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Coolant overflows when pointing downhill

I’ve had a problem for a while with my old (1982) landcruiser. In normal city/highway driving it uses no coolant, however if I do something (like offroading) and point the truck sharply down hill, the coolant will start to flow out of the overflow tube into the overflow tank.

Radiator cap and radiator are new (3000 miles ago). Headgasket is also new (2000 miles ago). Every soft hose in the system was replaced prior to my latest trip, and no visible leaks. Coolant doesn’t smell like exhaust, and the exhaust doesn’t smell like coolant. Heater works.

This latest time I parked the truck on a steep decline and noticed the overflow bottle overflowing. I pulled down the hill to flat ground and it stopped. About an hour later I let the truck cool down completely and found myself down 1/2 gallon. Then I drove home ~2 hours and checked again that evening - no coolant missing.

What do you all think???

Bad radiator cap. Cheap fix. Try that first. BTW, these can be bad when bought. QA is not perfect.

I agree. It has to be the cap.