Thermostat sticking closed??

[bob3dsf]

Ok so I seem to have a coolant leak issue. I drained and filled the coolant at 100k (drained the radiator and the water jacket) with SLLC. Filled it back up, filled the overflow, bled the air, etc. all went smooth as silk. I'm currently @ 135k. I can't remember if I've topped off the overflow since the service. I'm sitting at 1/2 between the two fill lines on the overflow (seems steady at that level). I'm getting crusty dry coolant at the overflow cap.

I've replaced the radiator cap, no evidence anywhere else of leaks, no funny behavior with the temp gauge, no overheating ever, no evidence at all of HG issues. No evidence of leaks at the radiator (but I haven't been able to inspect where the fan shroud hides the inner side of the radiator). However it is possible the crustiness at the overflow came from a top off... :/

In a couple other threads other possibilities mentioned were the water pump and gasket, thermostat area. Also, no evidence of air in the system.

With that background:
I noticed that after driving to work (a 45 mile drive, highway) a check of the radiator hoses revealed the upper hose was hot, but the lower hose (after the t-stat) was cold. Wouldn't the drive to work get the motor hot enough to open the t-stat?? If it is sticking closed, would that possibly cause a leak at the overflow cap??

I know the Haynes manual just says the coolant needs to be between the LOW and FULL lines, which mine is.... Am I just getting spooked by the HG failure threads? How can I check to see if the t-stat opens like it should?
Thanks...

 

[having fun]

Do like the old days, pull the thermostat out, (it's built in the housing). Put it in a pot of water and boil it. It should open up at temp rated. If it doesn't, replace it.

 

[JEL]

Head gasket failures usually, at least on the early 4.0 (yours being an 08 I wouldn't worry all that much) present themselves as rough running on first start with various malfunction lights, then smoothing out. Of course there's the classic holding pressure in the cooling system after cool down issue as well, common on a lot of HG failures on many makes and models. If you suspect your t-stat is sticky, just swap it out. It has 135k on it, so it's probably time anyway...assuming you have a V6 and it's never been done.

 

[Sungod]

If your top hose is hot, then your thermostat is open. Hot coolant flows to the radiator once the t-stat opens and cool coolant flow out the bottom hose back into the block. Your thermostat helps the engine warm up quicker. Once it gets hot, it opens and allows your coolant to flow. The t-stat doesn't regulate a maximum temp, instead it regulates a minimum operating temperature.

Your radiator cap maintains pressure in the system. Coolant expands as it warms and will over flow once the pressure exceeds the cap rating. This will allow coolant to flow into the over flow. Meaning you should see more coolant in the over flow when it is hot than cold. Once the system cools, it will draws coolant back into the radiator to replace the contracting coolant. That is why you have less when it is cool. This is how the system keeps air out of the flow.

Unless you are having to add coolant on a regular basis, there is nothing to worry about. Occasional addition of coolant is expected.

 

[bob3dsf]

If your top hose is hot, then your thermostat is open. Hot coolant flows to the radiator once the t-stat opens and cool coolant flow out the bottom hose back into the block. Your thermostat helps the engine warm up quicker. Once it gets hot, it opens and allows your coolant to flow. The t-stat doesn't regulate a maximum temp, instead it regulates a minimum operating temperature.

Your radiator cap maintains pressure in the system. Coolant expands as it warms and will over flow once the pressure exceeds the cap rating. This will allow coolant to flow into the over flow. Meaning you should see more coolant in the over flow when it is hot than cold. Once the system cools, it will draws coolant back into the radiator to replace the contracting coolant. That is why you have less when it is cool. This is how the system keeps air out of the flow.

Unless you are having to add coolant on a regular basis, there is nothing to worry about. Occasional addition of coolant is expected.

Yea, the more I've been watching it, the more it appears that it's working fine. As mentioned I've not been getting any signs of failure. My level in the overflow has been constant. Was near the dealer yesterday and wend ahead and got a t-stat. I guess I'll just hold on to it so I'll have it on hand when mine goes. A the very least, I have a better understanding of my cooling system

 

[Sungod]

T-stats can last for years, and replacing a good one wont necessarily extend the failure point. I had one in my old dodge truck that was over 20 years old and replaced it with one that lasted two. If you are lucky when yours fails, it will fail open and you will just notice that it never warms up. I had my OE t-stat fail on my 4 runner closed. Luckily I noticed the gauge spiking. I turned the heat on high and turned the fan up on high to get home.

Another item to keep an eye on is the clutch fan. It functions similar to your thermostat that it works when it gets hot. If it fails the fan stops spinning and you can quickly over heat.