TomZ

iTrader: (2)

I'm still trying to get the water pressure straight on my 502 MPI's.

Background... raw water cooled motors with CMI Etops and TRS tails. Water pressure is being read from the stock Mercury thermostat housings. Water is being pulled from the factory transom pickups.

I have shaved the pickups so that they're protruding an 1/8th below the hull. No change. I'm still seeing over 25 psi on the starboard motor and 30 psi on the port motor. I have had the circulation hose come off on the port motor so she definitely has a ton of pressure. On the hose I can rev both motors and spike 15 psi.

I'm thinking the tails are the restriction. CMI builds the tails with a very tight gap between the nipple and the water jacket. My guess is that's the problem. My pipes stay cool and the engines run at 180 degrees whether cruising or at WOT so she's cooling well.

I have some pressure relief valves from CP Performance that I'm going to t-into the line going from the raw water pump. They will be installed before the coolers (no space after the coolers). I'm going to dump the water to new thru-hulls that I'll install on the port and starboard sides near the stern. Good solution? I don't want to install these and have them cool my exhaust.

Thoughts?

compedgemarine

i had a set of tails on a triple sonic that had the same problem. when I pulled the fittings out of the tails the inner pipe was real close to the bung. I took a drift punch and knocked it down to open up the gap and the pressure dropped. after opening the gap a couple times I got the pressure down to where I wanted it.

Griff

My water pressure on my 502mpi's with CMI's has always been high also. I also have transom water pickups.
I'll peg the guage at 35psi at WOT with new impellers. Been like that since I owned it and never had a problem.
I noticed it on the sea trial 10 years ago with the prior owner and he said it was always like that as well.
I have often thought about adding pressure release valves, but never had any problems, so never messed with it.
My engines run 160-165 at idle and cruise rpms and 175 at WOT.

My thought has always been that its because the internal bypass in the TStat housing does not flow enough, but not sure.
I never thought about it being related tot he CMI's. I swapped from Gil's to CMI's on my Pantera and my water pressure went down about 5psi.

TomZ

iTrader: (2)

I've read something similar. Have you posted this before? I tried opening them up a little but it made no change.

TomZ

iTrader: (2)

Griff, this isn't the first that I've heard of this specifically with the 502 MPI. I called CMI to get their take, but they weren't really interested in talking.

Interestingly, the port motor has a new impeller, and it's the one that pegs the gauge. Now it did pop the hose off once, but I think that was because the clamp wasn't properly secured. It's run this was for three or four seasons, maybe I'll leave it alone for now.

Griff

I have also thought about switching to Globe impellers which people say don't flow quite as much.

TomZ

iTrader: (2)

Thanks Griff!

debcod2

I was having a water pressure issue and finally getting it under control. I rebuilt my 502mpi last summer and its a 558 or so now and I was getting 51psi at wot which was way to high and ended up blowing head gaskets but luckily it was out the side of the block and not into the cyls. it took some long hours of changing just about everything and the last thing to check was the intake manifold. found out it was warped as hell and cracked in 3 places! this was causing pressure from the motor to basically charge the water system and in reality water was dead heading in block. put new intake on that was straight and no cracks and water pressure dropped to 30-35psi at wot. just playing with springs in pressure regulator now to try and get it to around 25 psi at wot.

TomZ

iTrader: (2)

I've got water pressure regulators on standby for this, but I the problem is rectifying itself. The starboard engine is settling in at 25 PSI WOT, and the port (though still pegging at reentry) is also settling down. I think it's a problem with new impellers just like Griff mentioned. I put a good 150 miles on the boat this past weekend, and had no issues. Oil levels are good, and everything is clean,

ziemer

iTrader: (1)

We went the the top ends of mine a little over a year ago and my engine builder was baffled with the high pressure on both motors. Tried everything he could think of and finally we settled on restrictor washers in place of the thermostats. I run stainless marine manifolds and risers that extend thru the transom. Other than that, everything else is stock. Pressure is normal and never gets much over 20 psi.