Mercury XR4 Low Water Pressure

I have a '91 Mercury XR-4, 150 hp outboard.  The engine originally had a small gear case and it maintained 15 lbs. water pressure at 5500 rpms.  The gear case broke and was replaced by a big gear case.  Since then the water pressure would fall to about 10 lbs when run at 5500 rpms.  The water pressure falls when the engine reaches around 5000 rpms.  It maintains about 15 lbs until 5000 rpms is reached.  This problem has steadily gotten worse.  When the gear case was replaced the pressure would fall to about 10 psi and now it's down to 7 psi at 5500 rpm.  The impeller has been replaced with no improvement.    I also thought about the relief valve but if that was bad I would think it would always only hold 7 psi.

I appreciate your help




Replace the entire water pump assembly. You need to be sure and only replacing all the parts will verify this. If this fails to improve, then go to the bypass valve. First replace the plate/housing/gaskets/and impeller. Make sure they put the water tube grommet under the guide tube. If it were missing that would cause the problem. 




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Mark C.


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