I just replaced my heater core in my '88 f150 4x4 5.0 and now my temp gauge doesn't work. Could there be an air pocket. If so, how do I fix that? Also, I know there is a way test the temp gauge to see if it works, but how do I do that? Any help would be apppreciated. Also, I replaced the thermostat 6 mos ago, so I know that's not the problem.

 

If there was an air pocket, the temp gauge would peg. Double check your wiring.

 

That's what I thought but I wasn't 100% sure. I grounded my temp gauge and the needle pegged. Would that mean I need a temp switch or sensor?

 

That would mean the temp gauge works. If you've confirmed that the wiring from the sender to the gauge is intact, then it would appear that the sender is at fault. My only concern is that simply changing the heater core shouldn't have had any effect on the sending unit. Did the gauge die when you did the core, or had it been having problems?

 

The temp gauge wasn't working properly before the heater core, but it was reading above cold. It hasn't worked at all since the heater core replacement. I guess it was just coincidence. I'll have to get a new sender tomorrow. Thanks!!