Merc Mariner 150; less power in water


Old 03-13-05, 10:40 AM
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Merc Mariner 150; less power in water

Hello all!

Seems like a great place to get an answer! I have a mercury mariner 150 II; 1989. It starts great on land testing with a water hose attached. Starts powerfully. Put it in the water and it becomes hard starting. I can idle forever and run it up to the max and down with no problems once going. Stop it and it becomes hard starting with not as much power. Can anyone point me in a direction to look?

Thanks in advance!
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Old 03-15-05, 02:45 PM
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A few things to check


Could be a number of problems. First off , are you able to pressure test each cylinder? Do that first to make sure you have full compression on all pistons. If one is weak then you may need a rebuild. You will need to buy a compression tester. Get a merc manual for your motor to find out the proper compression per piston. Next, check the ignition. Make sure you are getting a spark from each plug - may be miss-firing from the ignition and while there is no load on the motor she revs up fine on 5 cylinders but once in the water under load she can't. How is the temperature? Did it ever overheat? Is the black paint on the block bubbling or damaged at all due to overheating? How is your fuel? If there is water in the fuel then you will have problems similar to what you are describing. Change the water separator or better yet, rig a 6 gallon bypass tank and test in the water. Maybe on land the water is staying clear of the fuel pick-up but when in water it gets mixed in. Finally, check your coil connections and ignition wires - I had the same motor and had a rusted ground screw on one of my ignition coils. It was causing the coil to short out and the result was the exact problem you are having.

Good luck!


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