Thermostat not opening after replacement

rubigold

rubigold

TJ Enthusiast
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Mar 22, 2019
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Connecticut
hey guys, so i just replaced my radiator and thermostat. used a mopar radiator and the stant 195 degree. it seems i'm having trouble getting the thermostat to open. i'm never overheating. i ran it for 1.5 hours the day i changed it, but the lower radiator hose never got warm. I've also driven it 30 mins on the highways several times, and still the lower radiator hose is not warm. There's gotta still be air down there, so i'm wondering what to do. anyone experience the same thing? plenty of heat coming out of vents too.
 
Had it happen to me once before and it turned out the thermostat I received was defective. Sounds to me like that may be your issue as well.

Either that or you could have an air bubble trapped in the system. I know those thermostats can be bad at self-bleeding due to the hole being too small.
 
Are you sure it’s the thermostat and not just an air bubble? Once the engine has cooled down a bit, open the radiator cap. If you see liquid to the brim, it should be close to full. If not, fill it to the top and go for a drive. Rev up the engine to redline a few times once it’s warmed up to burp the system. Let the engine cool and recheck the level. Also be sure the overflow bottle is properly filled.

Otherwise take the car for a spin and redline it a few times, both before the engine is fully warmed up and after. (Don’t rev it stone cold, though.) The water pump spins proportionally to engine RPMs, so higher RPMs increase water flow and pressure, which can help force air out of pockets. Doing it before the engine heats up will clear air out of the heater core. After it heats up it will flush any air out of the radiator and hoses.

If you still have issues, pull the thermostat and drop it in a pot of boiling (not just hot, but boiling) water. It should open. Hot water from the tap (or cooler) should close it. If not, replace it.
 
Thanks guys. I mean wouldn't this thing have overheated by now? Like wtf. Is this engine just so efficient that it doesnt need to circulate the coolant through the radiator?? LOL! I've driven it probably 60 miles since the change, but havent redlined it. Guess I'll be trying that. Might order myself a new thermostat too. WTF. Right now it's been running 2+ hours with the funnel kit on it full of coolant. It's only raised in level maybe 1 inch. Lower radiator hose still cool and temp gauge is sitting slightly below the 210 mark.
 
If you ran it on the highway for 30 minutes and it still runs, then it's cooling. If the thermostat was not open, then you would have blown it up due to zero coolant flow through the radiator.

If the lower hose was as hot as the upper hose, that would mean that the radiator is not drawing any heat out of the coolant. The new radiator seems very effective.
 
PatMc said:
If you ran it on the highway for 30 minutes and it still runs, then it's cooling. If the thermostat was not open, then you would have blown it up due to zero coolant flow through the radiator.

If the lower hose was as hot as the upper hose, that would mean that the radiator is not drawing any heat out of the coolant. The new radiator seems very effective.
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This has to be right. Although everyone I've ever spoken to has told me the hose should be at least warm. I ran it today sitting at work for almost 3 hours. Checked on it every 10 mins or so. Coolant temp never reached the 210 mark. It sat just below it. I would also pump the upper and lower hoses and some air bubbles would come up to the funnel. Just got home from the third 30 minute highway drive since sunday
 
rubigold said:
This has to be right. Although everyone I've ever spoken to has told me the hose should be at least warm. I ran it today sitting at work for almost 3 hours. Checked on it every 10 mins or so. Coolant temp never reached the 210 mark. It sat just below it. I would also pump the upper and lower hoses and some air bubbles would come up to the funnel. Just got home from the third 30 minute highway drive since sunday
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It should be warm...not cold...but not necessarily hot...and not as hot as the top hose. This is why I bought my thermal imaging camera...so I could see WTF was going on in situations like this.

The radiator determines the MAX coolant temperature, the thermostat determines the MIN coolant temperature. If you're not getting up near 210 idling for 3 hours, either your fan clutch is locked up or the thermostat is not closing.


I'd pull it and test it in a pot of water at this point.
 
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Any chance it's in wrong? Every thermostat I buy I boil first. Use a candy thermometer or similar and put it and the thermostat in a pan of water. Watch as the temp comes up to see at what temp the thermostat starts to open. Then you know it will work correctly when installed.
Obviously your system is working or you would be replacing a head gasket by now.
How is the heat in the cabin? Are both heater hoses getting up to temp?
It just may be the radiator is working extremely well.
 
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PatMc said:
It should be warm...not cold...but not necessarily hot...and not as hot as the top hose. This is why I bought my thermal imaging camera...so I could see WTF was going on in situations like this.

The radiator determines the MAX coolant temperature, the thermostat determines the MIN coolant temperature. If you're not getting up near 210 idling for 3 hours, either your fan clutch is locked up or the thermostat is not closing.


I'd pull it and test it in a pot of water at this point.
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my bottom hose if pretty cool. pretty sure any "warmth" to it is radiant heat from the engine itself. my temp gauge sits where it always has when warm, right below 210, and warms up to that temp in 5-10 mins. i ran it idling for 3 hours to see if the lower radiator hose would warm up. my fan is always spinning, so does that give any indication as to whether it's locked?

Goatman said:
Any chance it's in wrong? Every thermostat I buy I boil first. Use a candy thermometer or similar and put it and the thermostat in a pan of water. Watch as the temp comes up to see at what temp the thermostat starts to open. Then you know it will work correctly when installed.
Obviously your system is working or you would be replacing a head gasket by now.
How is the heat in the cabin? Are both heater hoses getting up to temp?
It just may be the radiator is working extremely well.
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Yeah i see now that i should have boiled it to test it. I bought the stant, and am reading that some people don't like it. I think i'm just going to buy the mopar one, and drill the bleeder hole in the flange if it doesn't come with it. The stant does not have the bleeder hole. heat in the cabin HOT, and stays HOT. both inlet and return from the heater core are equally as hot as the upper radiator hose. i couldnt keep my fingers on it very long
 
rubigold said:
my bottom hose if pretty cool. pretty sure any "warmth" to it is radiant heat from the engine itself. my temp gauge sits where it always has when warm, right below 210, and warms up to that temp in 5-10 mins. i ran it idling for 3 hours to see if the lower radiator hose would warm up. my fan is always spinning, so does that give any indication as to whether it's locked?



Yeah i see now that i should have boiled it to test it. I bought the stant, and am reading that some people don't like it. I think i'm just going to buy the mopar one, and drill the bleeder hole in the flange if it doesn't come with it. The stant does not have the bleeder hole. heat in the cabin HOT, and stays HOT. both inlet and return from the heater core are equally as hot as the upper radiator hose. i couldnt keep my fingers on it very long
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I've seen some STANT T-stats without the hole, but they typically have a dent of some sort that is supposed to serve the same purpose. If the radiator is full, and you think you got about the right amount of coolant in it, and it's not overheating, and it's warming up at a relatively OK pace...I'd probably leave it alone.

With the engine off, can you reach in and spin the fan? It should have some resistance...but not be locked.
 
PatMc said:
I've seen some STANT T-stats without the hole, but they typically have a dent of some sort that is supposed to serve the same purpose. If the radiator is full, and you think you got about the right amount of coolant in it, and it's not overheating, and it's warming up at a relatively OK pace...I'd probably leave it alone.

With the engine off, can you reach in and spin the fan? It should have some resistance...but not be locked.
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yeah definitely some resistance. i can spin it though
 
Rule of thumb. Radiator hose going into the radiator should be anywhere from 10 to 30° degrees higher then the radiator hose coming out depending on the health of your cooling system. You also have to factor in the capacity of your cooling system. Heater core should have almost the same concept but only Within a 10 degree difference being much smaller.
A brand new cooling system, water pump, thermostat, new radiator, would equal your cooling system working at its fullest potential. The amount of airflow would have to be factored in as well. The temperature you're describing would be normal. A good working cooling system should stay just below the 210° threshold. Anything higher than 210 degrees would be a little bit of a concern depending on how much higher and outside air temperature.
 
If you purchased a cap that had the safety mechanism it could have opened and locked open. I literally just had this happen and it wouldn’t warm up. Warranties it out and good to go.
 
MX952 said:
If you purchased a cap that had the safety mechanism it could have opened and locked open. I literally just had this happen and it wouldn’t warm up. Warranties it out and good to go.
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Have you actually taken it back out and looked at it?
 
Honestly if the engine isn't showing any symptoms of overheating, just run it. It may not be hot enough where you are to get sufficient flow through the radiator to return still-hot water. If you really want to see if it will heat up, put a piece of cardboard over half the radiator and run it really hard, ideally full throttle up a mountain. The cardboard will simulate higher outdoor temperatures , as it makes the radiator less effective at dissipating heat. (Obviously don't run with the cardboard more than just to test.)

If the flow through the radiator is slow enough, like would be found with cooler outdoor temperatures and high road speeds, it's likely it's just doing a really good job at cooling the fluid. Speed up the fluid by forcing the thermostat to open and the radiator won't cool the fluid as far (although the amount of heat dissipated will increase).
 
With the engine cool, remove the radiator cap. Start the engine and let it warm up. The coolant in the radiator should not be moving and remain cool. When the coolant nears 200, you should see the coolant in the radiator start to move. Quickly dio your finger in there. Hot? If it's hot and moving, the thermostat is opening. If you don't want to stick your finger, you should feel the top hose go from cool to hot when you start to see the coolant move in the radiator.
 
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Thanks for the responses guys. Honestly it's really hard for me to think something is wrong since it's not overheating, but at the same time it just doesnt make sense that the lower radiator hose isnt warm. I ordered an OE mopar replacement thermostat and will make sure it has the bleeder hole drilled in if it doesn't come with it. I've run the jeep both with and without the cap on the radiator. With the cap off, it doesnt seem like the coolant is as hot as the upper radiator hose. While using one of the burping kits with the funnel, the coolant in there definitely gets warm, but not as hot has what the upper radiator hose feels like. Obviously it's out of the radiator, so a temp change would be normal. I'm gonna swap thermostats for peace of mind and let it be. It's just weird. After driving the jeep for 20 mins on the highway, the top of the radiator is hot, and the bottom is cold. Either I have the best radiator ever, or theres something fishy
 
I just ran into a similar situation. I put in a stant 195 back in July when I got my jeep. Replaced the water pump, thermostat, and flushed the heater core. Purged all the air from the system and all seemed fine. Heat was never really amazing but I figured it is 27 years old, just be glad it works. Temp gauge always stayed around the line between 100 and 210.

Fast forward to a couple weeks ago. I put a new hood on the jeep, and then a couple days later, jeep won’t get over 100. Here I am thinking the thermostat is locked up in the open position. I’m headed home for work, thinking I’m gonna pull it out and replace it and now the temp gauge is back between 100 and 210. Now every day it’s a mystery to whether or not it’s gonna heat up or now. Idk what to do.
Starting to wonder how the performance will be this summer before I start replacing everything.
 
TrevorT said:
I just ran into a similar situation. I put in a stant 195 back in July when I got my jeep. Replaced the water pump, thermostat, and flushed the heater core. Purged all the air from the system and all seemed fine. Heat was never really amazing but I figured it is 27 years old, just be glad it works. Temp gauge always stayed around the line between 100 and 210.

Fast forward to a couple weeks ago. I put a new hood on the jeep, and then a couple days later, jeep won’t get over 100. Here I am thinking the thermostat is locked up in the open position. I’m headed home for work, thinking I’m gonna pull it out and replace it and now the temp gauge is back between 100 and 210. Now every day it’s a mystery to whether or not it’s gonna heat up or now. Idk what to do.
Starting to wonder how the performance will be this summer before I start replacing everything.
Click to expand...

Temp sensor?
 
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