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Mercruiser 3.0 knocking and oil pressure fluctuate

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Mercruiser 3.0 knocking and oil pressure fluctuate

Old 04-30-2019, 07:15 AM
  #21  
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Originally Posted by sjef View Post
Take the outdrive off. Put it in forward, remove trim cylinders front the aft, remove the 6 nuts and it will slide right off. Two motor mount bolts at transom and the lags on the front mount. Pretty easy on those motors.
thanks man I plan on doing it today I’m ready to get to the bottom of this
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Old 05-04-2019, 09:46 AM
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Originally Posted by Swdemp View Post

thanks man I plan on doing it today I’m ready to get to the bottom of this
update I pulled the motor and dropped the oil pan to find gunk and small metal shavings I disassembled the oil pump and went to spin it with my fingers it spun a little and then stopped the rods bearings and everything looked fine so I’m going to go with a bad oil pump would y’all say the same?
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Old 05-04-2019, 10:05 AM
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I wouldn’t put that engine back in there without at least pulling the crankshaft out, check the condition of crank journals and replace the bearings.
Where did the metal shavings come from?
Get yourself an engine stand and bolt it up. Makes work sooo easy. Mark every part as to placement and orientation before removing anything.
Bearing caps must go back on exactly how they were or you’ll be in trouble. Lifters in same hole and rods on the same lifters and right side up , etc

If the crank doesn’t look scored , install new bearings , oil on the crank and bearing mating surfaces only , put plastiguage strips under the bearings and torque to spec.
Remove them and check clearances..
or machine shop..
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Old 05-04-2019, 10:11 AM
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disclaimer: i know FA about your particular model / make motor ! .
did you check the mains and big end , small metal shavings don't usually come from oil pump wear..
pick up > oil pump > crankshaft > camshaft > and so on through the system .? how do the piston skirts look.
keep looking there will be a smoking gun somewhere. the oil pump may have collected some crap that has run its way through the system and that would possibly be what is causing it to jam..as posted ,inspect the check valve.
and why did you run it with diesel in the sump , ? were you trying to make a bomb.
dacman , you beat me to it , i type slow .
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Last edited by 1breakinit; 05-04-2019 at 10:25 AM.
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Old 05-04-2019, 10:36 AM
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Cold motors with bad bearings often have decent oil pressure, but everything changes when the motor gets to operating temperature.
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Old 05-04-2019, 11:02 AM
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Oh , and new connecting rod bearings , orientation of caps is equally important.
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Old 05-04-2019, 12:15 PM
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Originally Posted by 1breakinit View Post
disclaimer: i know FA about your particular model / make motor ! .
did you check the mains and big end , small metal shavings don't usually come from oil pump wear..
pick up > oil pump > crankshaft > camshaft > and so on through the system .? how do the piston skirts look.
keep looking there will be a smoking gun somewhere. the oil pump may have collected some crap that has run its way through the system and that would possibly be what is causing it to jam..as posted ,inspect the check valve.
and why did you run it with diesel in the sump , ? were you trying to make a bomb.
dacman , you beat me to it , i type slow .
the motor had a cracked head that I replaced and that is where I believe the metal is coming from
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Old 05-04-2019, 12:29 PM
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Originally Posted by Dacman View Post
Oh , and new connecting rod bearings , orientation of caps is equally important.
I did find a metal shavings in my oil filter as well
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Old 05-04-2019, 12:46 PM
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I don't think a cracked head would put metal shavings in your oil. The cracked head could cause the water to get into places where it shouldn't be and that could cause some big problems and metal shavings. You should probably resign yourself to a total teardown and see what you find.
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Old 05-04-2019, 01:00 PM
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Put a oil pump in and run it before you put it back in the boat.
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Old 05-04-2019, 01:44 PM
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Did you check the metal shavings with a magnet? Will help differentiate whether it is bearing material or not.
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Old 05-04-2019, 03:42 PM
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The main bearings are most likely wiped out and giving you the knock , since it’s there even with proper oil pressure.
Maybe it’s a stuck lifter if hydraulic type ? A slight tapping before oil pressure builds is different than a knock.

Either way, you have to flip the engine over to change the oil pump. Why not at least pull a bearing and see what’s up if you want to gamble with it being “OK” with just a new oil pump.

If you want this engine to last more than a trip or two, id go thru it and check everything. BUT

You can replace the rod and crank bearings without disassembling upper portion of the engine. Engine stand. May get lucky this way.
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Old 05-05-2019, 05:53 AM
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The oil pump is driven by the distributor gear. That gear is not very strong and will shear if the oil pump seizes. You've got problems somewhere else. Thrust bearing surface?
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Old 05-05-2019, 10:58 AM
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I'd pop the caps on the 5 main bearings and the four rod caps. Easier to do rods one at a time. Upper shell on rods is loaded, lower on the mains. If one was knocking, the damage will be visible. If they look ok, can put them right back on and retorque to spec. Rear main may need a little sealing goo.
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Old 05-06-2019, 10:50 PM
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Originally Posted by Corndog38 View Post
I'd pop the caps on the 5 main bearings and the four rod caps. Easier to do rods one at a time. Upper shell on rods is loaded, lower on the mains. If one was knocking, the damage will be visible. If they look ok, can put them right back on and retorque to spec. Rear main may need a little sealing goo.
does anyone have torque spec?
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Old 05-07-2019, 04:09 AM
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Originally Posted by Swdemp View Post

does anyone have torque spec?
Google it. All GM specs are available online.
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