Okay Ive been having this issue with my Evo for a while now, and there is alot of info so just bear with me. It all started one day when the car started sputtering, and losing power.I noticed my temp gauge rising and falling from the middle, up to the top and back to the middle. So I let it cool down, and it was fine for the rest of that day. The problem seems to come and go as it pleases, sometimes running flawlessly for several days, and sometimes not working properly at all. So I changed my coolant, replaced the radiator, and replaced the thermostat, and that made no difference. We hooked it up to evo-scan, and it showed that even when the temp gauge rises to the H, the actual temperature is staying at 175-180, so the gauge is just giving a false reading. So I took it out one day and the car just shutoff, and would not restart. I checked for spark and was getting none. So my local shop checked it out, and had to replace the fuse box, engine wiring harness (melted in a couple places), and the ECU was fried. After replacing everything the car ran, but the temp gauge issue, and sputtering were still there. It will run absolutely flawless, and then out of no where, the gauge will jump, the car will start cutting out, and sometimes cut off all together and not restart for several hours. But then it will be absolutely fine again, which is the part that is confusing me. Then one day I was out trying to troubleshoot and I noticed that the more electrical stuff that I had on, the more likely it is to act up. (Headlights on, climate control, radio playing, phone charging, rolling windows up/down) all tend to make the temp gauge spike, and the car start cutting out. It will sometimes be running fine, but then if I turn the headlights on, or plug my phone in, it will start acting up all over again. I am worried that the car will fry another ECU and wiring harness. This is extremely frustrating, has anyone had experience with anything like this?

 

alternator

check the whole wiring harness. easily could be a short somewhere

 

Could be a bad engine ground. If I were you, I'd check the ground strap from the block to the chassis.

 

Check EGR valve and solenoid. My car had loss of power and some sputtering. Most likely not since your issues with the harness/ecu but worth looking over.

 

I have the exact same issue did you ever resolve this ? Literally exact same problem temp gauge and all.