Ok, so I have a 1997 LT Bazer 4X4 4.3W with almost 200,000 miles on it. A few weeks ago the temp gauge went a couple notches above 210, then dropped to a notch above 100 and stayed there when it was warm. I switched out the thermostat, temp sensor, temp switch, water pump, radiator cap,and had the cooling system back flushed and filled. Now I have a whole new demon in the system. The temp gauge now warms up to 3/4 mark or 235 however you want to look at it. Then it drops down to 210 and stays there until it warms up to a notch below 235, then drops back down to 210. I checked the oil for coolant, no luck. Not leaking any coolant, and no white smoke. Is my gauge messed up, or did I miss something?
Any help will be greatly appreciated.

 

It could be your factory fan clutch if you're still running it. When your engine is cold and not running, try moving the fan by hand. It should have a fair amount of resistance to you trying to turn it. If it turns freely without resistance it probably is bad.

 

I would suspect air still trapped in the coolant system. Is the coolant level in the radiator all the way to the bottom of the neck and is the level in the recovery tank correct? To purge the air, raise the front end with the right side slightly higher and bring the engine to operating temp with the radiator cap off. Let it cool down and repeat the process, adding coolant as needed.

 

Checked the fan clutch, seems to be ok, has quite a bit of resistance while trying to manually turn it.

It could possibly be that there is still air in the system, since it was done at a mechanics shop I figured they would have it done properly, but I know thats not always the case. So the next step is going to be trying to release any air in the cooling system.

I noticed today that it stays at 210 or a little below if it is just sitting at idle. Once it starts to be driven then it goes up to one tick below 235, and then drops back to 210, not sure if this helps with diagnosis but just an observation.

 

Your new thermostat probably has no bleeder hole; a small (<1/16") hole that permits a small flow even when the thermostat is fully closed. This small flow allows some heat to come to the thermostat as the engine warms up thereby preventing 'overshoot' of the temperature. You could drill such a hole in the flange of the thermostat - inside the gasket diameter - and greatly reduce the overshoot. It also sounds like the thermostat control temp may be a little high or (more likely) your analog gauge reads high. There are non-contact temperature gauges that are accurate and becoming less expensive ($25 or so) that could measure the temperature of the thermostat housing and reveal the actual temperature.
Hope this helps, please post your eventual solution.

 

Went out and bought the non-contact temp reader and when my gauge reads 235 the temp reader says 202, when it is reading at 210 on the gauge the temp reader reads 182, that is a difference of 33 degrees and 28 degreesso I think it is time for an aftermarket gauge.

 

What are you pointing at with your non-contact gauge?

You do realize that outside temperatures of different components can be higher/lower temperature than the coolant inside. It all depends on the conditions. If the vehicle was just recently warmed up to the point where all of the outside metal was not up to its steady state operating temperature at idle, then you could get a cold reading. If you were to go beat the snot out of the truck for 30 minutes and then read the temperature, that same component could be hotter than it was before. All while the coolant temperature remained the same.

 

Pointing the gauge at the thermostat housing is where I got the readings from. I do know that I will not get an accurate reading if I just go shoot the thermostat housing before it is warmed up, therefore I waited until the thermostat had a chance to open up a couple times, so I let it idle for 30 minutes then took the readings. If you have a better way to get a more accurate reading, feel free to let me know, I want this solved, it's beginning to wear me down.

 

I finally got to work on the POS today, still not fixed. I found some coolant coming out of one of the heater core hoses. Where the hose connects to one of the heater core spouts wiggles alot and feels very loose. I am not sure if these are supposed to move that much or not. It is the heater core spout on the right side if you are facing the engine.

 

Drilled a hole in the thermostat and installed it. This seems to have taken care of the high heat reading so far, I will know for sure tomorrow after driving it. The old thermostat was a pain to get out, I actually had to pry it out, it seems like it got suctioned down. The heater hose was leaking because the clamp was not installed far enough on, so that is fixed, but I am still curious about what seems like excessive wiggle in the core tubes.