Hi All,
This is my first car I bought myself and I’m doing my best to get it running.
I picked up a T-100 a few months ago non-running (no motor)
I was able to save to get a rebuilt motor. Took me forever but I was able to get it in with the help of my dad and a couple of friends.
I installed it last week. Fired right up and purrs like a kitten.
One big problem. It stars to run hot within 15 min.
Here is the riddle.
I take out the thermostat and it does not over heat.
Great…I found the problem!
Replaced thermostat. It overheats.
Remove thermostat, it runs fine and does not overheat.
I replace the thermostat with another new one. It overheats!
I have now installed a new radiator, and water pump. It overheats!
Remove the thermostat and it runs fine.

Please, please help this dumb kid. With my limited mechanical skill I cannot figure this out.

Here is some additional information that may or may not help. It is a 96 T100 3.4L auto 4x4.
When I remove the thermostat I can feel both radiator hoses and they are both warm.
When I put in the thermostat and it runs hot, only the top radiator hose is hot. The bottom hose is very cool so it seems not to be circulating water.
Also, I’m not sure that the new motor came out of but it was not a T100. (oil pan was different)
So, I think it came from a 4runner. Again, I don’t know if that makes any difference.

Thanks guys!

 

What u are describing sounds just like a stuck thermostat. Also you may not have all the air out of the lines. Go buy yourself a new thermostat. Test it prior to installing. After its in jack the front of your truck up as high as you can get it. Remove radiator cap start the truck and keep a eye on the coolant and temperature. Add coolant as it drops. This will insure there's no air in the line. Let it run for at least 20 minutes with out the cap on the radiator. After 20 min install cap and test drive it.

Hope this helps

 

Thats exactly what I thought. I have swapped out three new thermostats.
All the same thing. Thermostat in...overheats. Remove thermostat and it runs fine.
I cannot believe I have three bad (brand new) thermostat fail.

Please help!

Thanks again
K

 

Did you replace the water pump when it was rebuilt? Could be a weak water pump that has a hard time pushing coolant through a open thermostat.

Just a thought, good luck

 

Is it possible you've installed the t-stat upside-down? The spring side should be toward the hot side of the coolant flow.

If that's not the case, check out this video:

You can know for absolute certain that your thermostat is functioning. If you can rule that out you can move on to other potential causes.

 

Yep...thermostat is working. I even replaced it three times.

So, I have replaced
thermostat
water pump
radiator

It still overheats.
As soon as I remove the thermostat it runs fine...re-install thermostat, it overheats.

I've double and tripled checked the thermostat. It is working.

At the risking of really embarrassing myself...when installing the thermostat, the large round part of the thermostat faces the radiator...right?

Thanks again

 

Air in the system is my guess.

You need to get it all out, as said before, top up you're coolant, leave the cap off, and run the truck to operating temp. Turn you're heat to high.

Let it idle, keeping an eye on coolant level, and temp, and top the coolant as it drops.

Increase the idle to 2500rpm and hold it once in a while, and hopefully, a stuborn bubble will work it's way out.

 

run it without the t stat... that's just extra parts anyways.... lol truck will run cooler but will take longer for the truck to warm up to operating temp. my pops did that with his 94 pick-up on the side of the road when it went bad. pulled it out chucked it never put a new one back in and the thing ran flawless for years then he sold it for a newer model just to upgrade.

try runnin a cooler t stat. different motors are supposed to run at different temps and to get that they use different t stats set at different temps. i think that can be your problem. it's not out of a t100 it could be out of a smaller truck or car that only gets so hot and you could be putting in a t stat with a hotter rating. who knows maybe i'm wrong.

 

Fan clutch failed?
You are running a Toyota OE 180deg themostat with the jiggle valve down when installed?
Exhaust gases getting into the coolant jacking up the coolant temp?

 

That's just a goofy statement, (just extra parts anyways....)

To the OP: I didn't see a post to the fact that you removed any air pockets/bubbles in system that was suggested.

Also suggested from "rworegon" Fan clutch?

Just my 02 worth, running an engine without a thermostat especially with coolant sensor's (computer controlled) is asking for trouble at any time.

Jack the front end up and turn on the heater and run it to see if air evacuates. Good luck

 

without the t stat it just means you've got free flowing coolant. motor stays cooler as it is constantly flowing through the block. heat is the number one cause of engine break down and can actually gain a tad bit more power in doing so with a t stat with cooler t stat ratings. but as i said with my dads 94 pickup. not a single problem from not having a t stat on the truck. the downside is it takes longer to warm up and will take longer for your heater to blow hot in the winter.

but lets exaggerate a little bit. lets say he's got a moter pulled from an older 4 runner and the operating temp on that motor is 500 degrees. and he puts a t stat in it for a t100 that's set for 600 degree operating temp. that t stat wont open up till it hit's 600 and his motor overheats after 500. but when he pulls out the t stat it don't overheat. see where i'm kinda goin with it? can be the same motor but because it's from a smaller car it's designed to only get so hot. not saying that is the problem i'm just sayin it can be the problem. water pumps rarely fail. there is no real restriction on the mechanism. i would suggest a cooler t stat be put in and see how that works.

 

Spring side toward the engine. Don't fill the system so fast that it traps air. Burp it to make sure all air is out.

Make sure it is the correct temp stat. Maybe one that is higher than what you had in the past puts the gauge at high end. Not actually overheating just looks like it on the gauge.