Rodger Clark Motorsports Subaru EJ Low Temperature Opening Thermostat

With the RCM thermostat in place, we noticed that our car runs a lot cooler, when the AC fans kick on the coolant temp drops to as low as 175 degrees.  Generally, the temperature dithers around 185 degrees and when really heat soaking in bumper-to-bumper traffic our temperature hit 203 degrees peak.  We also noticed that the fans would kick on a lot less in general.  When driving at highway speed the temperature ranges from 175-185 or 5-10 degrees cooler. So it seems like our peak temperatures were reduced by 6 degrees and the average temperatures in operation by 10 degrees.   This might be a decent cooling advantage for track driving. As a note, if you have a stock radiator this mod is probably not worth it but it is probably something to consider when you have a high-capacity cooling system.

This thermostat comes with the rubber gasket and fits all Subaru EJ20, EJ22, and EJ25 engines.


Flatirons Tuning



  1. Mike,
    Have you seen the cooling fan mod ? It is switching the primary fan cycling on the passenger side to have more exchange at the inlet versus the outlet. I believe at the outlet will cycle the thermostat more often between open and closed or partially closed.

    1. Sounds interesting but since the car never gets to 100c even on hot days I don’t know if it’s needed. The fans don’t come on too much unless the AC is on!

  2. Do you have the radiator caps on correctly? The one with the “ears” goes on the upper one, the one without the ears goes on the rad. just sayin lol

    1. We researched this and you are right. Not to many people know about this, not even experienced Subaru tuners. We are gonna do a story on what is going on soon,

  3. Having that RCM thermostat switched , will make the car to run rich at the time the car get warm up ?

    Also sti from 19-21 the fan get the signal by pulse so any type or upgrade on fans will be worthless unless is a variable speed fan .
    Have you guys hear about it ?

  4. I just had my 01 Forester Boxer engine rebuilt. Runs great except on hot days above 87 degrees. It will run a little hot but going up a steep grade the gauge goes up to a couple ticks from the red & Im not even over 3,000 rpm’s due to still in its breaking period. Radiator is new, fans work, the shop has double checked everything & can’t find the problem. After reading your experiment maybe it’s the new factory thermostat?

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