Exhaust in cooling system.

[76Red18]

1999 150 Johnson 60 degree block

Noticed running temp a few degrees above normal. Checked thermostats for opening temperature. 1 opens @ 145 the other @ 148: good reading.

Before reinstalling them, I figured I would start the engine and see what kind of flow i have @ the thermostst holes. Upon initial firing, exhaust came out of the holes before seeing any water. Plenty of flow.

This test was done in a tank not on muffs, so there was exhaust back pressure. I've never done this before and with limited outboard mechanical expierence (except for this motor ) my knowledge of closed systems says this shouldn't happen. Is it different with an open system??