Question about Thermostats

Hey Gang, quick question. It is my understanding that a thermostat is rated at the temp that it starts to open, IE a 195 degree stat starts to open at 195 degrees. This being the case it was also my understanding that it was normal for a car with a 195 stat to run 200-205… Is this info correct??

The reason I ask is that when I replaced the intake on my 98 Buick the other day I also replaced the thermostat as well. I did this as preventive maintenance, but also because I felt the car was taking a long time to heat up and thought the old one may have not been closing all of the way. With the old stat, the car would run at just under 200 on the dash gauge. With this new stat I am running about two notches under 200. So I am thinking I got a bad stat, HOWEVER I hooked up a OBD2 scanner with real time read out and took the car for a quick 7 mile test drive. The temp never got over 195, and you would see it fluctuate between 195 and 188 (IE the stat was opening at 195 and closing at 188). So this stat is working perfectly, however looking at my dash gauge It looks to running much cooler than before. So I guess my question is, is my 195 stat really a 185 stat OR was my old stat actually not working correctly and letting the car run hotter??

My guess is that your old stat was not operating properly. If I undersnat correctly, you’ve tested the new one and it’s working perfectly, so assuming you’ve purged the air from the system if this one is keeping the temp a bit lower, the old one must have been opening at a higher temp.

The rating on a T-stat is not exact. And I doubt if it gets more accurate with age. In truth, it sounds like your old one was working fine and the new one is just reacts to a bit cooler.

I should add that the T-stat and the temp sensor are responding to entirely different places. If you were to measure th inside of the combustion chamber you’d see perhaps 2000 degrees. Your exhaust manifold would probably be 800 degrees. Realizing that, the difference between the old temp and the new temp is, IMHO, really nothing whatsoever to worry about. They’re just normal variations between T-stats.

By the way, it might even help to realize that the whole function of a T-stat is to keep the engine hot, not to keep the engine cool. An engine without a T-stat has no problem staying cool, but would be unable to stay hot.

Your new stat is working fine. As thermostats age, the valve begins to stick. It usually sticks in the open position, but I’m guessing that your old one stuck in the not quite all the way open position. It wasn’t stuck closed, but it wasn’t fully open either. It would take longer to heat up and the engine would run a little hotter when it finally did heat up.

Maybe the old,sticky thermostat was not closing completely when it needed to be closed AND not opening wide when it needed to.
I’ve seen old thermostats fail both ways (open/closed).
I believe in changing the thermostat every 4-5 years (with coolant change) on a daily driver.

edit: wrote this before I saw Keith’s response. This happens when I open a zillion tabs.

That’s correct. The temperature rating of the thermostat indicates the temperature at which the coolant must reach before the thermostat opens. Once the thermostat opens it’s the duty of the cooling system to keep the coolant temperature under control which can get as high as 230 degrees depending on the ambient conditions.

If the hot operating coolant temperature is at or just above the temperature rating of the thermostat, it indicates that the new thermostat is defective and is probably stuck partially or fully open. You should be seeing a coolant temperature of at least 210 degrees with the engine fully warmed up.


Did you find any problem with your upper intake?

Sounds like everything is operating properly. I would believe the scanner long before the dash gauge.


ASE , yes I did… I did not even need the UV light, after I took of the TB it was just floresent green all over the inside of the manifold. Apparently the EGR burned a hold in the manifold right behind a gasket that was pressed on by the TB, so it was not gushing out but the Vac from the motor running was sucking it out. Thats my guess at least.

On a side note maybe you know this one as well, this car has the automatic HVAC, which has not been working properly since I did this repair. Apparently I was supposed to turn it to the off position before I disconected the battery, and it needs to re-learn it self which I have been told can take 50 on off cycles. Do you know anything about this??

Yup, typical intake failure.

I don’t recall off the top of my head about a relearn for the HVAC, but I’ll look in my service info at work tomorrow if I have time. LeSabre? Park Ave? Regal?

I never lose battery power unless I absolutely have to, even when replacing a battery.

its a 1998 Lesabre Limited with the automatic climate control.

Tester, you made an excellent point about the difference between the T-stat rating and the operating temp. I’m embarased that I didn’t think of it.