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1998 Mercury 150XL - Heat

Kingfish22

New member
I've been battling this for a while now... any advice would be gratefully appreciated!

Problem: Not enough water pressure (doesn’t get any higher than 12 psi on the gauge even at WOT) and boat runs at 200 degrees when mid to WOT. I have both temp and pressure gauges. Alarm has not gone off (assuming it goes off around 204 degrees or so?). Engine runs at 160-180 degrees at low and idle speeds (Idle it even runs cooler). Steady stream out the tell tale (water is too hot to keep hand under). Seems to be able to run all day long a low cruising (which made me think it was the poppet valve).

So far I have changed water pump (all new), have changed tstats (all new), have changed poppet value (all new). Last year I ran Barnacle Buster through the system b/c of overheating issues… didn’t really seem to solve the issue either.

I am assuming there is a blockage somewhere, perhaps the water jacket? How can I diagnose what and where the issue might be? Should I attempt to remove the head cover and check for any blockages around the cylinders? Is there any way to figure out approximately where the blockage might be within the system before I start trying to tear everything apart? Again, any advice appreciated!
 
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