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Yet another overheating question....


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So a little background...2008 Spec B with just over 180,000 miles. Radiator replaced with Mishimoto unit in 2015 with around 150,000 miles, Water pump was done at 171,000 when I did my timing belt (gates kit) and thermostat was just replaced. Fans seem to be working fine although its only overheating at highway speeds so that shouldn't matter. It is throwing an intermittent cylinder 3 misfire that I'm currently trying to track the fix too (Only seems to happens at start up or shortly after).

 

The car has been overheating at highway speeds, it started about 2 weeks ago, got off the turnpike to turn onto a slightly slower highway (still 50 miles na hour or so), turned off interior heat and shortly after the car heated up and wouldn't cool back down. I thought maybe there might be some air in the lines for some reason and I had some extra coolant with me so I topped off and bled the system, at the time I did notice that the system had pushed a whole lot of coolant into the overflow tank. Fired it up and drove down the road but it was still overheating. Did it once more and the car would sit at idle fine fans kicked on temp came down to normal, jumped back on the road to head for home and got about an hour and 50 miles or so into my trip (all highway driving) and it started to gradually climb and drop until eventually it just went to high and I had to pull over and get towed. Replaced the thermostat which I found out wasn't opening till around 190 and thought I had the problem fixed. Drove for a short bit at highway speeds and was fine same with slow speeds and idle. Heading out on an hour and a half long trip, about 25 minutes into the drive temp spiked and dropped and then would climb and fall, continuously slowly rising though. Got off the highway and it immediately went red but luckily I was where I needed to be, had to tow it home though.

 

My mechanic friend suspects it might be a head gasket, but he also says he found no sign of exhaust gasses in the coolant and when we pulled the cylinder 3 plug it looked fine (I suspect injector there). I have a new rad cap on the way as I figure I'll try the $30 fix before even thinking about anything else.

 

I just wanted to see if you guys could think of anything I might be missing before this issue drives me crazy or starts to get really expensive.

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Pull the number 3 spark plug and another random one. Now stick a borescope into the cylinder.

 

Then ask yourself if #3 looks cleaner than the other one.

I suspect it will due to the coolant steam cleaning it.

 

Very likely a headgaskets issue.

Make sure a good machine shop performs the resurfacing of the heads.

I'd also go with the newer black headbolts if not going arp studs:

https://legacygt.com/forums/showthread.php/psa-subaru-changed-headbolts-improved-2010-onwards-271129.html

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