Thermostat stuck open?

zscott

TJ Enthusiast
Supporting Member
Joined
Dec 22, 2020
Messages
110
Location
Brock, TX
Living in North Texas, we've recently been hit with a decent cold snap. We had our first really significant freeze about two weeks ago and I had noticed some liquid underneath the jeep the following day after that initial freeze. It was most definitely anti-freeze. I popped the hood and couldn't find where it was coming from. There was a little bit inside the fan shroud but other than that, I haven't seen another drop come from the engine compartment. Now, what I've noticed when I take it out is that the temp doesn't climb to its normal 210. It sits a hash mark lower and the heater doesn't get very warm. This morning, while on the road, with the ambient temp sitting around 25 degrees, it just wouldn't warm up. When I'd sit at a light idling, the needle would start to drop. My guess is that the thermostat is stuck open. It doesn't explain finding anti-freeze underneath the jeep a few weeks earlier but would likely explain why my jeep doesn't get to normal operating temp. What do y'all think?

The picture
Dash.jpg
was taken about 30 miles into my trip and 25-degree weather.
 
First I would start with the engine cold... check the coolant level in the radiator and ensure the overflow bottle is 1/2 full.
As for the problem with the thermostat being stuck; I had the same problem about 2 months ago and replaced the existing thermostat with a STANT 195*, drilled a 1/8" hole in the plate section to ensure proper air bleed thru when refilling the system with coolant and used a Felpro gasket with red sealant impregnated into the gasket surface. Clean the thermostat housing and head surfaces thoroughly to ensure a good seal.
 
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Cant speak for the leaks but I will say just about every vehicle Ive had if It got short tripped without a full warmup the thermo would become non functional and run cold. Why, I couldn't say. All the new ford F250 work trucks we bought were notorious for this as they operated in a small city going short distances. I'll add YJ noticeably runs cooler with less heater ops now thats its been in the teens or less in the am and maybe 45 during the day. Used to be just under the 210/1/2 wayish mark and now on a 15 mile run to town its more like 2/3 or the way up at 190 or whatever it is.
 
Then if that doesn't work, its Permatex time!
I'd never use any type of RTV in the cooling system and it doesn't take much to seal it perfectly well anyway. I once had to make a thermostat housing gasket by cutting it out of a piece of cardboard from an MRE box on the trail when my thermostat stuck shut on the trail and it had to be removed. That MRE cardboard gasket was leak-free for several months before I finally got around to installing a new thermostat and store-bought seal.
 
I'd never use any type of RTV in the cooling system and it doesn't take much to seal it perfectly well anyway. I once had to make a thermostat housing gasket by cutting it out of a piece of cardboard from an MRE box on the trail when my thermostat stuck shut on the trail and it had to be removed. That gasket was leak-free for several months before I finally got around to installing a new thermostat and store-bought seal.
Just as a clarification, "real" Permatex isn't RTV. There's 48 million different Permatexes out there now - very confusing. I use the old #2 - non-hardening black glop. As per the "What did you do to your TJ today?" thread, I had a coplanarity issue with a shitty aftermarket T-stat housing - the Felpro gasket alone wouldn't seal it. Permatex (with gasket) did. I've fixed several "impossible" leaks with #2 Permatex over the years, some of them weren't even automotive related. So I swear by it. I'm sure some swear at it! ;)

Oh - your T-stat stuck shut? That's far less likely to happen if you use a "failsafe" style! :D ;) *RUNS AWAY*
 
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I'd never use any type of RTV in the cooling system and it doesn't take much to seal it perfectly well anyway. I once had to make a thermostat housing gasket by cutting it out of a piece of cardboard from an MRE box on the trail when my thermostat stuck shut on the trail and it had to be removed. That MRE cardboard gasket was leak-free for several months before I finally got around to installing a new thermostat and store-bought seal.
Stay away from Nissans then :0 The pathfinder I once had was a rolling coolant leak compliments every joint was gasket free in favor of Nissan "sealant"
 
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I love that old brown #2 Permatex. Before I got into Jeeps my hobby was 50's trucks. I've made multiple t/stat gaskets from Cheerios boxes or similar and sealed them with old #2.
Hell, I even built an old Harley Shovelhead that didn't leak using Permatex.
 
I love that old brown #2 Permatex. Before I got into Jeeps my hobby was 50's trucks. I've made multiple t/stat gaskets from Cheerios boxes or similar and sealed them with old #2.
Hell, I even built an old Harley Shovelhead that didn't leak using Permatex.
Best stuff ever invented. I once had a polyethylene pipe connection that I fixed with it. That black pipe that comes in a roll and is hose clamped onto barb fittings. Had once connection that just would NOT seal up. Tried a second hose clamp, then cut the pipe again for a fresh end, etc, etc. Still leaked. Looked at the barb fitting, couldn't see anything wrong with it. Finally said Fuck it - slathered it with Permatex, put one clamp on it and it was perfect. Still in service the best that I know!

I used it on this T-stat housing as it wasn't co-planar. Yea, the "right way" would be to replace the housing - but the new one might have the same problem for all I know (it had already been replaced once before for unknown reasons), so out came the Permatex. Its not like its gonna wash out into the coolant.