I like to find out if that was actually the cause of the problem. It gives me more confidence when I finally do solve the problem that I actually did solve the problem. And it’s better than assuming I solved the problem and finding out on my next trip that I actually didn’t! 
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If the cooling system is operating too cool the thermostat gets replaced. It is obvious when examining the old thermostat if it is leaking or does not close, no need to cook the old thermostat. If I were spotted boiling thermostats in the break room they would probably send me for a drug test.
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1990 Camry 4-cyl was running cool…removed t-stat…looked fine…into the pot, it opened at 150 deg. Conclusion: the wax pellet can degrade such that the temperature coefficient of expansion increases.
;-]
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I’ve been boiling thermostats ever since I was a kid and my dad got a bad one, maybe 60 years ago. I can’t say that I ever really got a bad one but I always do it on a new one before going to the work of replacing it. I think I did get a weird one from NAPA once and took it back but not sure if it was really OK or not-just didn’t trust it. I use a candy thermometer but check it both ways-heating up and cooling down again. Opening and closing. Sometimes they’ll open ok, but not close down ok. Or so it seems to me.
I’ve had a brand new one that completely failed to open at all, up to the boiling point. There was no obvious visible difference I could see between it and the replacement that opened fully at the correct temperature.
Exiting update. I got the last spark plug replaced and found out that the coil for that cylinder is bad . Luckily my buddy had a spare coil pack for a GM 3800 Series 2 engine and we were able to get it replaced and fix the missing it was having issues with. Now it purrs like a kitten and I can use OverDrive again. And if I floor it it revs up to the proper spot healthily and works much better now.
Congratulations on a job well done.
Happy motoring.
i still have to fix the coolant leak, torque Strut Mount, back brake shoes and drums, and some other stuff.
Great job there OP. Best of luck.