

If you still suspect HG (highly unlikely) one option is to rent a leakdown tester. Keep it connected for atleast 20mins with rad cap off and see if you get any bubbles.
Using a Denso 221-9235 radiator. It's the exact same specs and materials as the koyo OEM, Japan company that OEM for Honda.

I am quite sure that the radiator is not the problem since the old one looks pretty good and the flow capacity when tested is the same. I just changed it out to be 100% sure. They both work the same. No clogs or problems.

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2010 Subaru WRX Limited Hatchback 5MT
892 Posts
You seem thorough but still, round cap is on radiator and tabbed one on reservoir?

Although a lot of work you can swap the old one back in to see.

· Registered
2 Posts
New to the forum. Please forgive me from any mistakes. Thanks in advance.

I am new to Subarus and essentially did not do enough homework to understand that I received a problematic car. It drives fine on the highway but begins to overheat after entering city (slower) driving conditions. The coolant reservoir always spits out coolant. I originally thought it was a coolant issue, now I believe it is a HG (potentially) engine block issue. I will do my best to explain the timeline in detail below:

  • Drove 2 hours home and started overheating once off exit
  • Pressure tested to check for leaks + coolant burp
  • Drove 1 hour on highway then temperature spike off exit at a red light
  • Towed home
  • Replaced radiator caps, radiator + hoses, thermostat, checked water pump (dated 2021) + new gasket
  • Drove for 15 mins (no overheat but coolant reservoir boiled out again)
  • Performed block test (liquid was dark green instead of blue or yellow)
  • Redid a burp and noticed TINY bubbles coming out of coolant overflow tank when using a burp funnel
  • Conclusion is MLS gasket has tiny air pocket that is still small but causing a vapor leaks into the engine

My research has told me that 08 WRX heads typically hold up pretty well with the MLS gaskets and if there is a reason that the car overheated, then the first step is to understand why it overheated (e.g. engine block warpage, ringlands, pistons, rods, etc.). The engine seems like it is still strong having been able to drive a couple hours and I do not hear the typical engine rod nock or any other odd sounds.

My first question is if people who have experienced (non coolant related) engine overheating resulted in the engine block being warped and have had to do an entire repair/rebuild. How often is this typically the case over (non coolant related) overheating?

My second question is that if I were to try and take out the engine myself to work on, what would be the steps that I should take in order to (1) better understand and confirm where the issue is coming from (2) what are the necessary things I would need to replace to ensure that this does not happen in the future
I had the same issue with mine on the way home from buying. Turns out the cooling fan fuse was popped.
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