For all the talk about how the ones these days are junk, and how hard it is to get a new one, the fact of the matter is that a thermostat is a fairly simple device.
There is just not a lot to go wrong.
You have a copper housing with a substance inside it. I have heard it called a wax pellet, but have never cut one open.
The spring holds the valve closed. When the copper part gets hot, the stuff inside expands, creating pressure that pushes the shaft out against the spring tension and opens the valve.
Obviously there has to be some kind of piston inside the copper part for the stuff to push against, and a seal where the shaft comes out.
Bad machining could cause rough spots inside that cause the piston to cock sideways or bind, a too tight seal could cause the shaft to drag, or rough edges on the valve part could cause it to hang.
Plus, the stuff inside could be the wrong composition, or have not enough of it.
Personally, if I had had 4 or 5 failures, I would first suspect something else wrong. Sure, you could have that many failures, but its not all that likely, and the fact that they all failed exactly the same way is suspicious.
The steps I took to solve the problem would depend if I still had any of the thermostats available. Probably they have all been exchanged. If not, I would take one and examine it carefully. Look for anything that could bind. Bring a pot of water to a boil, turn it off and let the bubbles stop so you can see. Using some kind of tongs, place the stat in the water sideways and watch carefully. It should open fairly rapidly, but smoothly.
Does it hang and jerk? Once opened, pull it out and examine the sealing surfaces of the valve for shiny spots, or anything that could indicate binding.
As it cools, does it close smoothly, or does it jerk?
Don't be concerned with the temp, the fact that it works sometimes says that the temp is pretty close.
If you see anything suspicious externally, can it be addressed? Say by sanding or filing something smooth?
Of course, if the shaft is rough inside the housing, anything other than a mild polishing will probably compromise the seal.
If it doesn't move smoothly, and especially if it sits for a few seconds then jumps open, and you cannot see any external cause, then I would try one more thing.
I would set up two pots of water. One ligltly boiling, and the other one ice cold.
Put it in the hot one until its open all the way, then plunge it into the cold one until its shut. Alternate between the two a few times, then watch it open again. If it opens smoother then, you may be correcting the problem by polishing up the rough spot.
If it keeps hanging, then a different thermostat is in order. Bear in mind, its been decades since I boiled a thermostat, and its entirely possible that even a good one might not move smoothly. But, if it hangs badly enough to cause your symptoms, it should be obvious that its not right.
Either way, once we have thermostat that we think might work, or if the only one is in the truck, then its time to examine the truck. Remember, all the 4 or 5 thermostats that have failed have one thing it common. They were all put in the same truck.
When you pull the housing off, before you take the thermostat out, examine things closely. Make sure the thermostat fits cleanly, and is not in a bind. A high spot under the flange could warp it when the housing is torqued down.
When its in the housing, its pretty much flush when closed. But, when it opens the valve part has to rise up. Look where it goes and make sure it cannot touch anything. Clean and polish all the sealing area, and make sure that the gasket cannot shift or interfere with the stat.
When you put the housing back together, do it slowly watching for any problems as you tighten it down. Anything suspicious, stop and investigate.
Hopefully this will eliminate the thermostat as the problem. But, it might not eliminate the problem. Before you crank it, examine the rest of the cooling system. Squeeze the upper and lower hoses to see if you see any soft spots. Spin the fan clutch if it has one to look for wobble and any oil leakage.
I don't know if I have been blessed or cursed, but I have seen more than my share of water pump problems. I don't mean the usual ones, weird ones seem to hunt me out. I have run into two with loose impellers. One of them, the impeller fell off, and the shaft walked out until the fan hit the radiator.
But it never leaked a drop of water!
The other one, the shaft stayed where it should, but the impeller would slip on the shaft. That one took a while to find.
That sounds suspiciously like your problem. The impeller could slip when cold, then when you shut the truck off and it heat soaks, the shaft could expand enough to bind it and it goes to work.
The third weird one, was the wrong impeller installed on the right pump housing by the rebuilder. The guy who replaced it, did not know much about water pumps, and never noticed the difference. When he called me in, I could tell it was not pumping much at all, but no idea why.
Finally, having exhausted everything else I could think of, I finally just said, well, it worked before the new pump, and not after, so lets swap pumps.
As soon as the parts house guy set the new pump on the counter, it was obvious what the problem was.
That is about all I've got. Hope it gives you some ideas and options.
