Merc 150 Four stroke

[NY OZ]

I have a 1999 Ranger 462VS with a 2013 Merc 150 Four Stoke on the back. During the winter I installed a Vance 4" manual jack plate. Ran the boat for the first time this past weekend and seem to have a Water Pressure issue.

PSI is fine at idle and climbs as I increase RPM and get on plane. Once on plane the PSI starts to drop to about 13 , and as soon as I turn the motor a little bit the PSI jumps right up to about 35.

Motor is set at 3.5" below pad.

I did have them change the water pump when they installed the jack plate.
Is this just cavitation and when I turn the motor slightly I am getting clean water?

[EuropeanAM]

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Sounds like you are now in aerated water. Check for transducers or other items forward of the engine- they can cause disturbances like this.

If no other "cause" is found, and the water pump is VERIFIED as fresh/pristine condition, you may benefit from a high-speed scoop kit. Consider this kit the "last step" after all other items are ruled out.

[NY OZ]

Thanks Don . Serial is 2B050813.

Water pump was installed by a Mercury Mechanic and was indeed new and in pristine condition. He was unable to run the motor after install as it was done here in NY in February. The boat sat until this past weekend. If the pump was bad would I have a consistent pressure issue?

I do not have any Transducers or other items forward of the motor..

[EuropeanAM]

Should be good, as long as gaskets were replaced.

Drop the engine 1/4-1/2" and see if pressure returns (if so, let's look into the scoop kit).