wierd problem / noise
 

A friend of mine has a small cruiser that's been acting up lately. The latest problem is a knocking noise that only occurs in gear, under load, at higher RPM (2500+). When you try to get on plane, it knocks really bad. It doesn't sound like a rod knock. I can't tell if it's coming from the engine or the drive.

It's a 25' Rinker with a 350 mag and an alpha one. The motor is a rebuilt unit, maybe 15 hours on it.

2 trips ago, it started occasionally loosing a little bit of power. Not too much, but needing more stick to maintain RPM, and the tone of the motor changed at a given RPM. Then it would come back. That same trip, we sucked something up, overheated, and got towed in. We hauled the boat and fixed the water pump. Ran it on the hose, everything seemed fine, put it back in the water.

Next time out, the power loss / RPM thing was a little bit worse. I suspected a fuel problem. Later that same trip, the knocking noise appeared. We stopped for lunch, and on the way back, the first time we tried to get on plane, it made the noise. The knock is not present at all at idle, so we idled back home across the lake. We suspected detonation, but that would have to be some pretty bad detonation. Anyway, we checked timing, set it to 8° base (it had somehow backed itself down to 4°base, but I didn't set it so I can't attest to the accuracy of either measurement) and replaced the fuel filter. We went for a test drive and the noise was still there.

Oil pressure is good, boat comes up to temp and stays where it should, and it doesn't make any funny noises at idle or with no-load RPM.

Might be wristpin, but I kind of doubt it. Last time I heard a motor with a bad wristpin, it made noise with no-load RPM, and it wasn't as distinct of a knock, IIRC.

Anything in the drive that could cause this?

Where should we start looking?

We're pretty well stumped. I told him to pull the spark plugs and go from there. I guess the next step if we suspect motor is a compression check, and if it's bad, the motor is supposedly under warranty. Could it be outdrive, though?

I would first try to determine if its the drive or the motor. Drain and check gear lube. A broken tooth on a gear is possible. It would get worse fast. Give the guy who built it a chance to hear if drive checks out. Good luck.
Jim

Sounds like a possibility. If it were a broken tooth, would it show up at idle speeds in gear?

Problem is, it's not free to pull the boat, since he doesn't have a trailer. If we drain and fill the gear lube, and that's not the problem, we have to put it back in the water to run it under load again. It becomes a $125 gear lube change.

I'll definitely suggest that though. Probably look at the spark plugs first, and maybe do a compression check. Those tests are free, and hopefully that'll rule out engine trouble.

it is a 15 year old alpha, so I wouldn't be surprised if the drive is fubared.

Well, after this last post, the boat sat in the well for another month then idled a few miles to be hauled out for the winter. The outdrive was pulled and no problems were noted. The outdrive seems to function fine when spun by hand, no roughness, it shifts fine, etc. Not sure if he drained the lube yet, but he did pull the plug for a second to check for water / contamination and found none. Normal amount of wear metal on the magnet and no water in the gear lube, the lube looked brand new (which it was, maybe 5 hours on it).

Gimbal bearing looks okay, so does the u-joint.

We're at a loss. I can't think of anything in the engine that would do this. No oil pressure loss, no indication that anything is wrong when the boat is idling. Temperature is fine.

The problem started with an intermittant low power condition, the boat would slow down and it would need more throttle to maintain RPM. Then it would go away a few minutes later. We stopped for lunch, and on the way home it started and idled away just fine, then made a horrific knocking noise when we tried to get on plane. Idled home, tried a couple of different things, still can't figure out WTF is wrong with it.

Is it jumping in gear like there is not enough engagement in to forward. Can you rotate prop shaft while moving linkage to determine how much engagement you have. I horrific noise should show it self.

It doesn't seem to be jumping in gear. It does it in reverse, too. It didn't do it right away when we put the drive back on, it started after a few outings.

Try a compression check. Had a similar problem with one of my motors. Temp, oil pressure, no Smart Craft warnings, etc. all fine. Boat ran good not much of a change, but noticed a knocking noise and it got worse under load. Did a compression check and the #8 cylinder had a bad piston and rings. No compression.

First thing that popped into my mind while reading post #1 was "Wrist Pin".
Sometimes even a compression test won't uncover it either.

take it out and brick it. you ll find the problem, along with a couple more you didnt know you had.

That thought did cross our minds. Take it out, firewall the throttles, and see what happens.

I guess it could be a wristpin, but in my experience a wristpin is more likely to make noise under no or light load. than under heavy load. This doesn't sound like the only other sloppy wristpin that i've heard.

I suppose it could be a bad piston / rings.

I told him to pull the plugs, inspect them, and then do a compression check. I don't think he did either. Maybe i'll be revisiting this thread again come springtime.

Are you certain its a nock and not a back fire threw the exhaust?Bad gas or water in carb will cause a backfire under load,drain the float bowls and see if there is water or debris.easy check.This will also hamper performance.

Take the filter out and check it for metal particles. Cost you nothing. My bet is on a spun rod bearing.

Ed, you and I had the same first thought!
Does the knock change as the engine warms.
Another thought, It is possible that the knock could be a broken piston skirt. I had that happen after a rebuild, a long while back. .02 worth...:D

who did the rebuild? do you know if the checked all the clearances properly? I have ran into this before. once was a wrist pin I knew that it was a pin problem I just didn't want it to be. The other time I did not check main bearing clearences, I was the only owner of the car sence new so when I rebuilt the motor I put all std. parts. It turns out the the motor from the factory had 20 over mains, and I did not check them. Take the motor out and start over. It's not that hard to get it in and out. I't could save you thousands. TOP FUELERS do a rebuild in 45 min. I'll start the clock GO.

It was a longblock bought from a company in florida and shipped. 1 year warranty, but by the time it's shipped, it's almost not worth it. No way to tell how they handled clearances. It was the cheapest motor he could find, and it was really all he could afford. ($4500 boat, $1500 motor, DIY motor swap)

I doubt it's a backfire. Too loud, too sharp, too consistent. I can't really say if it changes as the engine warms up, it doesn't seem to, but we haven't really run it hard enough to know. Once it starts knocking, he backs off the throttle. Hasn't gotten it on plane since it started. Generally warms it up at the dock before trying to get on plane, but I don't think it would go away if we ran the motor cold.

It's not a rod knock. I've heard enough rod knocks by now to recognize one of those. It almost sounds like it, but it purrs like a kitten at idle, and there's no oil pressure issues. Any knock i've ever heard is audible at idle, and sounds a bit more painful. We haven't checked the oil filter, though, that's a good idea. I suppose it could be a spun main bearing, I have never experienced one of those so I don't know if this noise is consistent with that type of failure. It does sound reasonable, since under light or no load the crank should hold itself in the center of the journal without a good bearing, but under load it would deflect and hit the edge of the journal, but like i've said, i've never heard a spun crank bearing to be able to diagnose. If the oil pressure is still good, though, who knows. I suppose a plugged or partially blocked oil journal could cause a spun main bearing without a loss in oil pressure..

pulling the motor out of the boat is very difficult. He doesn't have a trailer, but come spring, I can put his boat on my trailer. The first time we swapped it, we took it up to a friends shop with a 4 post lift. The lift only went up about 6 feet, so we couldn't get the boat under the lift. The best we could do was take a cherry picker and strap it to the lift... with the legs bent back. This gave us enough height to lift the motor up, but not enough reach. So while we were raising or lowering the engine, we had to push the engine forward (in relation to the boat) to clear the transom and drop into place. Luckily the cherry picker got us high enough that we could put a long chain on it and get enough room to push. All in all, it was a very shaky procedure. Took all weekend, and then some, by the time the drive was back on. We really, really don't want to have to do this again. We were really hoping for an outdrive problem.

http://www.mark8.org/users/sleeper/boat/dorislift.jpg

once it's out, the work on the motor is no biggie. there was no core on the new engine, so he has another motor with a presumably good bottom end. We can go through it and do it ourselves reasonably easily. It's just getting the damn thing in and out of the boat that's the problem.

I love that lifting rig you guys made up. I bet there's dead OSHA guys spinning in their graves! (Please be careful.)

I've seen a bad wrist pin make no noise at idle, but start thumping upon acceleration in neutral. It was on a "rebuilt" engine too. I isolated what cylinder it was by pulling a spark plug wire one at a time. The bad cylinder usually gets much quieter with no spark.

That's exactly what the (only) wristpin failure I heard sounded like. My buddy sold that car to some kid who put many more thousands of miles on it without failure, beating the piss out of it.

But this is different... makes noise under load but not in neutral. Which kind of has me stumped.

I think the crankshaft main bearing sounds pretty likely though. That could explain everything. Next step is to get back in the boat, check the oil filter, plugs, and compression, then go from there. I have a feeling we'll be pulling it and tearing it down.

My old school teaching always taught that a knock under load was a mainbearing....but unless it has a high volumne oil pump(or maybe a spun bearing) there should be quite a noticable drop in oil pressure(especially at idle). I will also add that on the few vehicles I have had with sloppy mains when 1st started always rattled pretty good or had a deep thlock,thlock,thlock sound until the oil filled the mains back up!

I had a simular problem, I thought what sounded like a rod knock was actually back fire. It was hard to tell with underwater exaust.

Would only happen some of the time, while trying to get on plane. The sound was rapid.

sleeper-dave, I don't know the answer to your engine knock but you should check this web site. In reference to your engine pulling thing, this site shows you why women live longer than most men.

http://www.gophergas.com/funstuff/womenlivelonger2.htm

It is very informative. Also, when you re-installed the engine di you guys make sure it was aligned properly so that you didn't damage the coupler?

yep, the alignment thing was quite the debacle. We played with it for a day and couldn't get the tool we borrowed to go in, so we finally said hell with it and towed it to a friends shop to align. He walked out of the shop with his alignment tool, slid it right in, and said "looks like you have it aligned just fine". So we tightened everything up and towed it home.

We'll continue diagnosis in the spring, or when the weather at least warms up a bit. Too cold and windy this time of year to do anything about it.

Another experienced person we consulted suggested that it was likely either a main bearing or a cam bearing. Sounds like we're going to be building a motor here shortly.

It sound as if it could be a lean pop. Caused by bad gas or intake leak. Had a friend have this problem before. Only under load and low rpm. Sounded like the engine was coming apart, but not too much like a backfire.

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I will let you know your problem soon I think I know have a simlar problem I think I have a small crack in one of my heads. When you lost power and rpm's did you r motor ever lock up or turn over real hard?