I have a 1999 Mercury XR6 150hp.
Sn# 0G883085
Last week I had an overheat issue so I decided to change thermostats, poppet valve and add a water pressure guage. The guage is installed at the top of the block about 3 Inches from top of block using a Tee fitting. I ran in the driveway using muufs and garden hose but idle pressure are mid 20s a d hi idle pegs guage. Water from the tell is warm enough to allow you to keep your hands in stream without getting burnt. Block is nice and cool to warm. My question will these number change when boat is in the river and prop engaged? Book says need to be over 12psi WOT and 1 to 3 psi at idle. What am I missing? Thanks