1988 Mecruiser 3.7L 2 BBL. MERCURY 224 I L4, power loss under light to moderate load

[ZilligMarine0311]

I am working on this motor trying to find the root cause of power loss that sometimes occurs under light to moderate load RPM for 15m or more. We are hesitant to run it at higher RPM with the problems we are seeing.

The water side of the cooling system is cycling water nicely, but the coolant side is showing signs of corrosion on the outside and the inside is corroded too from what I can see from under the fill cap. The temp on the outside of the exhaust manifold was 340F + and on the intake side 220+ right before he started losing power. I used a laser infra.

A compression test while cold got me 180/185 on all 4 cylinders but the oil pressure is fluctuating slightly while running. I'm concerned about the mechanical integrity of the motor.

 

[Scott Danforth]

first, welcome aboard

second, guessing this is a new to you boat

third, you have no cooling water. at 340F you toasted your rubber exhaust hose and flapper. until you fix everything, stop using the boat

with 180-185psi on the gauge on your pressure test, you either have water in the cylinders or oil in the cylinders or your gauge is bad. on a good motor with a good gauge, you should read between 145psi and 150psi

start with pulling the drive and replacing the raw water pump (the whole thing, not just the impeller)

while the drive is off, inspect the gimble bearing, the u-joints and the bellows. do an alignment

then take the heat exchanger in to have it boiled out and serviced. any competent radiator or heat exchanger repair shop can help.

once that is done, pressure test the cooling system. if the engine overheats by as much as 10 degrees, you can spit the head gasket. get a new mercruiser head gasket.

make sure you also service your points and go thru your carb.

 

[ZilligMarine0311]

That is wonderful information, thank you very much.

My second question is, if I do all this and fix the whole cooling system, fix/clean the fuel system, replace the head gasket and get good compression readings is there any guarantee this engine hasnt suffered more serious damage like lower end problems? Is it normal for the oil pressure readings from the analog gauge to oscillate seemingly with the RPM? could there be wear in the engine causing this or could it be normal? Im aware it could be a connection or sensor issue but the fact that this engine has been severely overheated who knows how many times makes me wonder if I could have even more serious problems waiting to be discovered...

 

[Scott Danforth]

the lower unit is why you dont have cooling. you have to pull it apart to fix, so you may as well inspect everything.

you most likely have water in the oil from the cooling system and you most likely need a speedy sleeve (google speedy sleeve 470 yourself. the pressure test will confirm this.

 

[Alumarine]

"Is it normal for the oil pressure readings from the analog gauge to oscillate seemingly with the RPM?"
Yes, as the rev's go up the oil pressure should go up.
Pressure at 2000 rpm should be a minimum of 15 psi.

 

[ZilligMarine0311]

I mean the lower end of the powerhead, the crankshaft, the rods and the oil passages that lubricate it

The oil pressure gauge noticeably oscillates at a high frequency (like hundreds of times per min) even when the RPM is constant.

I am trying to assess whether or not I might still have more problems with this motor after I do whats needed on the drive unit, the fuel system and the cooling system.

 

[Alumarine]

It's probably the gauge or sender.
Buy a cheap manual gauge and hook it up to where the current sender is now.

 

[ZilligMarine0311]

ok, thank you all for all the good info and the welcome, I think you will all be seeing more of me