This is Why Stop Leak Should be Banned

My brother and I work on Gen 3 Priuses acquired from disenchanted owners. The seller of this one said the vehicle "runs just fine...no problems".

Of course it rattled like a spray paint can with a blown head gasket, as most Gen 3's purchased from FB Marketplace do.

Replaced the motor with a different one rebuilt by the esteemed Scotty Partin, a trained Toyota hybrid tech in Rockholds, KY.

But after installing Scotty's rebuilt motor the vehicle still overheated on up-sloped highways, despite having a new thermostat and water pump in it.

The problem was revealed to be the stop leak additive poured in the cooling system by vehicle's former owner. It plugged the bottom half of the radiator.

My thanks to Scotty for his help on the project. He would not have known about the stop leak problem because he never saw the vehicle or its blown engine.

Surprisingly the vehicle's heater still functions like a blast furnace, though I imagine that its water passages are pretty gunked up.

BTW, Gen 3 dashboard's engine overtemp icon illuminates at 250 degrees F. But a cheap OBDII tester running in the front seat can help you stay below 210F by driving the vehicle slowly home on a cold day.

I cringed that Toyota set the temp threshold so high to turn on the overtemp light.