Coolant loss while towing gen 1 tundra

Any clues why my tundra is possibly overheating while towing a light travel trailer? I believe the coolant is bubbling up and coming from the overflow in the coolant reservoir

the vehicle has a new radiator, cap and thermostat

 

Do you have the V6 or V8? What cab type? Those are kinda important details to include.

Have you checked the fan clutch? You shouldn't be able to freely spin the fan on the front of the engine. It should have some resistance, and the hotter the engine is, the more resistance it should show.

What color coolant? (pink/red is the only correct answer here)

When the engine is cold, does it reach the correct cold line on the overflow bottle?

Are you 100% sure there's no air in the system?

When was the last time the radiator was replaced, and is the cap showing any signs of browning? (it should be fully black/charcoal, not tan or brown)

If V8, when was the last time the timing belt was changed (should be before hitting 100k miles or 10yrs), and was the thermostat changed recently? (if yes on t-stat, was the jiggle valve oriented correctly at 12 o'clock?)

 

Hello Shifty,
Here’s my response to your questions:

-My 2004 Tundra is a 4.7 liter V-8, SR5 four door.
-the fan clutch was recently replaced and the problem persists
-I’m using proper red/pink coolant
-when the engine is cold the coolant is at the correct line in the reservoir (unless is following an incident where coolant got past the radiator cap or reservoir overflow. After 3incidents of coolant loss I’ve used about 3/4 jug of coolant to replace the lost fluid.
-I’m not 100 sure that there is no air in the system (how can that be determined ?)
-the radiator and cap are new, replaced about 3months ago
The timing belt was replaced at 217000 miles, March 2025 (third belt in this vehicle’s history)
The thermostat was replaced by a Toyota dealer recently, along with the timing belt and water pump . I’m assuming the jiggle valve is properly oriented.
The independent Toyota mechanic I use is suggesting running tests to determine if the the head gasket is leaking. FWIW, I’ve only experienced the problem while towing in warm-ish weather (mid nineties)
Thanks for the feedback.
Terry H
Albuquerque, NM

 

By "four door", do you mean the access cab four door, or the double cab four door? Access cab rear doors are suicide doors, double cab has full sided doors, all four open like normal doors.


To your questions:

Did this problem exist before replacing the radiator (and coolant)?

Not properly burping as you refill coolant can lead to air in the system, which will lead to the exact system you're describing (as will a faulty radiator cap). Even the best of shops can screw refilling coolant up.

Did the problem exist before replacing the timing belt and water pump? A defective water pump would cause similar issues to what you're describing.

I don't disagree, necessarily, not replacing coolant at proper intervals can lead to head gasket degradation but it's uncommon to see head gasket issues in these trucks.

I guess next steps would be taking measures to ensure there's no air trapped in the system, burping the system. Several folks here on the forum can help with tips on doing so.

 

Read Shifty's Megathread and post some pics of the area around the radiator, thermostat and coolant crossover pipes.

 

Actually, I just remembered, and this is the reason I asked about coolant levels being correct, and faulty cap:

Too low a coolant level will result in similar overheat. And if the radiator cap isn't working and can't open, or is always open, it will either cause excess pressure in the system, or allow coolant to flow endlessly into the bottle.

There's so many stupid little things that can go wrong with coolant flow/coolant system that can cause only periodic overheats. I'm thinking that it happening only while towing, considering where you are in the (hot ass) world, it's prob not the head gasket.

 

What brand cap?

If not OEM or denso from a good source, I'd be testing/replacing it.

 

I don't know man, but running the combustion gas test inside radiator won't hurt and if anything, rule out head gasket. Anytime I seen the expansion tank bubble or fill to overflow was a head gasket. You probably have a very slow leak as the reservoir fluid bleeds back down to normal when cold.