Mercruiser 3.0 knock

[Oldpos]

Anything's possible. Seen it happen twice in my life. One was a screw came out of the choke plate, didn't totally destroy the piston, but hammers it up pretty good. The guy shut it down quickly when it happend.
The other was a guys motor he just rebuilt. His girlfriend put the carb on and dropped a nut. She figured it would be ok, upon starting it immediately destroyed a cylinder and valves. Funny part is he kept the girlfriend and I think they got married also.[/

haha she took his nuts and lost them, at least he knows where she hid them!

 

[Scott Danforth]

agreed the motor needs to come out

not much different between the 1995 and 2003 motor as the major change was 1991 with the crank and block change.

 

[Scott06]

just a thought, a guy is selling a fiberglass boat that is rotted out near me for $500 includes complete motor, drive, tilt system, and trailer. the motor is a 1995 3.0 lx. what is the difference between my 2001 3.0 and a 1995 3.0 lx? i see the linkage is different for the throttle cables , but nothing else that would stop a straight swap over, am i wrong? im thinking i strip the boat for spare parts, sell off the tilt system and trailer, and junk the boat i should come out pretty good money wise.

the guy doesn't speak English very well or at all, so asking questions is tough but he said it runs. im thinking if i check the compression and look at the oil it stands a chance of being good. The main question i have about doing this is that i hear tons about cracked blocks due to freezing. im in georgia and the boat has a south carolina registration sticker, should i worry about cracked block down south? would the compression check and oil inspection reveal a bad block? he said it wont start because of a bad fuel pump, i should be able to rip the one off of mine and put it on his to check it, how would you quick wire this just to hear it run? do i just need the yellow red and purple wire hooked to a battery ?

No real difference between 95 and 2001 3.0. Yes you should be concerned about freeze cracks down south, I’d be more concerned there vs up north as folks may not be on top of winterization. On a 3.0 common crack point is under the intake/exhaust manifold where head meets block.the manifold itself will also crack if not drained

 

[Oldpos]

I stripped the motor down today, remember nbr 2 had 0 compression. I took the bearing out and they all look brand new , no unusual wear groves, and no slop in the bearing caps. I took out nbr 2 piston expecting to see cracked rings but no nothing they were also in great shape, I replaced them anyhow. Cam looked good, head and gasket were good, valves looked like they were seating tight. I’m getting ready to do another compression check to see if the new rings hold, except out drive giving me hell, gear box won’t go on last inch.

anyhow I’m not expect on piston motors , could I have blown the rings and they will not be cracked?

 

[alldodge]

I'm old but sure sounds like I missed something
First compression test yielded zero compression on cylinder 2
So took motor down and found no visual signs of issues, so put it all back together

And once together going to do another compression test

If I got that right. WHAT!!!!!!

 

[Oldpos]

I did replace the rings , doing another compression check because I don’t see anything wrong with the old rings, I’m at a loss ,

 

[Grub54891]

Entirely possible that the valves on number two were set to tight and not closing.

 

[alldodge]

I did replace the rings , doing another compression check because I don’t see anything wrong with the old rings, I’m at a loss ,

Its not going to go from running fine to not running fine without something changing.

Things can loosen, but rare they would tighten. So if valves were to tight they would loosen and then compression would get better. They wouldn't go from good to worst without something showing signs (push rod real loose). Unable to jsut look at valves and say they seal without at least an acetone test

 

[Rick Stephens]

except out drive giving me hell, gear box won’t go on last inch.

Do you have, and did you use an alignment bar?

 

[Oldpos]

Ok got it running again this morning, and as you called still a knock sound, ran a compression check, nbr 2 still 0. Took of the valve cover and watched the rockers move, nbr 2 exhaust does a double tap when it move. I backed of the rocker nut and compression check was good. Took off the side cover and watched the lifters and sure enough the lifter does a double tap (moves up and down twice in the same amount of Rhône the others move once. I assume I flattened the cam as they say. That sucks!!!!! I may put the originals cam back in, lifters are still bagged for their original positions. Don’t know I will think about getting a new one once I’m over being pissed off

 

[wrench 3]

A "flattened cam" Would normally give a shorter travel on the lifter not a double lift. Also it wouldn't produce a knock.
Is it possible that your getting a single lift but a two step drop. From something sticking when the valve is closing.
None of this however, explains the oil loss.

 

[Simoniz]

Ok so I got around to the motor today, it’s storming so it’s hit or miss working on it. But I did run a compression check.
the results were taken at 1st reading cylinder dry as in nothing added since last run, second reading was after I put oil in each cylinder, third after I rotated the motor for 5 minutes with the plugs out and oil in cylinder.
nbr 1 140,170,150
nbr 2 0,0,0
nbr 3 130,200,160
nbr 4 120,165,150

obviously nbr 2 rings are blown and it explains the blow by and having to advance timing to compensate for a dead cylinder, but not so sure about the noise, bad rings wouldn’t make a knock sound. I did not run the engine but as I was spinning it with the starter with no plugs I did not hear any knock. Maybe good news? Head gasket is good, maybe rob bearings good, maybe just pull the one piston and put new rings on it?


thAnks again for all the help, any suggestions are always welcome

Probably a bit late now, but your readings on Cyl 2 do not indicage that the piston rings are buggered. If they were, adding oil would increase the compression, the fact that the addition of oil made no difference points to a valve problem, hole in the piston or head gasket.

But it looks as if you are past that point now and have diagnosed the problem as cam / valve related. Good luck on sorting it!

 

[Oldpos]

as stated before i found the second lobe on number two cylinder cam damaged. i replaced the cam and lifters and test ran on the hose , everything ran fine. first time out on the water i varied rpm up and down as recommended then first time at about 2600 rpm the knock came back. upon inspection the same lobe on the same cylinder of the cam was damaged again. the valve was doing the same double tap. im currently looking for another motor, but even the machine shops around here have never heard of something like that, have any of you ?

 

[Grub54891]

Well, back in the day.... Chevrolet had a bad run of cams, how do I know? Went through three cams before I found out from a dealer that they had an issue with them. I replaced them myself, always the same lobe that went south. Once I found a good cam the issues went away. Chevy 350.

 

[Scott06]

if you are wiping out the same cam lobe, I would think there is a blockage in the oil supply to that lifter which I believe is what oils the cam lobe.would also verify there is nothing wrong with geometry of valve train on that valve no coil bind, retainer to valve seal interference, slot in rocker arm binding on stud. Any parts mismatch that’s contributing to cam wear.

You lubed the cam and broke in with high zinc oil made for flat tapped cam break in ?