The Mystery of Subaru Radiator caps explained

After we had all of our overheating issues at Super Lap Battle, we got a message from James Hudson from Colonel Red Racing that pointed us to the note on all of Mishimoto's Subaru Radiators.

Specifically, the fact that the Mishimoto cap that comes with their radiator is a 2-way cap, and is not to be used on the radiator.  If you put the 2-way cap on the radiator it says, overheating may occur.

This was the first time that all of us here had heard about a 1-way radiator cap vs. a 2-way, so we then went and had to look into what the difference between the two types of caps is, and it was really interesting.

It definitely seems that there is not as much information about the difference between these caps out there as there should be in regards to using them on Subaru's.  It would also seem that having incorrect radiator caps on your car may play a part in overheating issues, which could be easily overlooked.

And the other interesting thing is that every aftermarket cap that we could find is a 2-way cap.  And if using two 2-way caps on a car can cause issues, that may require some changes to be made.

Thanks for watching and Stay Tuned!

Here is a summary of the key points discussed in the video:

The Core Mystery: Two-Way vs. One-Way Radiator Caps [00:15]
The Discovery: We discovered a critical note at the bottom of Mishimoto radiator installation instructions for Subarus. It stated that the provided Mishimoto cap should replace the stock cap on the expansion tank, and the stock cap from the radiator should be moved to the Mishimoto radiator.

Why? Mishimoto caps (and almost all aftermarket caps) are two-way valves, whereas the stock cap located on a Subaru's radiator is a one-way valve.

The Mechanics:

A One-Way Cap (the flat, round one) only opens to release pressure from the cooling system.

A Two-Way Cap (the one with "ears" and a small dome/valve on the bottom) releases pressure and opens under vacuum to pull coolant back into the system from the overflow tank as the engine cools down.

Why Does Subaru Use Two Caps? [04:09]
Most cars only have a single radiator cap (a two-way cap). Turbocharged Subarus uniquely have two: one on the expansion tank (near the intake manifold) and one on the radiator itself.

Historical Context: In 2002-2003, the WRX only had one cap (on the expansion tank) and the radiator was completely sealed. However, Subaru experienced high failure rates where the crimped-on plastic end tanks of the factory radiators would separate due to pressure.

The Solution: Subaru added a second cap to the radiator (which they officially call a "pressure relief valve") to bleed off excess pressure and save the plastic end tanks. They made it a one-way valve to prevent it from sucking coolant back in, maintaining the expansion tank as the primary point for the system to burp air and pull in coolant.

What Happens If You Mess Up the Caps? [07:03]
Flipping the Stock Caps: The stock expansion tank cap is 1.1 bar, and the stock radiator cap is 1.3 bar. If you accidentally flip them, the radiator will blow off pressure too early (reducing cooling capacity), and the expansion tank will no longer have the two-way valve needed to pull coolant back in or properly burp air out of the system. This leads to overheating.

Running Two Aftermarket (Two-Way) Caps: Because almost all aftermarket caps (like Mishimoto, Cobb, etc.) are two-way caps, many people unknowingly install two two-way caps of the same pressure on their Subaru. We now strongly suspect this dual two-way configuration prevents the system's ability to self-burp and pull coolant back correctly, leading to chronic overheating issues and overflowing coolant.

The Takeaway [09:11]
If you are struggling with unexplained overheating or coolant overflow issues on a turbocharged Subaru, take a close look at your radiator caps. Running two aftermarket two-way caps—or mixing up the locations of the factory one-way and two-way caps—severely compromises the system's ability to regulate pressure and purge trapped air.