Well I just replaced the engine in my bronco in August. Today while driving to work, which is just 3 miles from home, I was halfway there and my temperature gauge started climbing. It didn't get to the red and I wasn't sure if the truck was actually getting hot or if it was just the gauge screwing up. It's probably only 65 degrees here, so it's not hot here. Any ideas as to if it's the truck actually getting hot or if I need to replace the thermostat? it normally runs at about the R or M in normal. All of a sudden it started climbing and hit the last white mark before it goes red. I shut my truck off to see if it would cool down or anything but it wasn't acting like it was running hot. After about a minute, I started it back up and it was just above the L and it stayed there till I got to work. It's gotten a little high before when I've been sitting in stop and go traffic on a very hot day. The coolant was flushed when I put the brand new engine in in the middle of last month. And I occasionally have to add a little coolant but as far as I can tell, I never find a puddle underneath there's never cloudy exhaust either. It wasn't smoking or steaming either. My neighbor probably has a IR temp gun. Where exactly should I measure the temp and what should it be? I couldn't put coolant in tonight so I babied it home. Gauge went nuts again. Started low and after a minute or two, jumped a bit but was still below normal. Then as I accelerated from a stop light, the needle went up with the RPMs and then dropped some when my speed leveled off. But when I started to go up the long tall bridge, the needle started climbing and didn't stop until it hit the white mark, and at the exact same time, I started coasting down the other side of the bridge. It stayed high for the other mile and half home. When I got home, I popped the hood and it was warm from the engine but didn't seem any warmer than usual, but the top radiator hose was definitely hot to the touch.

 

I have a 1993 E150 5.8L that did the exact same thing and it had had the thermostat changed about a month previous. Took a new radiator (bottom of the radiator fell out when it was taken out of the van) Does fine now and gauge only goes up about a quarter of the way on the white line. Hope this helps.

 

Yeah I was on my way to the shop this morning to have it looked at, and it blew the intake gasket. Turns out the top of the radiator is clogged. So new gasket, radiator and thermostat is plan.