temp gauge going crazy but not coolent temp

[4BangerV8Killer]

i blew a headgasket and was overheating and stuff. but now that i fixed it my temp gauge still climbes up to the H but on my pocket logger my coolent temp is at a cool 199 at the peak. whats the deal?

 

[project_tsi]

I think your coolant temp sensor for the guage is going out. yes, there are 2 different sensors for coolant temp right on the coolant housing. one is for the mechanical guage in the car, the other is for the cars ECU. why they use 2 temp sensors, i dont know. autozone or another parts store should have the mechanical guage temp sensor in stock, and not too expensive. take 20 minutes to fix it and that should be your problem. hope this helps

 

[4BangerV8Killer]

ok thank you. just to confirm today i drove to my shop and was overheating a little then let the car sit for about 2 hours (it was totally cool) i checked the engine and it was cold. then i turned it on and the gauge peaked higher than when i parked it. and was in the red the whole way home but yet the coolant only hit a max of 204. soooo???? where is the coolant temp sensor? what if the thermostat isnt opening? and the reading is from outside the engine

 

[project_tsi]

yeah i guess it could be the t-stat not opening up, just replace that too while your down there just to be safe, they cost like $7.

In this pic on my 91 Laser, the green is for the cars guage and the red is for the ECU, yes my guage one is unplugged, the connector broke so it doen't read anything on my guage, I just have to get a new connector.