Ok so I have a PI swapped 97 mustang gt that I’ve had for a few years now and it has never given me this issue until now. What’s happening is that when the heat is on in the car, the PCM thinks the car’s coolant temperature is colder than the actual temperature, not allowing the fans to turn on until the car overheats. I have a glowshift water temp gauge, and the factory dash gauge. I am monitoring the pcm’s coolant temp through my bama tuner. For instance if I am idling with the heater on and the glowshift gauge is reading 215 degrees, the bama tuner is telling me the computer thinks the car is at 176 degrees the glowshift gauge will continue to climb to 220 and higher (I never let my car get hotter than 215 usually) and the bama tuner will just gradually go up slowly but the car is actually much hotter. The factory dash gauge will go past the halfway point aswell when this happens. As soon as I turn the heater off the temp on my tuner will climb fast and match the temp that my glowshift gauge is showing me, turning the fans on and cooling the car down. If I never turn the heater on, the temp on the tuner matches my glowshift gauge perfectly and there isn’t a problem, the fans work normally. I have replaced both the water temp sensors on the crossover, and the thermostat with motorcraft parts. My fans are set to come on at 198 for the low speed and 210 for the high speed. I didn’t have this issue last winter and the car stays right at 195-200 degrees when driving no mater if the heat is on or not. (I assume the motorcraft thermostat is a 195). I am very confused because even when both water temp sensors on the crossover are unplugged, the tuner still gets a temp reading, how is that possible? Is there another sensor I missed? Any advice would be greatly appreciated, thank you.

FYI. 220 is not technically overheating. Obviously this assumes that the fans are working on high and the temperature is not continuing to rise. This also assumes that the cooling system is actually holding PRESSURE with the correct mixture of anti-freeze and water.

You have verified that the cooling fan actually works on BOTH speeds right?

In my experience when a motor slowly overheats but otherwise runs well the problem has been traced to a clogged radiator.

Tell us HOW you connected both temperature sensors to the motor? Did you use a Tee or some other sort of fitting to double up? Note, if a Tee has been used this creates the possibility of air trapped in the top of the Tee. The air will make the sensor read LOW.

Has one of the temperature sensors been relocated to the side of the block?

Finally I have helped more than one person who thought they had an overheating problem that actually turned out to be a VOLTAGE problem causing the ECT to read high. Some of the people I have help changed virtually every part in the cooling system before reaching out for help. Only to find out there was nothing wrong with the cooling system at all that it was electrical. Usually a bad motor ground.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-for...-drop-test.56/