I have two supposed 180* tstats, one the JEGS brand #555-53211 and one local parts house brand. Neither one even start to open until about 190 (they're in a pot on my stove). At 190 they open a hair, 200 about halfway, all the way about 208. As the water cools, they both start to close about 190; the parts house tstat is closed about 185, and the JEGS is still open a crack at 180. I've done this three times, making sure to raise the water temp very slowly, in case the springs have some lag as they react to the temp (wife is looking at me like i belong in the nut house). The thermometer is spot on, Iv'e actually used two and cheked them both first.

Is the way these tstats react normal, or am I just unlucky?

180* stat won't open until it hits 180*.

The normal Start to Open range for a 180° thermostat is 177° to 184°. The tricky part is that STO is defined as an opening of .003", which you can't measure. It will be open all the way by about 204°. You can't measure it that accurately on the stove, because it has to sit at that temperature for a few minutes. There is a lag as the water temperature changes.

The spring has absolutely nothing to do with the operating temperature, which is controlled by expansive wax inside the bronze cup.

thanks for the "wife is looking at me like i belong in the nut house" line, i got a nice chuckle out of that .

If the initial opening is so small,
and since most vehicles coolant temperature
does stay pretty close to the 180 Tstat rating,
not near the 204 degree full opening
i guess that means that much of the time during lazy part throttle driving
the annular opening is only ten to thirty thousandths "high" ?

Thanks for the info. Sounds like what I have is working the way it is supposed to. Didn't know it was wax that operated the tstat, thought the spring was one of those bimetallic jobs.

What many people do not understand is that the thermostadt is not just simply an open/close water flow switch. The marked temperature is when it starts to open, and typically it takes another 5-7% of increased water temperature to open fully...

A 195 F degree thermostadt will start to open around 195 F but will not be fully open until about 205-208 F degrees. The real question is How much time? is required for the thermostadt to be fully open.. This is directly dependent upon several factors including:
Outside air temperature
Engine loads
Engine compression
Fuel octane rating
Amount of timing
Radiator capacity
Airflow through the radiator
Vehicle speed

Just my $0.02..

Probably 99% of the "defective" thermostat returns work perfectly, regardless of whether they come from consumers and jobbers (NAPA, Carquest, etc.) or from new car dealers!

In other words, the problem is usually with another cooling system component, or the installer.

Probably the installer.........guilty!

I installed a brand new thermostat in a DT 466 diesel engine last week that was new from the dealer and it did not work properly. I had to change it out Friday and now all is well.

Sucks for me as I had to do the job twice and not get paid for it.

Mine usually shows opening about 5 degress over the rating

In Chryslers original press release about the then-new iron V10 Ram pickup engine, they made these claims about the $35 thermostat used:

quote

A heavy-duty truck engine block cooling system and thermostat minimized low-temperature piston wear and oil consumption by allowing the cylinder walls to warm up gradually and expand uniformly. The truck-type thermostat opened in a smooth continuous manner because it had four times the working area of a passenger car thermostat. A conventional thermostat released bursts of chilled water during warm-up that distorted the cylinder walls and causeed wear and high oil consumption. The thermostat had a cylindrical valve element with an O-ring seal to assure smooth consistent operation. It was mounted in a molded plastic housing at the front of the engine.
unquote

Is the V10 thermostat a better design?

original at
http://www.allpar.com/mopar/V10.html

I guess Ma was saying that all of their other cooling systems sucked?

That top bypass design has been used by people like Mack and Detroit Diesel for years. The coolant flow is constant, and the thermostat directs the flow back into the engine, and into the radiator, and the percentage varies with the temperature. Pretty much just like any other thermostat. They might mix better, but I think they could have done the same thing with a more conventional bypass blocking thermostat.

Put the JEGS tstat in it and fired it up. Sure enough, gauge never went over about 183! Guess it works as advertised even though it didn't look like it on the stove. Who knew? Oh yeah, you guys did. Thanks to everyone who chimed in.