Radiator with twin fans

[Hatchet54]

The symptoms scream 'lack of air flow'. All signs point to a bad fan clutch.

With the engine running warm, shut it off. Pop the hood and see how easily you can spin the fan by hand. If there's little to no resistance, the fan clutch is bad.

That's a good trick

 

[NZurban]

Towed the boat up to where we launch from at the weekend, ran great, didn't overheat, didn't hear any bubbling in the overflow tank, a trip of about 25kms. When we got back later that afternoon drove slowly through town and then parked to do some shopping before heading home. Could hear bubbling into the overflow bottle. Drove the rest of the way home and once there the same old bubbling into the overflow. Truck sat for a few minutes then I had to tow a small trailer a few minutes out of town to drop something off. When I got to my destination sure enough it was bubbling some more. Popped the hood for a look and now the overflow bottle is completely full and the top hose is empty! I drained the overflow bottle and poured the contents back into the cooling system. Thinking one of the headgaskets must have let go. I had a Subaru that did exactly the same thing.

 

[Arkansas_V8]

Towed the boat up to where we launch from at the weekend, ran great, didn't overheat, didn't hear any bubbling in the overflow tank, a trip of about 25kms. When we got back later that afternoon drove slowly through town and then parked to do some shopping before heading home. Could hear bubbling into the overflow bottle. Drove the rest of the way home and once there the same old bubbling into the overflow. Truck sat for a few minutes then I had to tow a small trailer a few minutes out of town to drop something off. When I got to my destination sure enough it was bubbling some more. Popped the hood for a look and now the overflow bottle is completely full and the top hose is empty! I drained the overflow bottle and poured the contents back into the cooling system. Thinking one of the headgaskets must have let go. I had a Subaru that did exactly the same thing.

Sounds like exhaust in the cooling system(like you suspect). Happened to me with one of my old trucks.