Evo IV temp guage dropping while driving
 

Hey guys I just bought a 96 Evo IV, engine sounds healthy, pulls really clean, steady idle. The only thing is when Im cruising at highway speed or even sometimes doing pulls, the temp guage starts to drop. The lowest its gone is near the C mark, in which case I pulled over and let it warm up by idling. If im doing more stop and go driving in the city its no problem. I am wondering if this is the thermostat and also if it is a major concern as Ive literally only had the car for 3 days and don't want to lose it again for repairs right away. Will I be able to drive it without causing any damage for a bit until I hand it back over to get fixed? also on a side note even when the guage creeps down if I turn on the heat its still blowing out hot air which tells me the engine is in fact still at operating temperature, also it warms up very normally and never gets above where it should be temperature wise. The other thing is, when this happens and I come to a stop, the rpms are sitting where they would be if the car was warming up, so is it the ecu just telling the car that its cold or is the engine actually getting colder while driving? Thanks in advance, new member and stoked to get back into the evo world after a long time since I sold my evo X.

Thermostat is either too low of a temp, stuck open, or it may not even have one. Cold air across the rad is taking any heat out of it, the coolant can't warm up. Put a new thermostat in and report back.

Put new thermostat in and it worked! Now I can actually enjoy my evo for the first time aince I got it :)

Good stuff. Enjoy and stay safe!

I had a DSM with no heat and got a new thermostat. I pulled the old thermostat and boiled it (thermostat soup) with the new one. I was surprised to find how little difference there was. The old thermostat started opening slightly before the new one did. That little difference was enough to make a cold cabin.