I have a 2010 F-150 4.6L V8 3 valve truck. It has about 130,000 miles on it. Last time I drove it as I pulled into my driveway the temperature gauge on the dash was totally pegged out. It seems this happened between turning into the neighborhood and my driveway. Truck was running fine. I have no indications of coolant consumption or oil mixing into the coolant.
I came inside, grabbed my OBDII -> bluetooth adapter and went back out and hooked it up. The truck read 185 F. The next day I tried driving it while watching the temp. from the OBDII read out. The truck read around 200 F while idling and 235 F while driving.
The cooling system in this truck has always been really nasty, full of rust. My next step was to replace the waterpump and the radiator. During this process I flushed everything I could. The thermostat itself is relatively new. I tested the thermostat in a pot of water with a temperature gauge. It opens and closes as expected. After changing the radiator and waterpump I put in Zerex Gold coolant and distilled water.
After bleeding out the air from the system I am seeing the same behavior approximately. Driving around I can easily see temperatures of 230 F.
The thing is, I cannot measure anything near the actual temperature that the ODBII readout gives no matter what. When I start the truck, it seems to warm up but the indicated temperature goes up way too fast. It will say 190 F when the highest temperature I can measure is 160 F. The hottest spot I can find is the coolant hose coming from the passenger cylinder head. Conveniently, the cylinder head temperature sensor is in the same location although underneath the intake manifold. It leads to the heatercore. Even after driving and the OBDII temperature reading 220+ F that hose is only 195 F. 195 F seems reasonable. The hose from the thermostat to the radiator is usually around 185 F after driving. For measuring the hose I am using a non contact infared type thermometer.
Even measuring the aluminum of the cylinder head with a K-type thermocouple in direct contact I can only measure 195 F.
Is it possible the temperature I am getting from the OBDII system is just extremely far off? Is measuring the coolant hoses a good way to get engine temperature? Should I consider replacing the cylinder head temperature sensor? I am only asking this, because this job is completely non trivial. I have to completely remove the intake manifold it seems which is going to be a huge amount of work.