I'm in a pre-work rush, so I'll give as much detail as quick as possible:

Got the car running lastnight after finishing the work. Flushed the radiator to get the contanamites out of it. After that, refilled with 60water / 40fluid radiator fluid. Let car idle to running temps (~40-60% of thermo gauge on cluster). Shut car down, ate dinner.

Came back, cleaned up tools. Started car. Got to running temps. Drove car to car wash, (to clean dust off and whatnot real quick). Started to drive home (~5 minutes away), and the car's thermo gauge went all the way down. Got no heat coming through the heater either.

Checked plugs, wires, hoses. Nothing is loose that I can see/feel. No fluid spillage. There's fluid in the radiator (so much so you can see it when you pop the cap). I did burp the system before I drove it. Even still, it would read a temperature of some sort. (re: drove it once without fluid, which caused the issues, because rad hose blew without me noticing)

My only conclusion (actually, Smike's) is that the Thermo is stuck wide open. What else could cause this?

 

well, swap out the thermo, its a cheap solution if thats the case, but first things first.

 

Yeah, anytime the temperature gauge reads anything but normal, swap the thermostat right away. Most times that will be the fix.

I think you will need to visit the dealer for the stat. I've never seen an after market thermostat for the Evo. The parts houses have a listing but it turns out to be the thermostat for the Lancer which is smaller in diameter.

 

what about a motorad unit, it fails in the open position

 

I'm replacing the thermo tonight (hopefully). Gonna clean the sensor while I'm in there.

Gonna burp the system (of course) and run it for awhile as well. We'll see what happens.

 

I tried one of those thermostats that lock in the open position on my DSM. The thing worked for about 2 weeks before it locked open. I think the 4G63 engine dumps too much heat into the coolant for one of these to work on our cars. I wasn't otherwise having any heating issues.

 

I tried to get an aftermarket one around here to hold me over for the OE one.

Turns out nobody around here carries anything but regular lancer T-Stats

Local Mistu dealer is overnighting one to me.

And I found out - the t-stat was stuck open. It had sucked the washer into the opening and tried to close on it. heh.

 

There is the Ralliart 170* thermostat!

 

Do you mean the needle on the gauge just falls to its lowest reading or that it goes to warm to very cold and fluctuates? Sounds like a sensor is unplugged. That doesn't just happen. Did you just do a coolant flush or other work? How long was the car sitting?

 

Car sat for ~1 month. Did a head gasket, so I had the head opened up, in/ex mani's off, upper wire harness pulled, and fuel rail.

I double checked all of the wires the other night. They're all snug/clipped.

As for exactly what it was doing. It'd get warm, to running temp (which is ~40-60% of the gauge, or right near the water mark), and then as I got going down the road (40mph+), it would cool right down to 0-10% of gauge.

 

If it is going back to normal everytime you drive it then it is not a sensor. So yea I guess do what everyone else said and swap the thermostat. They are pretty cheap.

 

Yeah, picked one up from the local dealership (~45 minutes away) today. Did the swap, temp is fine.

Now I just need to fix my HVAC controls so I can get more heat. (It's binding, and is only going halfway into the heat zone.

 

Well, while at the dealer you should have picked up replacement (new design) levers for the heater box - about $15 tops. Look up and right of the accelerator pedal to see where they go.