Rodger Clark Motorsports Subaru EJ Low Temperature Opening Thermostat

We had noticed that the coolant temperature of Project STI was pretty hot considering that the car had a big CSF radiator and a Greddy oil cooler, in fact, the temperature was the highest of all of our project cars with modified cooling systems.  In bumper-to-bumper traffic, our coolant temps were going up to 209 degrees.  As soon as the vehicle started moving this would drop to around 190 degrees and dither from 185-195 degrees.  Although this was not bad, it was higher than our other project cars. Doing some research we found that the factory thermostat starts opening at around 195 degrees and is fully open at 203 degrees.  We decided to try a Rodger Clark Motorsports low-temperature thermostat from Flatirons Tuning to see what would happen.

The RCM thermostat is supposedly of higher quality than OEM and more reliable, but not being thermostat construction experts, we don’t have any way of proving this but this is what RCM claims.  The RCM thermostat starts opening at 158 degrees and is fully open at 194 degrees.

Changing the thermostat is easy enough, first, obviously you drain your coolant. Then remove the upper 10mm bolt holding the thermostat housing on.  This is located on the front lower driver’s side of the engine, not the top like an inline motor so it’s accessed at the bottom of the car.

Then remove the bottom 10mm bolt like so.

Boom, remove the stock thermostat and replace it with the new low-temperature one.

With the low temp thermostat in place, the engine coolant is allowed to circulate around the engine much earlier which allows the engine temperature to be reduced sooner than before. The coolant’s job is to carry heat out of the engine, take it to the radiator where temperature reduction happens, and then, back to the engine at a lower temperature ready to absorb more combustion heat. This cooling process is controlled by the thermostat opening and closing.  If the thermostat is opened sooner than before, then the coolant can get an earlier start on cooling the engine. With coolant flowing through the engine sooner at a, lower temperature, our CSF radiator can now hopefully start lowering coolant temperatures faster, more efficiently and at a higher volume than before.


  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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