Mercruser 4.5 Bravo 3 this thing is crazy. I tried new sensor today ran great temp never above 150 .Block pressure 10 to 8 above 3500. After about 2 hours got alarm low block pressure showing about 5 psi on gauge went into limp mode . Happen a few more times at rpm Then ok block pressure never above 8 psi temp never above 150 on engine temp . It’s racking my brain . 3 hours from any dealers.

what year is your 4.5 - this is a catalyst engine and requires full correct cooling at all times

the 4.5 used 2 different water pump impeller housings. change your impeller and check the housing for internal scoring and grooves. This reduces water pressure by alot and causes the issues

Next who and where did you buy the water pressure sensor from ? you also need the correct one as well for your app. If you bought it from ebay they are junk crap and do not work correctly. I see this alot from boaters. and they have issues with those ebay sensor from china -- yep the cost is very cheap but not cheap when they do not work right

Also at 3500 and above rpms -- water pressure readings of 8 to 10 psi is still low.