Bad Engine Knock

[HerbieJ]

Hi All,
I just bought a used pontoon with a 98 Mariner 50 on it. It's a 4 stroke engine and reportedly ran well since it was bought new in 2002. The seller states that on his last outing he attempted to drive it onto his bunk-type trailer at low tide and briefly revved the motor, apparently much higher than normal cruising speed. Subsequently, he found blackish-gray oil oozing from the exhaust port. When he tried to re-start it, it ran, but he could hear a loud knocking sound from within. A boat mechanic told him the engine wasn't worth fixing.

What could happen inside a 4 stroke motor that could cause oil to come out the exhaust port and also create an engine knock? If it runs now (Briefly, at least), is it likely the motor "...is not worth fixing"? I've worked on car motors quite a bit, but this is my first marine motor. The seller thinks the head gasket failed, but that doesn't explain the oil coming from the exhaust port, (does it?).

Love to have your thoughts on this and what I might do to correct these concerns.

 

[emckelvy]

Re: Bad Engine Knock

Some sort of mechanical damage due to over-revving the motor. Only a teardown will tell you the true extent of the damage.

First thing to do would be to check compression before you dismantle anything. This may help pinpoint the problem to specific cylinders.

After that, pull the cylinder head for inspection. Check rocker arms and the rest of the valvetrain for any physical damage.

Check the head for any cracks or unusual 'wash' patterns in the combustion chamber.

In other words, if there are nice, even carbon deposits everywhere but one cyl, which is all clean, likely there's something going on there.

If you can't find any issues with the cyl head, you'll need to pull the powerhead and split the crankcase.

Here's a good site to look up parts diagrams for your motor, it'll give you a good idea of what you're up against:

http://www.mercruiserparts.com/SelectModelType3.asp?class_id=2

HTH & G'luck...........ed

 

[HerbieJ]

Re: Bad Engine Knock

Ed,

Thanks for responding. I wasn't expecting good news. I plan to "pick the brains" of a couple local boat mechanics and see what they think. Your thoughts are an iimportant resource for me: I wouldn't otherwise know what to ask or how to assess what they tell me. Hope I'll get a few other responses as well.

That website you gave me looks pretty comprehensive. It'll be a resource I'll use often. Thanks especially for that. If you think of anything additional, I'll be monitoring this thread for a while to come.

By the way, your "lieutenant" symbol looks a lot like "Navy". Were you in?

Herb

 

[emckelvy]

Re: Bad Engine Knock

By the way, your "lieutenant" symbol looks a lot like "Navy". Were you in? Herb

Nope, although I've worked around folks in the Service for many years in my profession, I was never enlisted.

Those rates/ranks are solely based upon the number of postings. You'll notice others such as 'petty officer first class', 'lieutenant commander', and for the veterans of the site, 'admiral', 'vice admiral' and 'fleet admiral'. I haven't quite got there yet, working on it though!!

Glad the info helped, keep us posted. One other thing that comes to mind is, when you pull the head, you can check each piston for slop at the top of the stroke. Also run the piston down in the cylinder to check the cylinder walls for scuffing/scoring.

Cheers.........ed

 

[HerbieJ]

Re: Bad Engine Knock

Just remembered something else: The previous owner told me that he "eyeballed" the engine oil level when he added some just prior to his attempt to get the boat onto the trailer, and that he may have added too much. I checked the engine oil level: it shows more than two inches above the "full" level. Could this be a factor?

 

[j_martin]

Re: Bad Engine Knock

Just remembered something else: The previous owner told me that he "eyeballed" the engine oil level when he added some just prior to his attempt to get the boat onto the trailer, and that he may have added too much. I checked the engine oil level: it shows more than two inches above the "full" level. Could this be a factor?

If it was leaking water into the oil, it'll "gain" oil. Not a good thing for sure, but also a bit encouraging. Water in a cylinder could cause a hydraulic locking, or knock if it isn't quite enough to lock it. None of this is good on the engine works in general.

If it hasn't been run like that, it's possible a head gasket, and a couple of oil changes to flush things out would fix it. On the other hand, if it was run with water in the oil, it could be thoroughly toasted.

Too much oil can cause a lot of problems for much the same reasons as water in a cylinder can cause problems. It gets into the ventilation system, and ingested into the combustion chamber, gumming things up.

Drop the oil. If there's water in it, pull heads and look for trouble. If it's clean, try getting the oil to the right level, service the pollution control stuff, and try it out.

hope it helps
John