Well here's the basics on the car:

Fully built 2.0L motor
GT37R turbo
slim line radiator fan
factory radiator
factory thermostat
104K miles on the chassis

Last week car was running normal as I sat at work waiting for the Mrs. and no A/C running.. after about 10 minutes decided to kick the A/C on... after a minute or 2 notices AC not as cold as normal.. looked at temp sensor.. 3/4 way up.. FAWK!!
Coolant was boiling in the overflow bottle and out the tube... let it cool down looked over everything(carefully) let the rad fan run with key in on position but not running obviously. Drove home.. checked.. had a friend come over with a younger thermostat 40K miles Swapped out.. checked it no issue(coolant was added) ran car idle for awhile then A/C on... no issue.. got on it 20psi(low setting) no issue no white smoke. Figured just sticky thermostat since had alittle corrosion on it.
Fast forward to yesterday. Sitting waiting to pull into traffic car started climbing with AC off... ****! turned car off for a second..turned on.. and everything fine fan running no issue. Got home let it cool checked nothing in the overflow other than alittle coolant.
My theory is this:
Radiator fan relay possibly going bad intermittently? Was thinking about replacing it and the coolant temp sensor since both are 104K miles on them. Any other ideas? I thought headgasket since its got 60K miles on the built motor, but no white smoke no bubbles in coolant...

 

anyone?

 

Sounds like maybe the temp sensor for the fan control might be going bad. Or, like you said, the relay is going away.

 

My Lancer does the exact same thing , only does it going slower speeds at random after i had just been in boost with the ac on (100 degree weather) (16g turbo charger set up) i am going to try and replace my water pump to see if that fixes it since i still have my oem pump .

 

Well looks like I'm going to just keep an eye on it and see if it does it again... nothing as of yet and if it happens again I'm going to pull over fast and see if the fan is running or not.. if it does it and fans not running I'll replace the relay. As for now everything is ok and has been. Go figure.. damn gremlins.

 

If it helps mine are also running when it happens

 

Most of the time its the coolant temp sensor that signals the ecu.Make sure ther is no air in system because it wont be able to read the water temp and turn the fan on.

 

If all that fails, it's time for a leakdown test.

 

Are all of your grounds hooked up? Also log ur coolant temps, don't trust the dash gauge. We have seen the gauge on the dash go almost the entire way up while the ecu logged temps are 190 degrees because of a bad ground issue.

Cb

 

Figured it out. For WHATEVER reason the drain plug for the radiator (factory rubber one) is leaking.. completely screwed in but still leaks.. guessing the threads are crap. Do you think Teflon tape will work to better seal or any ideas? Not a nasty leak but a small slow one that after awhile enough air is in the system..

 

Time for a CBRD slim rad!

 

Sales Pitch!!!

 

Maybe if Chad hooks me up... he still owes me a shirt from back in the day before he opened the shop!

 

Lol PM me lol

 

I dont even work for CBRD! I just have one of their slim rad's and it is the ****!