Do not restrict inlet water flow (to pump).
Control water flow on the discharge of the pumping system.
Problem then becomes water pressure.
Control pressure with a pressure relief device.
Control oil temp with a thermostatically controlled device.
I have been through all of this.
I use the Mercury Racing water pressure relief valve and the Mercury Racing Oil Temperature Contolled Oil Filter mount.
I use a crossover with a bypass and a 142* T-stat with (3) 3/16" holes drilled in the T-Stat.
The bypass has a 1/2" brass pipe plug with a 3/8" hole drilled through it, jammed in the bypass hose. This is installed on a N/A 540.
No Problems! Good Temps across the board.

I checked my water pressure at the block and at the top of the intake manifold. Both of them were 2-3 PSI at idle and about 8 PSI at 4000 RPM. From these numbers it does not appear that water pressure from the crossover is the problem. Any other ideas?

These readings were done while on the hose. However our shop hose is at a fairly high PSI for that it has a booster pump on it.

Water pressure on the hose is alot different than water pressure while underway.

I just completed a test. 30 min with the exhaust going out the transom and got milkshake. 60 min with the exhaust going out the prop, and the oil looks great. The test was done on the hose, 700 rpm. I have done this test 4 times now and every time I get the same results. Any ideas? I know that this is completely opposite than what one would think with reversion happening when the exhaust has a clear shot out the rear and not when it is going out the prop. Any ideas??? THANKS!!!!

RLW, you are right on the money!
Listen to him.

The only thing I might add is that it's a good idea to plumb the relief valve =after= all coolers, so these have the benefit of the water volume...

Plus, I believe with any boat, a water pressure gauge is a good piece to have. It tells you a LOT about the health of the cooling system.
Kent

> I think the cold temps are making the engine run really rich

If the cold temp did anything, it would be to lean the mixture... that is, assuming a cooler block = a cooler intake manfold.

On my brother's 588 Merlin II, I'm looking to run a thermostat, but there's lots to consider before jumping in. It's been suggested I work the problem from the oil temp side (coolant by-pass thermostat on the oil-cooler), but I'd like to do both... but WATER TEMP first.

So far, I've done nothing. Waiting to learn more before I jump

jrc in SC

Is yout motor fuel injected or carbed? I know a stock 502mpi has to reach a minimum temp on the coolant sensor or it stays in warm up mode(runs extra rich). Procharger gives you a signal conditioner to "trick" the computer into thinking its warmed up. My 502 and my 540 (current motor) both run around 100degrees to 120 max (in warmer shallow water). I am using a crossover,no stat. I was skeptical about the low temps with 502 at first(i was always under impression you got rapid cylinder wall wear under a certain temp) but when i tore 502 down at 200 hours it had no ridge and no appreciable scuffing or taper,Smitty
P.S.-tyler crockett set efi program this time on dyno to account for low temps when operating so i have a spare signal conditioner if someone needs it for a really fair price.

Good info!! where would you u sugest plumbing a dash mounted water pressure gauge then?