Wow, an amazing amount of white smoke/steam
If the old thermostat was stuck shut, you'd have really heated the water in the engine and steam would have come out the exhaust. Replacing the thermostat or unsticking it should get water flowing again through the block and head and there should be no more steam. Check the water heater hose connections (pull the hose and verify that water can flow) at the engine before ya run it .. the QM that I know has the water heater exchanger in line with the block outlet and the block connections get plugged with stuff ... like the stuff that was sticking the thermostat.. Good luck..
Thanks Kloudie1 I think you're right, but not because it was stuck shut. Please read my rationale. (sorry about it's length). We don't have a water heater in line so that isn't a problem.
Was out to the boat last Sat, outside temp was about 55 degs. She fired up no problem. After a 5 min warm-up (no steam/smoke) I popped it into reverse and gradually up to 2000 rpm. Good water flow out the exhaust but more white smoke/steam than any time in the past 6 years we've owned her. I have a theory about what's happening and would like to test it out with the big brains on the board.
I understand how a thermostat on a gas engine (like a Chev 350) works; when the engine is cold the thermostat is closed so that coolant doesn't circulate through the block allowing the engine to come up to temp quicker. Temp comes up, thermostat opens, coolant flows, all is good.
I've attached a pic of the cooling system diagram from my Service Manual. On a 2QM15 the water come from the seacock, to the pump, to an inlet on the block. That inlet also has a bypass hose to the thermostat on the manifold and into the mixing elbow. There is an return outlet from the top of the valve cover into the manifold.
Like any thermostat, it is open when cold. However it's installation is upside down as compared to a gas engine. When the engine is cold, the thermostat is open so there is less resistance to the water in the bypass hose therefore more water goes straight to the elbow and less water flows through the engine block allowing the engine to warm more quickly. As engine and water temp rises, thermostat expands, closing off the thermostat body creating resistance to the water in the bypass hose which forces water through the block and out the return hose. All good. Whew!
The new thermostat does exactly what it is supposed to do; open when cold, closed when hot. My theory is this: the increase in steam is because now, when the engine is hot there is no restriction from the thermostat, so less water is forced to circulate through the block causing it to run hotter. I believe that semi-restricted water passages in the block are causing the engine to run hotter than with the blocked thermostat. Previously, the blocked thermostat
always forced water through the block keeping the engine cooler than with an properly operating thermostat.
Because the engine is running hotter this is causing more steam in the exhaust. Note that that the engine temp light come on (this may only mean the light doesn't work :neutral
Does scenario seem plausible?
