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2008 WRX (Overheating Issue & Diagnosis & Next Step Guidance Needed)

852 Views 24 Replies 8 Participants Last post by  dslover2000
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
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100% agreed here.

OP - I don't understand what you mean by non-coolant related overheating? If the engine is overheating, it has something to do with the cooling system. There is no other failure that would cause general engine overheating without some form of catastrophic local failure i.e. extremely lean condition or retarded timing creates extremely high EGT which can burn valves, turbines, headers, but won't cause "overheating" as defined by coolant temperature.

Head/deck warping, headgasket failure, etc. are the RESULT OF overheating, not the cause. Overheating is caused by either 1. low or no coolant 2. air trapped in the system 3. damaged thermostat 4. coolant passage blockage 5. or damaged radiator or fans

A severely damaged headgasket does introduce air into the system, but you should also notice severe coolant consumption/white smoke during engine operation and almost always significant "yogurt" accumulation in the oil sump.
I previously ran the car for a couple of hours while monitoring the radiator fans and coolant temperatures. Fans kicked on each temperature spike and never experienced overheating issues at idle so I believe they are in working order. I have also checked to my best ability that coolant flows all the way through so there isn't any blockage.

In regards to severe coolant consumption/white smoke/yogurt like oil, I read online that because MLS gaskets are very strong, they tend to only leak air pockets for a long time until the coolant actually begins leaking into the oil. I have checked the oil multiple times and it always looks good.

Makes me believe that there is an leak from the HG that i've concluded from the dark green block test and also the tiny bubbles I saw in the burp funnel when burping the system.
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I find this unlikely. Are you sure it's fully burped?
I used a burping funnel on top of the coolant overflow tank, overfilled it with coolant, and let the engine run for 45 mins. There were no big air pockets after a while and all that remained was a consistent stream of tiny air bubbles.

Keep in mind that each time the car was driven that the coolant reservoir would explode and spit out coolant.
OEM rad caps? The dual cap thing is odd, from what I remember you need very specific caps in each location and mixing them up can cause pressure buildup in the reservoir.

A steady stream of bubbles is not normal, you might just need more bleed time. Check for an AOS, if it's above rad cap height, it can cause local vaporlock and not bleed.
I purchased the radiator single valve cap for the radiator and the dual valve for the overflow tank, both OEM from Subaru dealership. There is no AOS on the car everything is stock.
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Everything I've read and saw says burping on the reservoir, not radiator, since it's the highest point in the system.
Yeah this is exactly where and why I burped it from. Any thoughts?
I've always done at both places - starting at the radiator and finishing at the reservoir at the end.
I am pretty sure that I burped it correctly and that there is no air in the system. Any thoughts?
Found someone resolved a similar issue by replacing their radiator. Yours is already replaced, but is it an OEM unit? How about the other replaced parts like hoses, thermostat etc?
I've used all OEM stuff.
Basic but easily forgotten: heater on high, fan on low when you burp?

Condition of radiator? Sure the fans are spinning fast enough? Dying motors often slow down & heat up.
The motor purrs while idling and pulls like a beast. Yes to heater on high and fan on low. Radiator is new and fans spin fast.... I'm really lost right now...
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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