Vehicle only overheats while driving. Cannot duplicate issue while sitting still. Thermostat was replaced prior to my friend bringing it to me to solve. During the many test drives, the gauge will quickly rise to the red mark. I'll pull over put in park and the gauge will then, after a minute or two, rapidly drop to normal range. During the episodes, I noticed that even tho the gauge reads hot, the cooling system did not show any signs of being overheated. According to the OTC scan data stream, coolant, oil, and cylinder head temperatures showed elevated (238F/245F/242F). My infared themometer shows no overheating of the coolant. Shot it several places, Top and bottom of radiator, thermostat neck, upper and lower hoses. The hottest I found was 192F. No codes have been issued. I went to replace the coolant temp. sensor to find that the ECM derives coolant temp. from CHT sensor and Oil temp. sensor. Which I've replaced both. With advise from another tech., I replaced the water pump thinking that maybe it was cavitating due to damage. Putting pressure gauge on coolant systems shows no sign of a blown head gasket. I shot both heads with infared and found an 8-10 degree temp. difference between the two, the passenger side being the hotter at an idle (which is the head the CHT sensor is installed). I thought maybe catalytic converter, but there is no noticable performance drop in the engine. Along with both converter temp. (shot on the ouside) was respectfully 409F @ idle. Any ideas, or am I in over my head?

Thanks

bumpety bump

PCM, cooling fan clutch, circuitry for CHT sensor, or gauge itself (shows hot, but not getting hot). Try using a candy thermometer in the radiator or degas bottle to get the actual coolant temp.

Thanks for the response. It has an electronic controlled clutch. It seems to be working properly. I have noticed that it will engaged seconds prior to the temp gauge rising. Which confused me slightly. I'm a diesel tech. (semi truck) so I'm more accustomed to air controlled clutches engaging "after" the temp is on the rise. But that maybe due to me being unfamillar with the system and how the PCM controls it. I will stick a thermometer in the coolant to get an exact temp. But that will be toward the end of next week, as I am on vacation at the moment. My biggest concern is if the cylinder head is getting hot while the coolant system is still normal, what would be causing it. I know of several things that could, but was wondering if there is any known issues in these types of vehicles. I know there are 2 to 3 TSB's on engine/engine cooling, but don't have access to the information contained in them, wether it be related or not.

Thanks again for the help!