The Mystery of the Round Subaru Radiator Cap

Having an incorrect radiator cap on a Subaru is a critical mistake that even seasoned Subaru builders, including us have made.  In fact, one of our sharp-eyed readers brought this to our attention when he was looking at one of our articles.  The fact that most high-performance radiators for Subarus come with high-pressure caps makes this issue even worse.  The problem is that the OEM radiator cap is a round one with no ears and we found that it’s important for this cap to be on the radiator, not the high-pressure cap that came on your performance radiator!

The round earless OEM cap has a unique feature, instead of the typical two-way pressure relief valve that comes on radiators, the OEM cap has a one-way check valve.

The OEM cap with the one-way check valve is pictured here on top, and the performance high-pressure two-way cap that came on our CSF radiator is on the bottom.  You can see the difference in the valves here.  A two-way cap is used on most cars so as the coolant expands with heat, the valve opens and lets coolant flow into the catch can.  When the engine cools down, the valve opens and lets the coolant returns to the radiator.  This helps keep some air out of the system and keeps the coolant level more consistent.

Normally a two-way cap is used on the radiator and this is how our car was. However, on a Subaru, you have a circulating expansion tank above the radiator.  This is to help ensure that all the air is bled out of the system and is a great OEM feature.   We convert most race cars we build to this sort of system for the same reason.


      1. “Cool”, I’m going to brag to my friends. “Rad” article lol. Glad to have helped and thanks for the shout-out!

  1. I spent 5 years as a certified Subaru tech and this was never explained to me. We were told not to mix them up but never given a reasoning. I even remember an instructor shrugging his shoulders in class when someone asked why!

  2. OMG, I’ve been struggling with air stuck in my cooling system (gurgling noise when I accelerate and no head gasket failure symptoms). I was having to repeatedly burp the system every couple weeks. After reading this article, I immediately went to my ’04 WRX and saw that I had the same two “eared” radiator caps on both the expansion tank and radiator. I just bought a round cap and am crossing my fingers that this’ll stop the entrapped air bubble issues that I’ve been dealing with.

    Thanks for spreading the good word!

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