heating (but not over-heating) on hills in 6th gear all of a sudden

Hello, thanks in advance to anyone for help.

I have a 2012 4x4 crewmax with 265k (bought it at 247k 3 years ago).
Truck runs incredible, I had no records when I bought it, had a "pre-buy-inspection" which passed with flying colors.

HISTORY:
Truck has never had the engine temp move past "half way" (I will call it) on the engine temp gauge...this includes up mountains, around town, etc. Yesterday when going up hills the needle on the gauge went up to about 3/4 temp within 15 seconds of hill, and comes back down when going downhill within 15-20 seconds. Drove about 15 miles total.

It has never leaked a drop of water or oil. Checked the radiator (cap looks good) when cold and I was able to put in 1.5 quarts fluid. Drove it again about 15 miles, and had the same results on the temp gauge. Parked, looked under the hood and looks like fluid is spitting out from expansion tank exit hose (expansion tank is empty, some coolant looks like it had blown out the hose with white residue). Radiator was again low, 1 quart filled it.

Checked oil, its perfect / not milky and no smoke from exhaust (so thinking / hoping its not the head gasket). Drives great, no loss of power. Only does this at 70+ MPH, doesnt do it on hills in lower gears.

QUESTION: I have looked around and know there could be many things, I am thinking thermostat can anyone give your $.02?

Thanks a ton
Trent in NC

 

Pull off radiator cap and post a picture of the rubber side here.

 

I'd flush the coolant and replace the thermostat. Easy and cheap to do. Replace the rad cap too.

Has the radiator ever been replaced?

 

Thanks @GODZILLA - dont know if radiator has ever been replaced but it looks fairly knew / entire top of engine looks new as well. Agreed on "flush + new thermostat".
Thanks @Half Assed, pic attached of the cap

 

pic attached

 

Also check that your fan clutch is still good.

https://www.popularmechanics.com/cars/how-to/a302/1772922/

 

Its possible the nipple on the bottom of the rad cap broke off and got sucked into system causing and overheating condition in part of the engine.

It happened on mine. Got stuck in a small heater line where the upper rad hose goes into the engine. I was able to remove it by removing the upper rad hose. Lemme search and find the old post about it...

Notice the difference between a new cap and yours:

 

@Half Assed that nipple certainly is missing, thank you...easy to get another rad cap and I appreciate you trying to find the old post on it, I am searching as well

@timsp8 thanks for the fan clutch suggestion as well, I had looked at that but 1. didnt have overheating at low speeds, only at higher and 2. there is no noise from the fan ...who knows though!

 

You need to find that piece as well as replace the rad cap. At least a couple members found that piece from the cap plugging a passage to the thermostat causing overheating.

 

Theres info and pictures in this thread about it

https://www.tundras.com/threads/running-hot.33183/

Considering your rad cap is broke, I'd start by getting a new cap, and then pulling the upper hose looking for little chunks of rubber stuck in those little coolant passages.

 

Already on it Half Assed, thanks for the help folks...love this forum

 

Damn nipples always causing trouble. I love 'em and hate 'em.

 

But do you snap them?

 

It’s a vacuum valve, not a nipple. If you suck on it you’ll see how it works.

 

Every one I can get a hold of.

Listen... I'm not a blushing, demure schoolboy. I've sucked enough neeples to fathom every aspect of their function.

Also, biting.

 

Well thanks for everyone's help...the damn nipple was the issue ;-) Pic attached

 

I don't have this issue on my 2007 - but you better believe I am buying a new Rad cap this week.

 

Where was it stuck?
I would second the option of replacing the radiator if it is original, it has given you 265k miles. Fan clutch should be rebuildable you can pull, remove the fan, open the clutch and then drain the fluid , measure what comes out and add new back in. Or buy new one.