Engine Knock

[winstonwilliams]

I have a loud knock coming from my engine. The knocking is very loud some times and almost goes away other times. It gets louder and faster as the RPMs increase. I could not find a stethoscope around town, but I went ahead and ordered one off of ebay. I tried to use a rubber hose to funnel the sound to my ear, but I could not find exactly where the sound was coming from. I did notice that the knocking was louder inside of the valve covers. I would think that if it was a lifter or valve problem the noise would be louder under one valve cover than the other. But to me they sounded the same... What do you guys think?

Thanks

Winston

 

[midway]

My first thought is connecting rod, that is usually where the loud solid knock comes from, lifters usually have more of a tic sound than hard knock. Louder and faster with rpm increase it is definitely a moving part that has pressure on it that changes, rod knock.

 

[Henry Boyd]

There are no 'good' knocking noises that are made by a gasoline engine. You have done all you can do at this point. Its time to take the boat to an experienced engine tech who can properly diagnose where the sound is coming from and what needs to be fixed.

Henry

 

[kindredspirit]

We need more info to help you out, lots more.

Your signature says you've got twin injected 502's in a 290. Wow! What's the compression ratio on those, I'm sure they aren't original? I'm guessing you can't run the cheap stuff through them. What octane are you using and how old is it? Bad gas will make a hopped up 502 knock.

Is this closed cooling? Milky oil? Change in oil level lately? Any oil samples? Have you cut the filter open and looked for debris/bearing material? Affecting both engines or just one? Do both engines run off a common fuel tank or seperate?

What happened just prior to getting this knock? Any engine work recently? Overheating or loss of oil pressure? Engines don't usually start knocking all of sudden for no reason. Something normally happens that causes this.

 

[BarryCF]

Loud knocking on a boat engine usually indicates very bad news, most likely a rod. Do yourself a favor, do not start it up again, as you can do even more expensive damage. Either take it to a mechanic for diagnosis, or, get an oil sample and remove the filter and cut it open as suggested earlier. If you keep running it, that rod will fly and you will do damage to the block and more.

 

[winstonwilliams]

I am going to try to answer some of your questions:

The boat only has one 502 in it... My info page must be wrong.

Bad gas was my first thought... so first I added some octane booster... then I disconnected the gas lines and put them into a remote tank with new gas in it.

The engine has a open cooling system

Oil looks clean but I have not changed it since the knock started... Maybe I should try that? I guess something could be blocking the oil filter restricting the flow of oil... although the oil pressure on the dash says it is ok

Do you guys think the timing could be off? I could do a compression test and if all the numbers are low that could be the problem... right?

I have done a lot to the engine recently... The motor has had some trouble starting while warm, so I have changed a lot of the sensors, the distributor cap and rotor, the plugs and the plug wires... I have changed all of these back to the originals (except the plugs and wires) to see if that was the problem but nothing

Thanks for the help

Winston

 

[fc3]

Are you 100% absolutely positive that the plug wires are routed to the correct spark plug?

 

[kindredspirit]

Bad gas was my first thought... so first I added some octane booster... then I disconnected the gas lines and put them into a remote tank with new gas in it......How old is the gas? When you ran the engine off the new tank did it still "knock"?

The engine has a open cooling system...O.K. You're in Eastern VA. I'm ASSuming you're in salt water, right? How old are the risers/manifolds and when were they checked last? Any chance of a problem here? Risers and manifolds deteriorate pretty quick(+/- 3 years) in salt water and water can get into the cylinders if they fail.

Oil looks clean but I have not changed it since the knock started... Maybe I should try that? I guess something could be blocking the oil filter restricting the flow of oil... although the oil pressure on the dash says it is ok....If oil looks clean, and not "milky", I wouldn't change it....yet. I'd Not likely something is blocking the oil filter, you'd see a DRASTIC drop in oil pressure. I would remove the oil filter and cut it open, just to be on the safe side. This would be the first thing I'd do just incase a bearing is spinning on the rod. If you've never done it just follow these instructions:

1)wrap oil filter in "sandwich" bag and remove. The sandwich bag keeps oil from spilling into bilge and making a mess.
2)Preferably use a filter cutter to cut top off of filter. If you don't have a filter cutter use a razor blade, depending on who made the filter you may or may not be able to do this. . Do not "saw" filter in half with anything, you won't know if the shavings came from your engine or from sawing the filter in half.
3)Using a razor blade cut out a section of the paper filter that is about 2" X 2".
4)Collapse the section you just cut out like an accordian and wrap that section in a CLEAN cloth and put it in a vise to "squeeze" the oil out of it. If you don't have a vice, vice grips or a channel lock will work. The clean cloth catches the oil and prevents whatever tool your working with from contaminating the filter sample.
5)Take a pix of the filter sample when you open it up and post it here. If there is bearing material or shavings present you'll know it if you know what to look for. Keeping the filter sample clean is very important, that's why you can't "saw" the filter case in half with anything. Any "crap" will be stuck in the filter media....that's why you squeezed it with a vice or channel locks, etc. Anything floating in there was forced into the filter for you to see and doing this "squeezed" the oil out making contaminates easier to see.

Do you guys think the timing could be off? I could do a compression test and if all the numbers are low that could be the problem... right?......Yes, I think the timing could be off. You just replaced the cap and rotor-any chance you moved the distributor? If you do a compression test you would want all your #'s to be within a 10 to 15 psi of eachother. They wouldn't all be "low" or "high". I'd wait untill you answered a few questions about the oil filter, age of gas, etc.

I have done a lot to the engine recently... The motor has had some trouble starting while warm, so I have changed a lot of the sensors, the distributor cap and rotor, the plugs and the plug wires... I have changed all of these back to the originals (except the plugs and wires) to see if that was the problem but nothing.......what was the original problem? Hard starting? Hopefully the problem is somewhere in here....not in a spun bearing, etc. Double check and make sure you've got the correct plugs, gapped correctly. Double check the wires are going to the correct plugs. Both of these could cause a "knock" and are easy to mix up.

If it were me I'd check all these things before I attempted to start the engine again. Post what you find back here and we'll help you out.

Chris

 

[bushway9172]

About a month ago we were experiencing loud knocking as well. I'm not sure if it's the same problem that you're experiencing, but our mechanic has diagnosed our problem as missing gear teeth. Maybe this will help you? Maybe not? Anyway, good luck.

 

[Woody]

I have a loud knock coming from my engine. The knocking is very loud some times and almost goes away other times. It gets louder and faster as the RPMs increase. I could not find a stethoscope around town, but I went ahead and ordered one off of ebay. I tried to use a rubber hose to funnel the sound to my ear, but I could not find exactly where the sound was coming from. I did notice that the knocking was louder inside of the valve covers. I would think that if it was a lifter or valve problem the noise would be louder under one valve cover than the other. But to me they sounded the same... What do you guys think?

Thanks

Winston

Does your engine performance otherwise seem acceptable? When does the noise change? When does it almost go away, when does it come back? Is it different when the engines are cold vs hot? Are you giving gas, backing off, turning the steering wheel, triming the drive, etc? If your engine noise sounds the same at both sides of the engine it could indicate a lower end brg problem. While the noise from them can change with rpm they don't normally go away once they're there. Engine noise descriptions are always fun but I'll ask you if it is a knock or more of a thump? I'm wondering if perhaps you have bad u-joints in the drive instead of an engine problem.

 

[Jimmy Buoy]

If the boat is out of the water now, you could remove the B3 drive and try the engine without the drive attached. This will ensure that the knock is NOT from drive u-joints (since they are attached to the drive).

You obviously won't be able to use "Muffs" to provide cooling water. Instead, disconnect the raw water cooling hose off of the out-drive coupler on the inside of the transom and stick the water hose into it while supporting it vertically. Water will pour out until you fire up the engine - it will suck the water usually faster than the hose can provide.

If the knock is gone - it must be the u-joints on the drive.

 

[winstonwilliams]

f3c-

I checked and made sure the plug wires were routed in the correct firing order. That was one of my first thoughts!

kindredspirit-

The gas in the tank is a 3 or 4 months old... but the knock did not start until that tank was almost empty. I added octane booster to the tank and the knock was still present, so I disconnected the gas lines and placed them into a bucket of new gas and ran the engine for a bit. The knock was still there. So I don’t believe it is the gas

The engine is run in salt water.. But the risers were replaced two years ago. I did not even think about this... How can I check to make sure this is not the problem?

I will go ahead and cut the oil filter open this weekend and see what I find... Thanks for the step by step!

I just replaced the cap and rotor and I guess I could have moved the distributor but in my experience it is pretty hard to advance the distributor. but maybe I am wrong.. How can I check this?

I did notice that the plugs I took out of the motor where not the ones that the engine recommended... so I replaced the plugs with new ones that were recommended... I wish I had kept the old ones to put back in... I don’t remember what model they were. I did double-check the plug wires and they are in the correct firing order on the distributor

Bushway9172-

Your missing gear teeth were in the transmission? Did the knock occur in neutral or just when the boat was in gear?

Woody-

As far as I can tell the engine performs acceptable. It is hard to tell because I have not left the dock since it started. I am scared to push the engine at all in fear that I might hurt something else.

The knock occurs most of the time but every once in a while it dies down to almost nothing. But it does this randomly. It seems to be the same if the engine it hot or cold. The knock gets loader and faster as the rmps increase. I have not tried to move the steering wheel or trim tabs because the boat has not left the dock since it started

Bad u-joints in the drive would cause the knock only when the boat was in gear? Correct? This knock seems the same in neutral forward and reverse..

 

[winstonwilliams]

oh thanks again for everyones help! Hopefully we can figure this out!

Winston

 

[winstonwilliams]

Jimmy buoy-

The boat is in the water and I dont have a trailer for it, so I would have to get it hauled

 

[stevea661]

U-joints and gimbal bearing spins with engine running, in gear or in neutral. The trim of your drive would affect the noise level if it were one of these.

 

[bushway9172]

Bushway9172-[/FONT][/COLOR]

Your missing gear teeth were in the transmission? Did the knock occur in neutral or just when the boat was in gear?

Remember, this was over a month ago, so I'm not completely fresh on each individual occurrence; although, if I remember correctly, the knocking only occurred in gear. This does not mean that the knocking was completely inactive when in neutral, because it very well could've been a much quieter knocking, which would indeed make sense. The higher the RPM, the louder/faster the knocking. :huh:

 

[Woody]

f3c-

I checked and made sure the plug wires were routed in the correct firing order. That was one of my first thoughts!

kindredspirit-

The gas in the tank is a 3 or 4 months old... but the knock did not start until that tank was almost empty. I added octane booster to the tank and the knock was still present, so I disconnected the gas lines and placed them into a bucket of new gas and ran the engine for a bit. The knock was still there. So I don’t believe it is the gas

The engine is run in salt water.. But the risers were replaced two years ago. I did not even think about this... How can I check to make sure this is not the problem?

I will go ahead and cut the oil filter open this weekend and see what I find... Thanks for the step by step!

I just replaced the cap and rotor and I guess I could have moved the distributor but in my experience it is pretty hard to advance the distributor. but maybe I am wrong.. How can I check this?

I did notice that the plugs I took out of the motor where not the ones that the engine recommended... so I replaced the plugs with new ones that were recommended... I wish I had kept the old ones to put back in... I don’t remember what model they were. I did double-check the plug wires and they are in the correct firing order on the distributor

Bushway9172-

Your missing gear teeth were in the transmission? Did the knock occur in neutral or just when the boat was in gear?

Woody-

As far as I can tell the engine performs acceptable. It is hard to tell because I have not left the dock since it started. I am scared to push the engine at all in fear that I might hurt something else.

The knock occurs most of the time but every once in a while it dies down to almost nothing. But it does this randomly. It seems to be the same if the engine it hot or cold. The knock gets loader and faster as the rmps increase. I have not tried to move the steering wheel or trim tabs because the boat has not left the dock since it started

Bad u-joints in the drive would cause the knock only when the boat was in gear? Correct? This knock seems the same in neutral forward and reverse..[/quote]


Your drive shaft and u-joints are always turning whenever the engine is running. Doesn't matter if it's in gear or not. If it were me I would start the boat and then turn the steering wheel from lock to lock and move the drive up and down with the trim switch. If it is a u-joint problem it would be rare not to have some noticeable change in the noise. I would suggest doing this test both in neutral and in gear. Sometimes a little torque on the u-joint can make the diagnosis real easy. If this "knocking", as you describe it, is from the engine you've already got damage and so I wouldn't be hesitant to do the other quick experiment I suggested. If your boat is on a trailer you could hook up the muffs and do it, personally I think it would be better done in the water so the drive is under a load when you put in gear.

 

[winstonwilliams]

Bad news :-(

I started the engine today and trimmed it up and down while I turned the steering wheel and there was no change in the knocking... so it is not the u-joints..

Then I cut the motor off and removed the oil filter and cut it open.. There are metal shavings in the filter! This means the rod bearings are shot right?

 

[BarryCF]

Depending upon how you cut the filter, the metal shavings are bad news. If you cut the filter with a hacksaw, the shavings might be the cannister. I assume you did it the correct way, most likely bearing material. You could get an oil analysis done which will pinpoint the metal type and determine if it is bearings or something else. Doesn't really matter at this point I'm afraid. Whatever it is has been circulating in your system. If the bearings aren't shot they soon will be.

I'm sorry to hear this. Good luck with it.

 

[winstonwilliams]

I popped a hole in the side of the filter with a large screw driver and then cut the top of the filter off with a pair of tin snips.

Where do I get an oil analysis done? How much does it cost? Should I even think about trying to fix the problem or should I just think about repowering at this point?

Thanks

Winston