Water Pressure Bouncing!
 

Hi :drink:
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.

Any suggestions for good Thermostats? High flow or whatever? Or just original 160F ?

Test drove her yesterday without Thermostst.
No fluctruating Water press. Idle 3psi, went up to 4200rpm @17psi. Engine cold, Manifolds hand warm.
I'm wondering if the thermostat was bad or if the fluctruating comes back with the new thermostat.

I agree that 35+ is too high, and even questioned it when I bought my boat years ago, but my stock 502mpi's have close to 900hrs and haven't had any issues related to it. I have transom mounted water pick ups, so can't plug drive holes.

I would try a new 160* OEM thermostat first. If it is still bouncing, I'd drill a 3/16" hole in the tstat and test again.

Drilling a weep flow hole in the thermostat, and getting some water flow in the engine as it heats up is good to have.

Cummins 5.9 engines has this weird temp fluctuation as the thermostat cycles. It's unnerving. I think Dodge changed the gauge to respond less.

Tried today. I took the original thermostat, drilled 2 holes 1/8. The water pressure does not fluctruating anymore but she would peg the gauge. Reved 4500 and had 35psi. Pulled back the stick then.
so whats the best solutuon now? Bypass?

Like I posted above, my 502mpis will push 35psi + also. When I first got my boat with 250hrs, I was going to install pressure relief valves. I wasn't having issuies and never did install them. The engines have almost 900hrs on them.
You would need to either reduce the amount of water coming in or get a pressure relief valve to get it out faster.

What happened to trying a new 160° thermostat?

I would like to install a pressure relief valve. Whats the best place to tee in? CPPerformance has the T in 1-1/4 inch. Looks like it should be between seawater pump and oil cooler? I weld the T by myself so i can size it how i like it. so whats the best place to tee in?

Thanks!

After the oil cooler. You want full flow to the oil cooler.

Not sure what pressure he's popping of to, but wouldn't 35psi would pretty much be 'full flow'?

Then he's not getting surges on his oil cooler.

If there is 35psi on the oil cooler, wouldn't the engine have 35psi also?

alot are on the sea strainer before the raw water pump.