1978 Toyota Landcruiser Sudden Overheating

Hello Team,

Thanks for the quick response.

The viscous coupling fan had been replaced and observed to come on at the appropriate time.
The thermostat has been replaced twice (OEM) with no change.
The radiator is a new 4 row aftermarket radiator. I installed an old 4 row radiator I had in storage that I had replaced in 1999 because it had a small leak. The old radiator worked the same as the new radiator i.E. No change on the temperature gauge.
I replaced the water pump because of a catastrophic failure. No change.

I tested for exhaust gasses at the radiator with the cap removed using a blue dye. No color change. Then I took the truck to a radiator shop that performed the same test. Again no color change. There are no bubbles in the top of the radiator or in the overflow tank with the cap installed. I torqued the head bolts to 90 ft/lbs - nothing moved and I checked the valve lash. Lash is good (I had adjusted the valves just 1000 miles ago).

There are no leaks in the intake manifold or from the manifold gasket. I checked with propane, carb cleaner, WD40, and smoke - all with the fan disconnected.

The temperature gauge sending unit was changed - no changes.

There is a sending unit for the carb cooling fan near the temperature gauge sending unit on the head as well. The carb cooling fan is coming on regularly. It rarely came on before. This observation supports that there is excess heat not being scrubbed away and it's not a faulty temperature gauge.

An infrared thermometer tells me that the cooling system is working well. The temperature at the temperature sending unit gets as high as 199 F.

I'm guessing a blown head gasket, yet there are no obvious indicators that this is the problem other than a hotter engine. As a diagnostic tool, I put some Stop Leak in the radiator. This helped the symptoms. It quit running higher and higher on the temperature gauge at idle as it did before the Stop Leak. Now the gauge will stop (but still too high).

The water pump is working well. I installed an inline filter in the upper radiator hose - no deposits or clogging materials in the filter (1/16" square mesh). I have flushed the cooling system extensively, in reverse flow as well.

So uh, yeah. I'm about out of options. I don't want to pull the head only to discover the problem was something else. The truck starts instantly, idles well, and accelerates well. On flat ground the truck runs at normal temperature. The fact that the climb on the temperature gauge was sudden and the carb cooling fan comes on might be a clue to this puzzle.

Thanks for your help. I hope a solution is near. Maybe it's running lean? How would I check this?

Dan