Engine knocking under load

[zsites]

I just pulled my engine last weekend (merc 120) due to a very loud knocking noise. There was no/ very little knock at idle but when the boat was in the water and under load the knock could probably be heard from the bank with the engine cover off. The floor is in need of replacement as well so I figured it had to be pulled anyway. This is my first engine pull and teardown. So far I have started from pan up and it appears the rod bearings look ok, however 2 of the main bearings have some groves in them. The journal of the crank however looks very smooth. I have not had a chance to take the head off yet to look at the pistons but judging by the loud noise it was making i dont know if the bearings are the culprit. Is it possible for a wrist pin to make a noise such as this? I should add I did have oil pressure during this.
My plan was to sell the boat this year for something more powerful but now that the motor is out and in need of work, are there any recommended alternative engines out there that work without too much modification?

 

[coheej]

Re: Engine knocking under load

Are you sure the noise wasn't from the drive area. From what you describe, it's doesn't sound like it came from the lower end or upper for that matter. Since it out look under the valve cover just to be sure. Make sure nothing came loose in the flywheel, coupler, and starter area.

 

[Bondo]

Re: Engine knocking under load

for something more powerful but now that the motor is out and in need of work, are there any recommended alternative engines out there that work without too much modification?

Ayuh,... A 3.0l, 140hp should be a Plug, 'n Play swap for ya....

 

[zsites]

Re: Engine knocking under load

Coheej I'm beginning to think you may be correct. I took the head off last night to take a look at the top end and everything seemed to be normal there as well. There was one wrist pin that was a little tight but nothing to make that noise I heard. I may have jumped the gun a little on pulling the engine, but I'm going to replace the bearings now that its apart. (Plus I was curious just to tear it apart). Do you have any ideas on what may have made that noise in the drive? Is there any way to tell if the coupler is bad?

 

[coheej]

Re: Engine knocking under load

Possibly U-Joints, but I'll have to let the other guys help you there, I don't have much experience with that.

When you put the motor back together you may want to loosen up that tight wrist pin.

 

[ussoldier_1984]

Re: Engine knocking under load

my gimbal bearing made a noise similar to your discription. almost non extistant at idle but under load it would make a knocking noise, replaced and its good now

 

[Bluestream]

Re: Engine knocking under load

Before spending more money on that engine, you may want to check the bearing clearances with Plast-guage. They are usually fine.

Remember the crank spins on a film of oil. and never really touches the bearing under normal use.

 

[OldNBold51]

Re: Engine knocking under load

having spent the last 6 weeks chasing a loud engine knock, even though I had only 55 hrs on a rebuild, my advice would be to keep looking until something obvious rears it's head. If it's as loud as you say, it'll be apparent.

I initially thought I was getting water past the intake gaskets into #2 combustion chamber evidenced by a super clean spark plug while all the rest were carboned. I changed the intake gaskets and still knocking. A leakdown test yielded 7 good cylinders tested with 1 cylinder with big leakdown #'s. I pulled the engine expecting to find a broken or cracked piston. Took the piston out and it was fine. (maybe the ring gaps were aligned....dunno)

Mine turned out to be a rod bearing chewed up (didn't look spun) with scored crank. My best guess is i had some abrasive trash hidden in the oil passage of crank evidenced by the wear pattern on the bearing/crank.

My point is, my knock was loud and it took some time to locate but once located it was very obvious.

 

[zsites]

Re: Engine knocking under load

I pulled the drive off last night. My u-joint bellows did have a tear in it. The gimbal had surface rust as well as the u-joints. I couldnt get much grip but just by reaching back in I was unable to turn the gimbal bearing. The u-joints seemed to turn ok but I'm not exactly sure what I'm looking for there. I did replace the gimbal bearing as well as all the bellows 2 years ago. Any ideas why the bellows tore so quickly? With the drive in forward gear i can turn the prop by hand and dont hear/ feel anything out of the ordinary. I should add I did replace the impeller this year as well. (At this rate I will have the entire boat rebuilt in a year)

 

[coheej]

Re: Engine knocking under load

I pulled the drive off last night. My u-joint bellows did have a tear in it. The gimbal had surface rust as well as the u-joints. I couldnt get much grip but just by reaching back in I was unable to turn the gimbal bearing. The u-joints seemed to turn ok but I'm not exactly sure what I'm looking for there. I did replace the gimbal bearing as well as all the bellows 2 years ago. Any ideas why the bellows tore so quickly? With the drive in forward gear i can turn the prop by hand and dont hear/ feel anything out of the ordinary. I should add I did replace the impeller this year as well. (At this rate I will have the entire boat rebuilt in a year)

The inside race of the gimbal bearing should turn easily and smoothly.
inspect the u-joint at each cup to feel for looseness and side ways movement. they also should be smooth. Some say that you can't check by feel. If you can break them down and look at the needles for flatness or corrosion that would be better. For sure the u-joint in a boat get much more use than a car's u-joints.

 

[ussoldier_1984]

Re: Engine knocking under load

The bellows could of torn from leaving the outdrive in the up position while in storage. It puts alot of stress on it, Like I said in my earlier post my mercruiser 4.3 was making the exact knocking noise you discribed and my gimbal bearing was seized like yours is. if you can move the u-joints smooth and you don't feel binding and its not too loose it should be good. My universal joint had a little surface rust but was good. The gimbal bearing should be very easy to turn if it is good.

 

[6meter]

Re: Engine knocking under load

It could have been a piece of carbon got loose in the combustion chamber. It will sound just like a spun rod bearing.

 

[zsites]

Re: Engine knocking under load

Pretty sure its the gimbal bearing. I still havnt had a chance to get the bellows off but it seems Tight. So I'm going to replace it and put everything back together. (Lesson learned there) Now that the motor is completley tore down, is it worth the money to try to bore it? As I said earlier I was planning on selling it for something with a little more power but I'm thinking now that I am doing the engine, floor, and part of the drive I really should keep it. Im just wondering if doing anything to that small of a motor will give any kind of noticeable effect?

 

[familyguy]

Re: Engine knocking under load

As stated earlier:

The inside race of the gimbal bearing should turn easily and smoothly.
inspect the u-joint at each cup to feel for looseness and side ways movement. they also should be smooth. Some say that you can't check by feel. If you can break them down and look at the needles for flatness or corrosion that would be better. For sure the u-joint in a boat get much more use than a car's u-joints.

The kind of noise you're describing doesn't sound like an iffy gimbal bearing. If that bearing is the culprit, you'll know it when you try and spin it. Pulling the u-joint appart to get a look at those needle bearings would be a major pain in the *** unless you have the right tool. When checking the u-joints grab the splined driveshaft and move the shaft in a cranking fashion so both u-joints are pivoting in every direction, push, pull while doing this all the time feeling for play or binding. The whole time it should feel silky smooth. If they're bad, you'll know based on the sound described. If the sound occured even while you were in neutral chances are it's somewhere in the engine if the gimbal bearing and u-joints test out okay. I would check the crankshaft bearings with plastigauge like Bluestream reccommended. When you described that sound in the first post a damaged crankshaft bearing was the first thing that popped in my mind.

Not sure what state of engine repair you are in, but I would take the valve cover off and turn that engine over by hand with a socket wrench attached to crankshaft pulley and watch all your valves and feel for anything binding. Everything under that valve cover should play out smooth...like instruments in an orchestra.

Good luck

 

[zsites]

Re: Engine knocking under load

The head gasket I removed was copper. Does the replacement gasket need to be copper as well or will some other material work just as good?

 

[GA_Boater]

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