Rod Knock

Recently I inherited a 19’ Boston Whaler with a 1986 Merc V150. The engine ran great at full R’s but wouldn’t idle. I pulled the crabs (of course they were all gummed up) cleaned them and the engine ran at idle but at full throttle seized up. I let it cool down and was able to start it and head back to the dock but again this time at about 2/3 throttle the engine seized. This time when I got it started I heard a rod knocking at the top of the engine. Should I tare down and rebuild or not waste my time and just spring for a rebuilt power head. From looking at Mercury’s parts express if there is crank damage a replacement is $2200.00. Boatmotors.com offers a rebuilt power head for around $2800.00.

Re: Rod Knock

In my opinion you have nothing to lose by tearing it down or having it taken apart so you can get a visual on the damage. You may just have a damaged piston and the crank is OK.<br /><br />If the crank and block are useable, then most likely it is less expensive to rebuild but the down time may be longer. To order a replacement powerhead from a stocking rebuilder should be quicker but more expensive. <br /><br />If the crank is bad, thats another story.<br /><br />Good luck!

Re: Rod Knock

Thanks for your optimism Dhadley. What do you think caused the seizure? The oil tank under the cowl was full.

Re: Rod Knock

Knowing that your carbs were "gummed up" makes me think you may not have been geeting sufficient fuel/lubrication to some bearings/pistons. Just my 0.02.

Re: Rod Knock

The engine seized after the carb rebuild.

Re: Rod Knock

Of course it's all guess work from where I sit, but from your first post, you claim engine ran full revs prior to carb clean... at least for some period of time. Is it possible that some engine damage was done at this time, then progressively got worse after? Of course it could have been damaged prior to you receiving it, or a number of other things. ??? Full revs under load on the water? or did you rev it up without a load on a hose? "should not exceed 3000 RPM's without Load on engine" quote from Merc.

Re: Rod Knock

Certianly thats a possibility. You also mentioned you had to let the motor "cool down". The damage may be from heat (water temp). Increased combustion heat also comes from other things like old fuel (more than 30 days), too high octane, wrong heat range plugs or wrong brand. Electronics comes into play also. <br /><br />The first step into determining why is to get it apart and see what exactly went wrong.<br /><br />Good luck!

Re: Rod Knock

The engine was run in the water on 3 occasions for up to 3 hours prior to carb rebuild. If damage was being done at that time I would have herd or felt something then. Is the thermostat a possible culprit even thou there was a constant stream of water from the tell tale?