Merc 200 Water Pressure


I recently purchased a 1988 Vegas XT with a 1984 Merc 200.  As far as I know the only mods to the engine are tuned exhaust, Merc 150 heads and low water pick up.  Currently I am running a 4-blade 25 pitch stainless prop.  I had to have one of the cylinder's sleeved and I am currently in the break in process.  My question is about water pressure.  When the boat is running at 3200 RPM's the water pressure is 18 PSI.  At 3500 RPM's the needle on the gauge fluctuates rapidly between 18 and 12 PSI.  At 3600 RPM's the gauge is constant at 12 PSI.  During all of this I have been keeping my eye on the stream of water coming out of the engine.  There is a steady flow of water.  As the water pressure decreases the stream pressure is reduced slightly. There is always water coming out of the tube.  It never drips out or fails to come out at all. Before I had the work done on the engine the pressure was about 20 -25 PSI above 4000 RPM's.  I have not turned the engine any higher than 3600.  I have a break in schedule that the mechanic gave me and I am following it exactly.  I do have an audible alarm for the engine temperature, it works but it has not been set off.  I plan on getting a temperature gauge as soon as I can.  Any ideas about the water pressure?

Don Robinson



The pressure fluctuation is from the spring loaded bypass located below the switch boxes. It is popping open too early. I've seen the wrong base gasket do the same thing. Check both of these out as you should be back to your original 20-25psi. Have your mechanic verify this as he'll be pissed if the motor burns down and you did nothing wrong. When the valve opens it dumps the water in the center section. That is why the stream shows different flow. 

                                                           Good luck,             Randy



Please submit whatever questions you have to  All questions and answers selected will be displayed on this page as they come in.  Note: though all boaters are welcome, priority may be given to I.H.R. members.  Randy can not be held responsible for any advice given.  Though his information and expertise is second to none, he has no control over what you do with your boat.  It is up to you to boat safely and act responsibly, and his advice is only to be used as a guidance for your high performance boat/motor of which you are the one responsible for the risks involved.

Mark C.


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