Alright guys hopefully an easy question. Having some slight overheating issues. Came on suddenly and hasn’t gone away. The factory gauge is obviously a suspect, but I wanted to think simple first.

What ive noticed:
temp starts to climb in town, while driving slow, with heater fan turned off. Doesn’t peg out the gauge or anything, but climbs well past where it was stable a few weeks ago.

If if I turn on the heater fan and let that run it brings the temp down to mid-range for a while, then temp creeps back up.

While im on the freeway cruising it comes back towards the middle and hangs out until I slow down.

all the classic signs of a radiator issue, thermostat issue, water pump.could be lots of things.

Things i I know or believe to be true.

after it gets warm and gauge is near 3/4, with a laser thermometer I can read 220 on most of the block and the inside of the thermostat. On the rad side of thermostat it reads 195, the upper hose reads 195, lower hose is about 170. There are still some hot spots around the back cylinders and of course exhaust manifolds are well over 200.

I can tell that the thermostat opens because the level in the rad drops and I can tell that coolant is flowing.

I know my gauge works through the range by grounding the wire.

What i I believe is happening is that the thermostat is either not opening fully, or it is clogged due to some crud getting loose after putting in new coolant.

Im not a physicist, but it makes sense that if coolant were flowing through the thermostat normally the temp would be the same, or at least close, on either side of the housing. Please correct me if I’m wrong and let me know if there are some more tests I can run before replacing all the things.

im waiting for the truck to cool off before I pull the thermostat and check it visually.

thanks for all the help guys.

 

I suggest installing a real gauge with numbers! then replacing the thermostat and at the same time flushing the radiator and the block. On an FE, pull the pipe plugs to get the crud settled at the bottom of the block's coolant passages which cannot be flushed out during typical flushing procedures. The pipe plugs are either recessed Allens or have a square head and located about two inches above the pan rail below cylinders 3 and 6.

If nothing flows out, then probe the opening with a stiff wire and stand clear because a torrent of muck will likely flow out like with my 70's FE. That is the proper way to flush an FE.

 

And if you cannot remove those plugs then remove the starter and knock out the 2 rear core plugs on either side of the engine. Do not do this on a clean driveway or in your garage.
The brown casting mud may be really packed in the rear of your block. Your radiator also may have a lot of the same in it.

 

Also, drill a 1/8-inch diameter hole on the theremostat flange and install the hole at the 12 o'clock position (cuz it mounts vertically on the intake manifold) to minimize trapped air.

 

Those all sound like great ideas and I will for sure get around to an actual flush as you described. But I figured it had to be something simple and I think I was right.

Im still going to keep an eye on it, but I think I figured it out. Of course, I didn’t take pictures of anything but I’m pretty sure I’m just an idiot and when I changed the thermostat months ago I think I put it in backwards. I was trying to pay attention just now when I pulled out the housing but it all came out at once. half the gasket was stuck to the housing and the other to the block so it was tearing and I was trying to deal with that. When it all came loose the thermostat dropped out on its own so I’m not sure what it’s orientation was. But I’m fairly certain it was not the right direction.

Since the weather was a bit cooler when I was driving it more regularly and my first ride in some time was after it was heating up I think it finally just showed me what was wrong.

With the thermostat oriented correctly and new coolant to top everything off I ran it until thermostat opened, made sure the coolant was moving, capped the rad and let it run. Just inside the thermostat housing stabilized around 198, and the rad side of the thermostat was sitting about 185. I still think it’s odd that there is a difference in temp right there, but, like I said, I’m no physicist.

thanks for everyone’s help. Next side project is cheap exhaust modifications.