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.

 

start by replacing the switch. did you install a new water pump? did you install a new Tstat?

does the clutch fan appear to be working correctly? Perhaps the radiator needs flushed.

 

It ended up being that the radiator was blocked up. I was pulling up to a friend's shop on Sunday and we measured the temp with the IR temp gauge. Top of the radiator was 220* while the bottom was only 150*. And while it was sitting there idling, it blew out the intake manifold gasket. We ended up replacing the radiator, fan clutch since it was wore out, intake manifold gasket, thermostat, and then it's been needing a new serpentine belt and belt tensioner so we tossed those on there too. The old radiator was so filled up with crap, it weighed twice as much as the new radiator.