Was driving my '86 Ranger across town for a Highway Patrol Inspection, first long drive since my cab reconstruction project. It was cold and windy and as I got 15-20 minutes into the trip( surface streets, not highway) the heat gauge rose ominously to the top and stuck there. The Inspection had to be rescheduled and I drove home still at the top of the range, but with no malfunction or steam. At home I raised the hood, radiator cap was cool, as was lower radiator hose. Upper radiator hose and both sides of the radiator were warm, but not baking hot. Plenty of heat in the cab. Any suggestions on how to trouble shoot? is it possible air is trapped in the engine block? Could the sending unit be bad or the gauge? I replaced the thermostat but not the water pump or radiator during my project. I'd be most grateful for any observations or suggestions. Merry Christmas and Happy New Years to all.

'86 2.3L Auto 2WD, no AC

 

While air may be trapped in the system, I would suspect that the thermostat is not opening. First, check to see that the thermostat is installed correctly, namely that the copper expansion unit is pointed toward the engine. If that doesn't work, I'd replace the thermostat and in the process of refilling the system, work on seeing that the cooling system is properly bled.