Overheat only in 4 high

I have a 2006 Toyota tundra TRD that is overheating only when it's in four-wheel drive high anybody have any answers why that would be happening thank you in advance

 

Additional load? Have you gone through the cooling system?

 

I just had timing belt replaced with a new water pump and it was doing it before all that

 

And no load just 4 high is the only time it does it i can put in 4 low and it will cool back down

 

How does the radiator look? Are the fins messed up? Are you losing coolant?

 

Check your fan clutch.

 

no lose of coolant and fins are in good shape

 

Increased engine load from turning front and rear axles. 4 low offers a significant mechanical advantage making it easier for the engine.

 

So is this a common problem

 

No

 

That's what i thought even going down the highway at 50 in the dead of winter it does it

 

i will do that i think that is the only thing i haven't replaced

 

betting it is the fan clutch. Id check that, your radiator cap, those are easy. If still an issue the radiator may need to be cleaned/replaced.

 

Thank you so much I got a new fan clutch on the way i will let you know what happens

 

None of those are 4wd dependent.

 

is the tranny cooler or a seperate transfercase cooler built into the radiator on the gen 1? 4 hi typically raises tranny temps quite a bit maybe its indirectly making the coolant overheat?

 

Nothing in the cooling system is.

 

Exactly! Need to isolate what the cooling system and 4hi have in common. One dude mentioned the front having more resistance, which 4lo would overcome but it should be a trivial amount. Though I can't think of anything else that would connect 4wd and the cooling system besides engine load.

As I recall you can't put the tcase into neutral... So another pseudo test would be, OP when you start from a stop in 4hi how does the engine rpm compare to 2hi to get moving? And when you're moving and let off the gas how quickly do you come to a stop? Is there any evidence that the front differential or tcase has excess drag on it? I'm thinking like it ran out of fluid and just getting insanely hot so you're using way more throttle than you normally would. Still would be surprised it'd overheat, but not sure what else cooling temp and the diff/tcase have in common...

 

From my personal experience with the fan clutches, they do wear out and cause the temperature to raise especially at idle when there is less air flow. There is a viscous coupling in the fan clutch that could be worn out especially since the truck is 15 years old.

OP should check the resistance on their fan clutch when the truck is off.

This.

We have a transmission cooler built in to the radiator then an auxillary cooler.

I'm curious what type of driving they were doing in 4-HI and at what speed.

 

Ok to answer a few questions there is in inner cooler in the radiator and an external in front of the radiator the overheating comes in 4HI at a little past ideal and no more then 30,000 RPM and like i said when i put it in 4LOW same situation no overheating it has me completely baffled the driving situation is EASY 4 wheel drive trails in colorful Colorado and i have noticed that elevation does not make a difference

 

In 4 low, the final gear ratio goes from around 14 to around 38. It’s a huge difference.

ETA those are 2nd gen numbers but I’m sure it’s similar on a FGT.

 

I'm still going to replace the fan clutch this weekend that is the only thing I have not replaced

 

I think thats the key here, if you are doing heavier wheeling in 4 Hi when you should be in Lo, its going to get tranny temps up alot. Or if you are using 4 Hi when you really shouldnt be and going fast and shifting allot it would cause elevated temps as well.

OP are you sure your radiator fluid level is correct and there is no air in the system? Maybe try to burp it for a few minutes?

 

yes levels are good when i start and the wheeling is a slow pace that you can actually do in two wheel drive

 

^^^ For some easy climbs, even if you don't feel like you need the lower gearing, 4-LO is better just because the truck won't have to work as hard to get up.

I second what Sniv said though regarding the fluid. Servicing the transfer case fluid is easy, you just need a 24mm socket and

$39 75w-90 GL-5 Limited Slip, Ultra Long Drain 500k Miles, Manual Transmission, Gear/Differential Lubricant (1 Gallon)

75w-90. Loosen the fill plug first, then the drain plug. Don't do the drain plug first, just in case the fill plug won't come out. Once it's drained, fill it up until excess fluid comes out the fill plug.

Your front diff uses 75w-90 too.

 

Slow pace = 4 low for me. 4 high at a slow pace means the engine is working at its hardest turning both axles without the advantage of low gear and with very little air flow.

 

No, they are not but 4wd can add some additional stress/load. And the fact that generally, if you are in 4wd, you are traveling slower so less airflow. If the system is in marginal working order, any additional stress/load can be just enough to not effectively cool. I mention it because I have personally had this occur in a jeep I owned years ago. I have also had auto that had marginal cooling overheat with idling in hot weater, and even the load from turning on the AC could make them overheat. I still think it will be fan clutch and radiator replacement for fix, one or both.

 

he also mentioned 4wd at 50mph in winter being a problem. So that takes the should be in 4lo out of the equation.

 

So where should he start then?

 

Simple. Don’t use 4wd!