Poppet valve '99 Merc 150

[Blue_Runner]

More poppet research. Commentary on someone elses problem:

Did you check the poppet valve and springs?? Double-circular cover on the lower starboard side of the block, held on by 4 (maybe 5) bolts. I believe you will find it helpful to remove the lower motor cover on that side in order to gain access. The poppet controls cooling water flow for hi-speed operation, is supposed to unseat and flow max water around 2500 rpm's (?? I think).
-John

This sure sounds like the poppet valve problem. When that valve is working correctly, you can watch the temp gauge climb to about 3/4 at high idle speed. At around 2500 rpm when water pressure builds, the gauge will drop rapidly down to 1/4 when the valve opens up, and the temp will stay down there up to WOT. Your new water pump eliminated that as a problem source, leaving other things to look for. Other than poppet valve, the other possibility is some kind of blockage in the cooling passages. Pulling the heads off and checking the water jackets is called for there. With a motor that old used in salt, replacing the head gaskets is not a bad idea anyway. Because it is easier to replace or clean the poppet valve assembly than it is to pull the heads, I would look at the valve next. Not sure about the big blocks, but with the small block V6s, you have to remove the lower engine cowl to get to the valve assembly. Problems like this are a process of elimination. With a used motor of unknown history, replacing these components as you go through the elimination process leads to peace of mind and confidence in your power plant.