MIKE H

I boat in fairly cool water, anywhere between 60 to 75 degrees in the hotest part of the summer, I have the Imco Power Flow Plus exhaust system with the custom pipes, I am presently runing the fresh water through my oil cooler first then through a cam driven water pump to the exhaust manifold to preheat then to the block front and out through the risers and tail pipes. I still have the two front plugs that are constantly black and carbon covered and then they get cleaner towards the back. Is it a good idea to blead off some of the water from the back of my manifolds and plumb it through the block at the drain plug area, or should i just feed the fresh water through the block first and out through the manifolds, risers and pipes? I find that my manifolds are constantly wet with condensation and was wondering if the inside of the manifold could be wet with condensation also?

thanks

Gary Anderson

What's your water temp? You could try adding a bypass and thermostat.
I'm running a preheat setup and couldn't heat up the water or oil enough without a thermostat.
Also consider your carb may be affecting the carbon fouling.
Gary

MIKE H

Thanks for the reply. I get up to 140-145 when running moderate to hard. I tried the thermostate route but found my water presure was too high.

Mudball

Mike, you can run a Merc Racing pressure relief valve for when running a thermostat with a crossover (with bypass).
I installed one and no more pressure problems. These mount on the line after the water pump, and should be mounted after all the coolers.
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I am running an EMI crossover with bypass, 140 degree thermostat w/three 3/16" holes drilled in them (for steam venting), and a Merc relief valve. I mounted the dump on the port side. I would have dumped it on the drive, but there was not enough room to mount it on the transom...

I now run 140-150 water temps, and water pressure of 25psi at 2500 rpm and up. Oil temps are 190-200 at cruise and 220 at WOT.

Before the thermostat was installed, I never could get any water or oil temps, or water pressure to speak of until maybe 10 miles of running, and had condensation problems, too. No problems at all now, and the motor sure seems to run better.

Take care,
Kent

dckrafft

Does anyone have a part number for this pressure relief valve?

CIG3

The pressure relief is one way to go. You could also dump more water from your tails over-board. Your tails are the restriction. As for pressure you really only need about 12psi of water pressure. Flow is what cools everything not the pressure.

PatriYacht

If you run a thermostat with a crossover, the thermostat becomes the greatest restriction when it is closed. Having a bypass hose to cool the exhaust manifolds is required when running a thermostat but that may not relieve enough pressure. That's when you need to use the relief valve.
Make sure you have the bypass hose or you'll burn up your exhaust.Here's a couple of pics that show the bypass on top of them. housing. The hose goes down to the pipe fitting on top of the crossover.

Mudball

PatriYacht is right.
The thermostat is a huge restrictor also when it's open! But closed is dangerous...
A fellow on another forum had his motor milkshake TWICE on the dyno before he checked the water pressure. His head gaskets went with 35+ psi of pressure. He installed a relief valve and all was good.
I was lucky in that I blew a hose off before any gaskets failed.

Also, if you run too low of water pressure, steam pockets can form in the heads and actually cause overheat problems from that.

Most think that around 20-25psi max is about perfect. The Merc valve limits it to 25 on mine, which is slightly on the high side.

The Merc relief valve P/N is 863208A3.
Here's a pic.

dckrafft

Thanks for the part number and posting the picture.