Hi
Yesterday i Test drove my new to me boat the first time.
Boat is refit by my self, never run before here. Engine is 502 MPI with coolfuel, except of roller rockers Crane gold, and stainless marine exhaust it is stock. Engine is rebuilt and first time in this boat. Bravo leg.
I have a water pressure gauge, it shows 4-5 Psi @ idle. yesterday i got it on plane the first time, 3000rpm. 150F steady all times. Then i watched the water pressure from 25psi going over 35psi straight to the end of the gauge. Staying there about 10 sec, and falls directly down to 15psi. stays there about 10 sec, and rises directly to the end of gauge. 10 sec.....all the time.
I think what i see there is maybe the thermostat working? all original, no crossover.
Shouldnt be normal for a original engine to do this? Maybe something is wrong here? What could cause this?
Pressure gauge is old version, mechanically, hooked up to block winterize outlet. All new hoses.


Thank you guys!

Philipp

Does it have a pressure regulator or blow off valve? The fluctuation Could be the cycle of it building pressure and dumping it overboard once it hits the relief.

Mine bounces all the time in waves or wakes.

Are you running in water calm enough to to oscillate the water feeding the raw water pickup?

I have transom pick up, 1/2" below the water line and bounces like ICDPL discribed.

It does not exactly bounce, it does fluctruate. Water was mirror calm, i was cruising steady 3000 rpm. The needle went all the way up, maxed out the gauge, stayed there about 10sec, and fell back to 20 psi. stayed there about 10 sec, then maxed out again. All the time. when i took back the throttle, it went down and stayed there. For me it looks like i see the thermostat working here. but i think its not normal that the working thermostat sets the block water pressure over 35psi @ 3000rpm ?

Where are you tapping into the water system to get the reading?? My 502mpi's both hold 100% steady water pressure. The water pressure has always been very high on them in the upper rpms, especially with new impellers and will push past 35psi, but at 3000 rpms, it is around 15-20psi depending on the age of the impeller.

I am tapping in at the port side block drain. Mechanical gauge with all new hoses. Impeller and housing is new. Bravo with standard and low water Pickups.

35 is to high. With pressure getting high the water is not getting out of the engines fast enough. You could end up with intake or head gasket problems. I had this problem with my boat. Mercury had a solution and put out a service bulletin because of high water pressures. The solution was to put set screw in the side pickups on the bravo drives. My top 3 hole are plugged and the dropped my pressure down to 15-20 psi running wide open and 10-12 cruising.

Yesterday i took out the thermostat. Now i have about 3 PSI in idle. Reved it up to 3500 and were about 17 psi. Didnt drive it longer, but seems it could have been the beat of the Thermostat i was seeing. It was a 140F Thermostat wich is the wrong one, I try again with a new 160F thermostat. Maybe its getting me where i need it.