Knocking noise and subsequent power loss

Stephen Kenna

New Member
Oct 4, 2018
27
Hudson, Fl.
Boat Info
2002 Sea Ray 2600 Sundancer
Engines
5.7 GM 350 w/ bravo III duoprop drive
Hoping someone else has had this issue and can point me in the right direction.
My 2001 SeaRay 260 Sundancer with a 350 EFI/Bravo 3 has a knocking noise that starts almost exactly after 45 minutes of cruising and has now made this noise on last 6 outings.
The motor starts effortlessly and runs super for 45 minutes then it starts to make a intermittent knock/clunk noise and then boat starts to slow down as if something is binding. (driveshaft/ujoints?) I slow down and can idle in gear with no noises but if I power up to plane out the knocking starts again. If I stay put for a half hour or so and then start up again I can plane out and cruise at 15 to 28 mph for about 25- 35 minutes and then it starts clunking again. I then idle down and make my way home thru the channels to my dock at idle speeds. There is no noise at these low speeds. No clunking when shifting gears and no apparent noise when turning the boat.
 
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I had a similar problem once with my last boat. It had a 350 as well. Once it got hot the engine would start tapping and had no power. Turned out a marina mechanic had changed the oil and used a low viscosity automotive oil. It caused the hydraulic lifters to not hold pressure once it thinned out when hot. An oil change back to proper Mercruiser oil fixed the issue. So if you have had a recent oil change, that may be the issue.
 
Creekwood, thanks for responding but do not believe that is my issue. The knocking noise is not like a lifter noise but a banging noise that is intermittent once it starts where it bangs once then maybe 6 or 8 seconds later then maybe 10 or twelve and so on. There is also a definite power loss like the engine is going to stall.
 
My two cents....I'd stop running her until you have someone take a look. You don't want to push it to the point of catastrophic failure...

Ohh and perhaps make sure your tow boat US membership is paid up for the year as well :)
 
For sure. It has gotten a little worse since I first noticed it even though it runs normal for first 45 minutes.
 
For starters, I don't have outdrives, so no first hand experience. It seems to me from your description that the engine is running (sounding) fine, it's just not pushing or driving the boat. Have you check the gear oil lately? Good level? Not milky? Also, do your drives have that rubber hub? Could be stripped. This would explain low power propulsion but not at high power. Just spit balling.

Jaybeaux
 
Engine starts and runs fine. Boat gets up on plane quickly and easily cruises at 30 mph on medium to light chop. After about 45 minutes of cruising it starts to make the clunking noise and then strains the engine slowing it down as if it was going to stall. If I keep it off for a few hours (anchored) she runs great for another 45 minutes only to repeat the earlier issue. This has happened the last 6 times I have taken her out. (Just bought this boat in October and due to some other issues I have not used it very much) Each outing has had this issue occur at almost the exact same location about 45 minutes out. Oil level is fine, engine purrs, mechanics have had the engine out and checked the gimble and u-joints while repairing some oil leaks and said they looked and felt fine. My feeling is that perhaps they look fine when "cold" but if under load for 45 minutes the u-joints may have dry grease in them that heats up and starts to bind causing the shaft to knock. My plan is to pull the boat out this week and pull the drive off to inspect the joints and drive shaft. Will also drain the oil and inspect for metal chips. Hopefully I find something definitive.
Thanks for your input.
 
Is the fuel tank vent blocked or restricted?

Simply removing the fuel tank fill cap when this happens can confirm or rule this out.

It’s an easy thing to try before pulling the boat and drive
 
i wonder if the temp in the engine room is building up while running at higher rpm for a while and possibly affecting the electronic ignition system....or it could be an ignition coil that is starting to go bad....this could explain the engine shutting down but not the knocking sound you are hearing....

cliff
 
i wonder if the temp in the engine room is building up while running at higher rpm for a while and possibly affecting the electronic ignition system....or it could be an ignition coil that is starting to go bad....this could explain the engine shutting down but not the knocking sound you are hearing....

cliff
I ran it for a while with the engine hatch open to try and determine if clinging/bang noise was engine or drive. Even with hatch lid open is still made the noise.
 
Is the fuel tank vent blocked or restricted?

Simply removing the fuel tank fill cap when this happens can confirm or rule this out.

It’s an easy thing to try before pulling the boat and drive


this a good possibility....the 'clunking' noise you are hearing could be coming from the engine being starved for fuel thus leaning out the fuel/air ratio until the engine stops completely.....i once had a fuel injector go bad which caused a lean fuel/air ratio in that cylinder and caused a knocking sound from the engine...i thought the crankshaft bearings had spun....as soon as the injector was replaced the knocking sound stopped...

cliff
 
Engine starts and runs fine. Boat gets up on plane quickly and easily cruises at 30 mph on medium to light chop. After about 45 minutes of cruising it starts to make the clunking noise and then strains the engine slowing it down as if it was going to stall. If I keep it off for a few hours (anchored) she runs great for another 45 minutes only to repeat the earlier issue. This has happened the last 6 times I have taken her out. (Just bought this boat in October and due to some other issues I have not used it very much) Each outing has had this issue occur at almost the exact same location about 45 minutes out. Oil level is fine, engine purrs, mechanics have had the engine out and checked the gimble and u-joints while repairing some oil leaks and said they looked and felt fine. My feeling is that perhaps they look fine when "cold" but if under load for 45 minutes the u-joints may have dry grease in them that heats up and starts to bind causing the shaft to knock. My plan is to pull the boat out this week and pull the drive off to inspect the joints and drive shaft. Will also drain the oil and inspect for metal chips. Hopefully I find something definitive.
Thanks for your input.
Next time you take the boat out and it starts to stumble slow the boat down put it into neutral increase the RPM in neutral and see if the engine still makes the same noises that way you can eliminate whether it’s in the out drive or it’s in the motor then report back to us .I have a feeling this might be a fuel issue if it turns out to be the motor
 
I see you mentioned that the mechanics check the gimbal, but it doesn’t take a lot for it to bind up. May look good while cool and expand with heat after running. I had one do just that on a previous boat with an Alpha 1 outdrive. Sounded like what you’re describing. For as cheap as it is to replace, I would consider it depending on when it was last done. I rented a gear puller from autozone and did it in an afternoon with a six pack of beer.

Check your bellows for and cuts or holes. It doesn’t take a lot of water to mess it up.

All the best,
Nic
 
Have you inspected the plugs and checked compression?
Wonder if high deposits in one of the cylinders could lead to hot spots and possible detonation under extended loads? Is it possible the engine is going into limp mode?
A cylinder with higher than normal compression could identify heavy deposits.
 
Next time you take the boat out and it starts to stumble slow the boat down put it into neutral increase the RPM in neutral and see if the engine still makes the same noises that way you can eliminate whether it’s in the out drive or it’s in the motor then report back to us .I have a feeling this might be a fuel issue if it turns out to be the motor
I have done exactly what you said and no noise was heard. Thanks for your input.
 
I see you mentioned that the mechanics check the gimbal, but it doesn’t take a lot for it to bind up. May look good while cool and expand with heat after running. I had one do just that on a previous boat with an Alpha 1 outdrive. Sounded like what you’re describing. For as cheap as it is to replace, I would consider it depending on when it was last done. I rented a gear puller from autozone and did it in an afternoon with a six pack of beer.

Check your bellows for and cuts or holes. It doesn’t take a lot of water to mess it up.

All the best,
Nic
Gimble bearing was replaced by previous owner a year ago. Checked by mechanic in February when engine was out and told it was good. But...like you said the engine was cold so it still could be binding when under load and hot.
 
Have you inspected the plugs and checked compression?
Wonder if high deposits in one of the cylinders could lead to hot spots and possible detonation under extended loads? Is it possible the engine is going into limp mode?
A cylinder with higher than normal compression could identify heavy deposits.
Compression test done in February and all cylinders were good. Engine purrs like a kitten and easily moves boat up to 35 mph on calm water with no hesitation.
 
Have heard the flat top style distributor caps are prone for problems causing cross firing as well. Would think you have started with the little stuff though. Is limp mode a possibility with your engine? Would explain the sudden drop in power. Could your clunking be popping in the intake or exhaust?
 

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