engine compression

[mr steve wayne]

My Sea Ray with a mercruiser V8, when I purchesed it for restore it has set for 4 or 5 years. I found the compression very low between 70 and 90 PSI. with dry compression test. only 2 cylinders were up to 120 or higher.
I was told by a friend that I should soak the cylinders with Marvel Mystery oil for several days, rotate the engine and soak the cylinders again. then do a wet compression test on all cylinders. Has anyone had experience on doing this with a Mercruiser 5.7,Mercury 150ELPO?
Or is there something better I can try to raise compression?
all answers are appreciated
Thanks

[mr steve wayne]

I for got to mention the hour meter only shows 638.3 hrs. is that a lot of hrs for a boat motor?

[mmwjr]

A small block GM marine engine that is raw water cooled should be good for 1000-1500 hours, however I’ve seen them die at 500 hours and go to 2000+ hours. Really depends on how they were cared for and used. The rings may have frozen from sitting and or the seller knew the engine was worn. I would soak with marvel oil as your friend said and after a few days get the engine running. Once running repeat the compression test but dry, adding oil will only give a false health of the engine. If the compression numbers are still down I would perform a leak down test to determine if the issue is a valve vs rings.

[mr steve wayne]

Thank You! that is the 2nd time I was tole about Mistery Oil. I have A some in my garage and will try it today.
Thank You
Steve

[shrew]

If the engine sat for 4-5 years, how did you do the compression test? More to the point, what did you do to the pistons and rings before moving the pistons in the cylinders? If a piston and ring are sitting for 4-5 years, I would have shot mystery oil down the cylinders every few days and let the engine sit for a week before I attempted to move it.

What can happen is the piston and ring and 'cold weld' together over time. If you don't divorce them first, you can effectively break the edge of the ring as you move them and break them apart. That alone can cause compression issues as there will no longer be a good seal between the ring and the cylinder.

Mystery oil is what I would have done FIRST. I'd be looking at new rings and pistons at this point. Oil isn't going to magically span the gap and restore compression.

[mr steve wayne]

Thanks for the advice I pretty much did what you said. This motor was free and easy to turnover. Mystery oil had been soaking for about a month. I went to do a wet compression test today and 1 cylinder that was testing at 68 psi with dry test a month ago tested out today at 130 psi. I am encouraged that the rest of them will come up also.
After testing 1 cylinder the starter went out so I have to replace that before I can test the rest of the cylinders.
I think all of you for your advice!
Thank You
Steve

[mmwjr]

Good to hear there’s improvement. Keep us posted.

[mr steve wayne]

Well i WOULD LIKE TO thank you all for your input. After soaking a couple weeks in cylinders with Marvel Mystery Oil. I did a wet compression today and I only one cylinder at the low of 105. the rest of them was 125 or better. T thank that is enough to start the engine.
The is hope for my restore project.
I am finding before I should try and start my 1984 Sea Ray I need to do some electrical work for the boat to be safe. I will post my electrical questions in another post.
Thank to all of you men that helped. Your advice is truly appreciated!
Steve Wayne