The For the plug, you can coat the threads with Permatex Aviation or OMC/Evinrude gasket sealer Or Mercury Perfect seal the last 2 are the best for these applications and are really the same product. The Permatex works fine for that too. I use either the Permatex or the OMC stuff depending on what I have at the time. They don’t get hard and prevent corrosion. Very good on thermostat housing bolts and gasket, all the engine and manifold drain plugs and bolts that go into the outdrive as well.
As far as the engine not warming up, it is usually some piece of rust, sand or even a bit of an impeller keeping it slightly open. For the engine to warm up it has to be able to close to keep hot water in the block/heads & intake manifold. I’ve had this usually every 2 years or so . The only permanent solution is a closed cooling system because that prevents internal engine rust. On
mine every time this happens it’s a tiny piece of rust from inside the engine. If you drain out the water when you change the stat next time wipe out the inside of the intake manifold I bet you’ll
see some flaking rust there!
