Very Low Compessions on Mercruiser 5.0 MPI Engine. with Exhaust Manifold, Thermostat Housing Removed.


New Member
Mar 23, 2024
Boat Info
Mercuriser , Sea Ray
Dear All, I have a Mercruiser 5.0 MPI Engine. I did a compression test with the left hand Exhaust Manifold and Riser removed, Sea Water Pump removed, Alternator removed, Thermostat housing removed. I removed these items because they was rusted or sezied and need replacing or unsezied or removal of rust. The Engine has not been started for over 4 years and some items have developed corrosion. (See the attached Photo of the intake manifold under the thermostat housing.

On the left hand side (Port Side) of the engine the results was Cylinder 1=100, Cylinder 2 =0, Cylinder 3 = 0 and Cylinder 4 = 170, this is a very low/bad result I have not tested the right side as yet. Does it make a difference that the hand Exhaust Manifold, Riser, Sea Water Pump, Alternator, Thermostat housing are all removed. As the results are bad, should I just pull of the head and inspect the head gasket and pistons, wall, values etc? Thank you in advance. Rob


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The pieces you removed have nothing to do with combustion.

Sitting for that long in that condition and not having the engine run recently is probably not a true representation of PSI - but most likely things are as bad on the inside as they are on the outside.

I have a feeling you're in for a complete teardown here. Be forewarned that this is going to be a rabbit hole. If you can do all of the work yourself, good. If not, you'd likely be better off sourcing a reman. Truthfully, you may be better off doing that, regardless.

How does the oil pan look?
First give us all 8 cylinders.
Next 2 & 3 are adjacent so could be blown head gasket. But after 4 years more likely stuck valves.
If you want to continue next diagnostic would be leak down test
I would start by soaking the cylinders with some oil first. Loosen the rings and clean surface rust of the walls.
Bore scope would be useful to see walls and valve condition.
O PSI is probably a valve stuck open. If you put 1 QT ATF in the engine you may loosen things as ATF tends to clean things. I used it once in a car and it helped. If it does not help you probably have valves that are burnt or hole in pistons. Not a mechanic. Just my opinion.
I would put marvel mystery oil in every cylinder let it sit for 24 hours and see if it will free the rings up once it gets running I'm sure the rings will continue to free themselves up and hopefully gain compression if you don't have a bad head gasket
Spray WD-40 into each cylinder crank the motor over by key or by hand to get it into the rings, let set over night then spray a light oil into each and turn engine over again. Had my one boat merc 305 sit for 8 years and it wouldn't turn over did the above and its been fine and running great for last 8 years, compression is back to normal. When a motor sits some of the valves are open and you get moisture in the cylinders and a light coating of rust on cylinder and rings and you need to get things loosened up, once you get motor running you'll see things come back to normal.

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