new thermostat stuck closed?

[GhettoPop]

my 78 blazer 350's stock temp gauge runs at about 3/4 towards hot. this is the way it was when i got it a few weeks ago so assumed the temp is ok just an inaccurate gauge and/or sending unit. took it on 2 long drives, each for 2 hours, no overheating. gauge gets up to 3/4 and does not go up or down.

i decided to freshen up the cooling system since i was not sure how old everything was, new hoses, coolant and since i was in there i change the thermostat and coolent temp sensor. afterwards, the gauge now climbs all the way to max. since i changed 2 things i'm trying to pinpoint the problem, which i'm guessing is either a faulty temp sensor but more likely the new t-stat is stuck closed. i made sure to install it in the correct position and not upside down.

i let it idle it with the rad cap off and i believe when the stat opens up the coolant level should drop and the upper rad hose should become pressurized. i did not see that happen.

whats the best way to tell if the t-stat remains closed?

FYI- the parts i installed were a Mr. Gasket 195 degree (#4365)
http://www.summitracing.com/parts/MRG-4365/

Standard Temp Sender (#SMP-TS6) http://www.summitracing.com/parts/SMP-TS6/

 

[Hogback Fabrication]

Take the t-stat out and put it in a pot of water and boil it, see if it opens

 

[K5dreamer]

Take the t-stat out and put it in a pot of water and boil it, see if it opens

.x2

 

[GhettoPop]

good idea, thats what i was thinking. i pulled out the new t-stat and put it in a pot of boiler water, along with the old t-stat. the old one started opening at around 200 (stant 195), the new one didn't open until about 210+ (mr gasket 195).

i put the old t-stat back in the truck and let it get hot. the t-stat opened properly and the water level in the radiator dropped big time, didn't really see that happen w/ the new t-stat. the gauge now stays between 3/4 and HOT.

so i had the double wammy. the new t-stat was opening about 10-15 degrees hotter than the old one and the new temp sensor outputs a higher reading on the gauge. at least now i know i'm not overheating.

any ideas how i can get the stock gauge working more accurately? i figure they are junk but any ideas?

 

[Hogback Fabrication]

Make sure you are getting all of the air out of the system, refilling it after each time you let it warm up all the way.

Aftermarket gauges are pretty cheap.

 

[SnackPack]

Also make sure it seats properly. I put in a new thermostat last year and it moved on me when I bolted down the outlet, keeping the thermostat closed at all temps.

 

[GhettoPop]

I ended up ordering a new AC Delco 195 t-stat. I even boiled it on the stove prior to install to see if it opened up properly and at the right temp, which it did. I also pulled out the new temp gauge sending unit and put the old one back in. The t-stat opens up great and I bled out the air and added coolant after a couple of trips to the rad and overflow tank. Stock temp gauge reads at 3/4 then stays there, which was how it was when i got it but at least now everything is fresh. i put in one of those mr. gasket radiator caps w/ the thermometer on it for ___ & giggles and it reads 200 degrees on 90 degree days in traffic and stays right there. looks good to me. thanks for your help.