Merc 350 engine knock..help!
 

hey guys,

Little help.. my 350 mercruiser is knocking. Tried pulling each spark plug wire from the distributor to figure out if and which cylinder was causing the problem. Engine kept on knocking.

Here's a video of the sound.

Can anyone tell me where to look and how to fix this?

http://www.youtube.com/watch?v=VaDmeNrxidY

start with compression test

I'd use a stethoscope or large pry bar cupped to the ear. Either way, it's coming out. What color are the flakes in the oil? Copper - rod bearing. Aluminum- Piston issue. Seems to deep of a sound to be valve train related.

Maybe a stuck lifter?
If it was a rod bearing it should have got quiet when you pulled the plug wire (or the 2 wires for a particular main bearing). Hows the oil pressure?
One time I started a 350 not knowing there was water in a cylinder, it hydrolocked bending a rod which caused the piston skirt to hit the crankcounter weight, also sound like that.
Just some other ideas

Stuck lifter- nope wouldn't make that sound.
Rod Bearing- of course it would still make that noise. Not under load, it's just flopping around. This isn't a car in gear.
Bent rod- you had water in the oil and still ran the engine?

Dude, pull the engine and stop making it worse.

Its sounds like its in the lower end, so the engine will have to come out. That is too loud of a knock for the valve train.

Stop running it and pull the engine.

Typical winter time OSO, jumping all over someone with a simple response. The water was in a cylinder from a leaking head gasket, no water in the oil at that point(never said water in the oil either). Usually a rod bearing will get quieter when you pull that cylinder plug wire, maybe not completly go away.
Proceed to bash away.......

Ditto.. .by continuing to run it and listening to noise ...potential for catastrophic failure rises...

Pull it out, I agree that is not valvetrain noise. Slight chance it is loose coupler bolts or maybe even a really bad u joint in the drive but chances are bearings are hammered.

Sounds like piston slap or a rod bearing. Like it was said above, it's time to pull her out.

Piston slap or scuffed pistons isn't usually that consistent in my experience. Unfortunately I've dealt with it a few times

good news is its only a 350 .. .. just saying ...

Sounds like rod bearing. Diego's has a nice set of 383s for sale. Cut another whole and make twins. Lol

Bent rod, pull it.

I would pull the valve covers and take a look, probably comng out but. I would say lose coupler bolts, bent rod,or valvetrain. To me it does not sound like a rod bearing.

Does anyone know where I can find a short block?

You can buy gm crate motors pretty cheap Change gaskets cam and have a brand new motor for about $1800

http://www.youtube.com/watch?v=K7Z8I0CvEVs

Sound familar? We did the same thing--pulling spark plug wires and couldnt isolate the problem. Checked the valvetrain as best we could and just couldnt locate the problem so the engine was pulled.
It ended up being the number seven wrist pin was bad and causing piston to score.

I've got a drop in ready big block I can sell cheap

\\\\why not just pull the valve cover and check if a rocker arm got loose hammering the lifter. Happened to a friend's 67 Mustang fastback recently. It started with the hammering I could hear driving next to him. Told him to pull over to the curb and pull the cover but he ran it for another 5 minutes and kaboom, rocker stud broke. Luck has it I still had some studs from a 429 at home, so I machined one down to accept his rocker, so we could fix it on the road taking the car home to check the whole engine later.

Mine made a sound like that when one of the exhaust valves let go and trashed 3 more cyls.

If its a rod knocking it won't have any oil pressure. What's your oil pressure at idle? It don't sound like a rod knock but its hard to tell threw speakers.

Sounds like a rod bearing to me. I spun one in my old truck with a 305 (originally I thought maybe it was the transmission because it was waaaay low on fluid, turned out to just be a coincidence), here is a couple of my videos for comparison:
http://www.youtube.com/watch?v=yx57kMkpIXM
http://www.youtube.com/watch?v=mQZFcyccfb4

The squeal is the rod bearing working its way out between the rods I think. Made for some good pictures when I tore it down.

Sometimes I would lose oil pressure until I rev'd the engine, then the pressure would come back, other times it was just fine. When I pulled the engine and tore it down the crank was wore down about .040; I knew the engine was bad and needed replaced but I figured what the hell, one more run to the landfill won't hurt it any worse....:D

I also did the pulling of the spark plug test, no change was heard. The knock was the piston slapping the head in my case.

Oil pressure is normal. No issues there.

I guess u will have to pay $75 bucks to get it looked at by a mechanic at the closest marina. It could be lots of things. If u can take some stuff apart and put them back then go with some suggestions givin here. Pull valve cover look around. Use a long screw driver with your ear on the end and listen around with it. If u got good oil pressure it can't be a rod knocking since its that loud!!

I have not read the entire thread.... If it has not been said, the first thing you need to do is too is take off the oil filter and cut it open. If you have any metal, more than likely you have a bearing going bad.
And yes you can have good oil pressue and have a bad rod bearing/s. Seen it several times, you might notice a small drop in oil pressure.

Yep had a 350 with a rod out the side of the pan and 30 psi oil press at idle. Throw that theory out

Ok there big boy what ever u say!!

Ok im very confused over here.

We pulled the engine and started it out of the boat and the sound disappeared. We also removed the outdrive too. When we put it back in the boat the sound came back. Removed flappers and its still knocking. Did the spark plug pulling and still loud as ever. Removed the belt and started her up and still knocking. Open the valve train all ok. What is going on here? I messed around with the screw driver and my ear and it sounds like it is coming from the intake. No leak from exhaust manifolds either.

Mechanical fuel pumps can make a noise like that. Sometimes sound can travel through other components and confuse you as to its source. I would cut the oil filter open. Fairly simple process - any engine shop should have the proper tool to cut the case open.

you certianlly do have an attitude problem,he is 100%correct,i have seen spun rod bearings and loud nocking noise with 30+ lbs of oil pressure.you like to argue when you are wrong but people are starting to see what you are.you are not making any friends with your attitude dude.if you want to argue with me have at it,im your huckleberry.

Dang this guy is taking a beating , think he ruled out lower end noise , even the elusive cracked piston skirt noise that beat me up once . Why is the noise gone with eng on stand ? The exhaust was baffled somehow yes ? Bad u joints or coupler slip fit ?

Just watched that video again damn that's loud at idle seems to subside when goosing it wonder if the noise would still be there with the flywheel mounted biscut coupler removed

when the engine was run out of the boat was it on a run stand or just on the floor,did it still have all the marine stuff on it.need more information to try and help you figure out what is causing the noise.

Check filter first as stated above first. Then maybe it's something in the drive making the noise. If its the fuel pump rod I would not see how that would change from in the boat or out but stranger sh!! Has happened!!

I ran the engine in the boat with the drive removed to see if that was the cause. Sound still there.

Engine was on the floor.

All accessories on including the waterpump.

Well if my engine had 30lb oil pressure I would be concerned. That's not enough to suit me. The loud knocking noise is not from a bearing going bad. The bearing is already gone. That's why I asked about oil pressure. I don't give a hoot about making friends. I'm not looking around to do that. I have been called worse things by better people than YOU!!

Maybe since its down in the engine bay it makes it seam louder than it really is. Maybe u can't hear it real well on the ground???

Bell housing, coupler still on? Sounds too loud but maybe pieces of old flappers in the y pipe?

il bet you have been called worse things on a regular basis.every time you open your mouth you prove your ignorance!glad you are not trying to make friends.