1986 Four Winns with 170 h.p. Merc...If I've Thrown a bearing in my boat motor, will it still run great?

Question

On the lake today and with no warning at all the most obnoxious rapping sound started... I mean LOUD! But, there was no loss of oil pressure, no loss of power...in fact, that particular trip of the day it was running as strong as it ever has... I was told it sounded like it was in the top end, maybe a "thrown bearing" or a "wrist pin...." either way, I obviously can't run it the way it is, but.... it WILL still run and run strong at that.. makes no sense to ME, but... then again, what do I know?!

Answer 1

Is this the in line 6 cast iron motor? Do you have a book? your going to need one.That set up was pretty strong. However you may have lost a lifter stud. In which case you will see little dents in the top of the valve cover.  

Next; You can't isolate a noise on a boat motor very easy. This could be a bad intermediate brg, or a broken u-joint. So if you have any question separate the engine from the gear box. to see if it gets quiet.(spray water up the pump hole in the face of the gimble housing. If you don't trust that slip the long "raw water in" line over the garden hose.

When you drain the oil, look for metal in the oil.

If you do decide to re build- get auto parts as they are exactly the same manufacturer as marine with the exception of the head gasket.

I'm thinking you don't have an engine noise. A wrist pin letting go is catastrophic. A bearing going isn't that loud-till it is ready to come out the side of the block. So I'm betting drive plate, intermediate bearing or ujoints.(also an automobile part)

wish you the best

Answer 2

Well the first thing for you to know is that whoever told you ,"it sounded like it was in the top end, maybe a "thrown bearing" or a "wrist pin....", doesn't know what they are talking about, seeing as how those are both bottom end problems.

If this is a V-6 engine, first determine which side of the engine the noise is coming from if you can, an in line motor don't worry about this step. Next get a broom stick or similar piece of wood. Hold the end of the stick against the engine block where you think the noise is coming from and put your ear against the stick up higher. The stick will amplify the noise if it's internal and you should be able to determine which cylinder the noise is coming from. After you determine what cylinder it is, remove that plug wire. The engine will miss, but if it's a wrist pin the noise should stop or at least quiet down somewhat. A spun bearing will keep hammering loudly.

If you don't hear the noise amplified through the stick, you simply have a blown exhaust manifold gasket, which in a boat motor sounds exactly like a rod knocking.

Answer 3

First thing that comes to mind is failed con-rod bearings, the most common internal failure on that motor.  Sticky lifters would make your motor sound like a lawnmower, rod bearings will make it sound like a lawnmower on crystal meth.  Any internal failure on a motor that old is not worth the repair bill.  The good news...... fall is the best time to buy a newer boat.  good luck

Answer 4

What you should know it that you DON'T run it without determining what's wrong!  Take it to an expert.

Answer 5

Get a mechanic to look it over as what ever it is won't be going away> Don't sound good or in expense>

Answer 6

The main problem with this engine is the fact that the cylinder block is made of aluminium and the cylinder head is cast.If you overheat this engine you will have a major problem.Anyway you are right it could be a wrist pin but it could also just be a collapsed lifter and this would be very noisy to.If it is a wrist pin you can either take it apart and fix it or keep driving it until it blows up.(Your choice) Good luck.