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you're low on coolant - should up at the top of the inner seal. did you install an OEM-style thermostat? make sure there's no build-up of grass/bugs type debris between the rad and the ac condenser. confirm fans are OK cut the bottom of the o'flow tube at 45* angle. If the o'flow bottle is dirty, take it out and put some pieces of ice in it and shake like crazy. until you think this is solved, check coolant level IN THE RADIATOR , the o'flow bottle is an untrustworthy reflection of total coolant volume.

I’d think if there was a hole there it would push out coolant until the level in the radiator couldn’t get to the hole any more. This would do two things - first, the cooling system would never pressurise, leading to poor cooling performance. Second, reduced coolant volume will circulate the coolant quicker through a cooling system at or above normal operating temp. And if that’s your original radiator you’ll probably find it’s full of crud. If that hole isn’t your issue the partially blocked radiator could be. Last thing, do you still have the air dam/engine undertray piece still fitted? This is designed to develop a low pressure system in the engine bay at speed to draw air through the radiator. Without this you could find your engine running warmer than normal in hot weather but I doubt it will allow the cooling system to boil. Cheers Bennie

Appears to be a defect in the molding process in the radiator neck and I can't really see how that would be an issue as it has probably been there from day one. If you haven't been losing anti-freeze constantly then that defect is nothing to worry about. It is below the port to the overflow tank so that would not affect the expansion and contraction of the coolant either. I've seen several cases of people here concerned with their temp gauges reading higher than normal and I was one of them. In my case, the needle was at the red line but never boiled over or showed any other signs of running hot. When measuring the radiator hoses or the thermostat housing with an IR temp gun, things were normal. I had a bad temperature sensor where moisture had wicked inside and internal corrosion was raising the resistance of the circuit and causing the gauge to read high when all was actually normal. Are you noticing any other signs of a hot engine? Do you have a way to measure the coolant temps with another device? I used a cheap Bluetooth remote BBQ thermometer to confirm the temps while driving. I temporarily attached the temp sensor to a coolant line under the hood and then watched the readings on the base unit inside the car while driving for a few days.

Looks original and old. If you’re not seeing losses or leaks then it’s not leaking externally. The only failures I’m aware of are external leaks, damaged necks preventing the cap from sealing, and interval clogs. I’ve never heard of internal leaks but failure of the plastic is no surprise on 10+ year old radiators and yours is 20 yo. If you have tractor or equipment repair shops or one of the few remaining radiator repair shops around they can test the radiator. But for the time you can just buy a new one. Ebay is the go to radiator source. They’re very inexpensive and aftermarket is a surprisingly decent option for Subaru radiators. I’ve seen H6 Subaru radiators for under $70.

Good to hear that these were replaced. When buying genuine cam cover seals I’ve always needed to order the spark plug tube grommets separately. Next question: how did you clean out all the oil that was in there from before the grommets were replaced? Not an easy task! I had my engine on the floor when I did this and I still found it difficult! I used some torn down rags made into strips, shoved these in and twisted them around the spark plug as much as possible using a long screw driver, then did it again with a fresh rag strip until I was confident the tube was pretty clean. Then I removed the spark plug and used a fatter rag strip to clean out any last bits. I could’ve used some Metho at this point to give it a thorough clean but I don’t recall doing that. The results? Well the project is still sitting there waiting to be finished off and used so I can’t report on it yet. Cheers Bennie