So before I overthink all this and do more work than necessary, I wanted to get your thoughts on this little problem!

My Evo 9 has been somewhat stationary for the last few months with the occasional drive. I typically just start it every couple of days to keep the fluids moving.

This weekend I popped the hood and noticed a slight build up of dried coolant under the radiator cap and slightly at the seam of where the larger hose attaches to the radiator (the "s" shaped one. I popped off the cap to see that the fluid wasn't filled to the neck, but I could see it at the top of the fins (looking down in). I checked the overfill reservoir and it was about 1/2 inch filled from the bottom. There is no leaks of any sort anywhere else.

Would this be a sign that I replace the cap? if so, can I top off the fluid for now or do I need to top it off and burp the system with a funnel, etc. Or can I clean out my overfill and fill it partially with new fluid? Forgive my lack of experience with this, coolant is the one thing I haven't ventured into yet

 

Yes, a new cap.
Yes, you can top off the radiator with 50/50 mix. The system only needs purged of air when drained.
Yes, clean and fill the reservoir.

 

That's good news! Should I fill the radiator or the reservoir? Or both? Do I fill the res to the minimum mark?

 

Makes no sense.

You mean purged of air when filled.



They purge themselves when the head gasket isn't leaking.

 

Fill the radiator and fill the jug to the full mark.

When drained and refilled a large amount of air is trapped under the thermostat until the thermostat opens. Then the level of coolant in the radiator falls until no coolant is visible in the radiator. I never bothered to measure the volume but a fair amount of coolant can then be added.

Just driving the vehicle in this condition means driving with a partially filled cooling system. Yes, the system will nurse off of the reservoir at the first cool down but will probably empty the jug before the radiator is full. This means refilling the jug again and the radiator is still not full. The maker intended for the radiator to be full so start off that way.

 

Thermostats have a bleed hole in them.

It's to let the air out and the coolant in.


If it doesn't have a bleed hole, it's the wrong one.

 

This information is specific to the 4g63 engine. Yes the thermostat has a bleed hole and yes the thermostat traps a large amount of air regardless. Upon initial coolant fill and as the engine is idled, small bubbles will appear at the filler. These are from the bleed hole. However, the engine still contains a large amount of air. That is why Mitsubishi recommends the use of a funnel that connects to the filler. The air expands as the engine warms forcing coolant out that would otherwise be lost. When the thermostat finally opens the coolant in the funnel disappears and the remainder of the air is expelled. Fill and cap and you are finished.