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Old 09-09-2008, 12:43 PM   #1
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Default Loud Knocking at Startup

So things are apparently going downhill.....thankfully it's late in the season. I have a 1997 2800 SCR with a single 7.4L. I started it up last weekend and had a loud knocking noise. The freequency/speed of gteh knock corresponded directly with engine RPM's. I let the boat warmup and it was still persistent. I took the boat out to get teh engine under some load and see what happened. After a few min. the knkocking stopped. The next day I ran the boat and it didn't make the noise again.
I fired up the othe rday and the knocking is back. This isn't a light 'clacking' like a valve, but a loud knocking. I checked oil level and it was fine.

Any thoughts?
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Old 09-09-2008, 02:45 PM   #2
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my first thoughts are rod bearing or wrist pin....but it maybe water injestion into the cylinder and finally clearing after warm up or something..since I'm not there ...I can't give a real difinitive answer...


do you think the noise is way down low in the engine or up on top....??


here's how you can isolate the noise...while the engine is idling....take a long pipe or stick....and touch the engine with one end and the other end stick your ear to it...being careful not to hit any moving fanbelts or pulleys...move the end on the engine and listen ...from place to place..you'll get different noises...cam ...tappet .....piston..etc....you will be able to pretty much islolate the noise to a cylinder and figure out what is going on........

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Old 09-09-2008, 03:34 PM   #3
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I'll give that a whirl and see how it goes. As always, much thanks.
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Old 09-09-2008, 08:40 PM   #4
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Default Knock knock

Well, lets just say that your description sounds very very familiar. I would describe my situation, but I think I can sum it up by saying, ditto (except this happened midway through boating season).
Did a compression check and 2 cylinders were at 0 and 20. Further inspection yielded blown head gaskets and block damage. Solution??? New long block.

Hopefully yours turns out to be an easier fix.
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Old 09-10-2008, 12:20 PM   #5
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I've also had someone outside of the forum suggest stuck lifter and 2 other people say it could possibly be the main bearing.
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Old 10-20-2008, 01:06 PM   #6
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This is a follow-up on this post [I personaly hate when folks post questions, then don't bother following-up ]

The sound is in the bottom end. So the verdict from several mechanics is wrist pin, or connecting rod. Whether it's a wrist pin, connecting rod, or main bearing, the motor needs to be pulled and completely disasembled. I'm told if were a lifter it would be more of a ticking, which this isn't. This is more like someone rapping a hammer on the engine. The short story is, we know for sure until the engine is pulled and completely taken apart, at which point since you're already there we might as well rebuild it.

So, the long and short of it is, we're looking at dropping in a new 496 MAG HO. we'll go from 300 HP to 425HP and get Fuel Injection. I don't need to fly in a cruiser, but it'll be nice to get on plane quickly with a boat load of guests.
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Old 10-20-2008, 02:54 PM   #7
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well....sorry to hear of this but I think I mentioned it above that what came to mind was the wrist pin or rod bearing....
Michiganmotorz.com sells long blocks for a decent price and offers a pretty good warranty with it....might wanna check them out....in the mean time...if your upgrading to a bigger motor....might also check the drive...make sure it can handle the higher horse powered engine...

a b3 drive would do it and offer better handling as an added benefit.....

keep us informed...thanks for the followup...


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Old 10-20-2008, 07:27 PM   #8
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It's funny SP, you pretty much called it based on the description.

I have a B3 now, which is rated for 400HP. Technically the HO is a 425HP. When I called Mercruiser they said they had some problems with the B3's on this motor and now only recommend the X & XR version of the drives. After some additional discussion, apparently this is only an issue for those who are jumping the boat and/or forgetting to come off of the throttle when the outdrive is out of the water. Well, I know better than to leave the RPM's screaming when the drive is out and don't typically find much opportunity for this with a 2800 SCR. I simply don't run the boat this way.
I SHOULD be ok, but we'll see.
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Old 10-20-2008, 08:13 PM   #9
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great...keep us informed....the drive hp rating would be my only worry with a larger hp engine...


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