Well, I'm back! We fixed the leaking water pump last week, and all was well for two days. (this is a CAT 3306 in a boat, by the wat) Yesterday, I looked up and saw water running from the expansion tank. It filled the 3 gallon tank from 1/4 all the way to the top, and started pouring out the cap (13PSI cap). Normally it'll go from 1/4 full when cold to 1/2 when it gets hot. Now that expansion tank is about 3 feet above the engine expansion tank. What's weird is that when we pull the cap from the tank on the engine (below it), it bleeds off air, not water, and then the level in the other tank (it has a sight glass) drops down. What I don't understand is, how is air getting into the system? We just changed a water pump that was leaking water into the oil. And it started right after that. I hope someone here can help, because chasing it is gettin' old
If it has pre-combustion chambers in it with glow plugs, one or more of them is leaking. They get pinholes in them, or they can get loose but not common, or they can start leaking at the washer(seal) where they screw in the head. They get holes in them from the coolant not being maintained with cavitation additive. I've never been around marine engines, so I don't know exactly how they're cooled with sea or fresh water. This is probably why it started leaking out the water pump seal, because it over pressures the cooling system with combustion gases.
Check that the cap is holding 13 psi, my 977L started to run hot, the cap was bad.