Working on a 4.2L right now. I'm at my wits end. The truck started showing that it was running hot. It would be fine then all of a sudden it would jump. Just peg out. I've done everything I know to do. Here's the list of what has been done:

New water pump
New thermostat
Radiator boiled out/checked
New head temp sensor
Heads removed/checked, reinstalled with new gaskets (they were fine, no blown gasket)

You may be wondering why I didn't try changing the water temp sensor. Well, that's because this truck does not have a water temp sensor. The port where the books show a water temp sensor to be was plugged from the factory. I installed a regular temp sensor and the truck is definitely not getting hot. The stupid thing still show hot, though and even goes into the limp home mode. I tried unplugging the head temp sensor and then that screws up the timing, etc after a while (I think the truck thinks it's cold all the time). If anyone has any clue what I should do next, please let me know. The only thing I can conclude, is that the portion of the PCM that handles the temp is fried and is misinterpretting the signal from the head temp sensor. I haven't replaced the computer....YET!! I even got the LCD odometer to go into the diagnostic mode and it will show the temp to be fine for a long time, then within about 5-6 seconds it will jump to 120 degree Celsius, which is 248 degrees F. Correspondign with this, the gauge in the dash will peg out. All the while the manual gauge will be stuck at 185.

JJ

 

Pull your thermostat and put it in a pot of water on the stove. Put a thermometor in the water and observe the temperature as it heats up. See if the thermostat opens at its given temperature. If not,new thermostat needed. Also,try replacing the de-gas(radiator) cap.

 

I already replaced the thermostat. I can't believe that a brand new one would be acting in exactly the same sqirrely manner as the old one. I will check/replace the cap though. Not sure how that could cause problems though????