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1995 7.4 Merc KNOCK

C

colwoodshane

New member
Hi Guys,

Last night I took my boat out. Started both motors and observed 45psi on both, warmed up and we headed out. After about 10 min on a plane I noticed my rebuilt motor (less than 10 hours on it) started to drop oil pressure slowly. It came down to 30 psi so I thought i should stop and check. No oil in the bilge and the dipstick was reading full. Started it back up and low oil pressure alarm sounded after startup for about 8 seconds. Shut it down, went back into engine compartment to check remote oil lines etc. All looks good so i start it back up, getting about 25psi of oil pressure, I rev the motor up and NOOOOO. i hear a nasty clanking/knocking sound.

Limped back to the marina on other motor. Started it up when i got back to port. The clank/knock is not noticable at an idle and I now have 40psi at an idle. As i rev the motor up i can hear the clank/knock. Any ideas?
 
my guess a spun rod bearing. Oil from the pump thru the main bearings, thru the crank to the rod bearings.Lose a main bearing and oil pressure drops, no oil thru the crank means no oil to the rods.
 
Last edited:
Bt Doctur said:
my guess a spun rod bearing. Oil from the pump thru the main bearings, thru the crank to the rod bearings.Lose a main bearing and oil pressure drops, no oil thru the crank means no oil to the rods.
Click to expand...

A guy on another forum just suggested the same thing. What would cause this to happen? I did just remove break in oil and put proper merc oil in with new filter last week but I ran it to warm at marina with no issue right after then again last night when this issue happened out of no where
 
Typically loss of oil pressure is a result of a rod or main bearing issue.
Assuming oil pump did not fail.

Reason? As long as you did not rev engine to high which when freshly rebuilt and not broken in properly could result in this issue but not very common.

This in my opinion is the engine builder/ assemblers fault.
Either a tolorence for the rod/main bearing was wrong or possibly a fialed bearing.

Although not common it does happen.

Engine will have to come out and should be sent back to engine builder for inspection and resolution. Sucks but it is what it is....

Just my opinion
 
"This in my opinion is the engine builder/ assemblers fault."

Agreed! The rebuilder of my old 360 Mopar had such tight bearing clearances it spun a bearing when a rag clogged the intake and water flow stopped--and it took less than a few minutes.

Terribly sorry, but the motor has to come out.

Jeff
 
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