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Mercruiser 74 MPI

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David John

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I have a 1998 Mercruiser7.4 MP

I have a 1998 Mercruiser7.4 MPI that has a deep knocking noise when reved over 3500rpm. I was driving it at about 3000rpm and the warning buzzer went off so I shut it down. It started again but I have not been game to continue using it. Could it be the knock sensor ? any suggestions please.
 
"yeesh. Doesn't sound like

"yeesh. Doesn't sound like opportunty is knocking at your door. Won't be the knock sensor. Sounds like a "rod knock" as the result of an oil pressure issue, water ingestion or other issue causing warn out connecting rod bearings. Check for water in the oil (milky or excess quantity oil) and check oil pressure. If it is a rod knock, you will likely have low oil pressure. Rebuild is the only solution.

That's all i can think of at the moment. It could be the drive making noise i suppose. Hopefully you'll get a better opinion here."
 
"That warning buzzer is a grea

"That warning buzzer is a great thing, but wouldn't it be nice if mercruiser would back that up with a light that actually told you which errant parameter was causing the buzzer to go off?

Dave,
You didn't happen to notice what oil pressure and/or water temperature was when it was 'alarming' did you?"
 
"Thanks guys,oil and oil level

"Thanks guys,oil and oil level seems OK. it was running at 175 deg and about 35 psi. One other thing on start up it makes a knock for about 1/2 a second."
 
"It knocks for a sec until the

"It knocks for a sec until the oil pressure pumps up. Still sounds like rods.

My suggestion is to warm it up and, using a test gauge, check oil pressure against specs for the engine."
 
"David, am tending to lean you

"David, am tending to lean your way "rods" I did a compression test today all between 125 and 135, do you know if these engines have a history of doing gudgeons or more likely rod caps? I am hopeing to just do the bottom end, rod to crank caps and replace the oil pump,should I do crank mains as well ?"
 
"Are you going to try to do th

"Are you going to try to do the work in place?
If you have that much room, I'd do everything I could while the oil pan is off.
If you have to pull the engine, why waste the opportunity to do everything that can't be done with the engine in the boat?
You still haven't found the root cause... it may have spun a bearing, but why? If was oil starvation... why? Oil pump...why? Why only one bearing starved? If more than one, what's the extent? Why wouldn't the cam bearings be affected as well? etc. etc.

See where I'm headed? You never know what the extent and cause of the damage is until you start taking it apart... unless you pull the first rod cap and can say "Ah-ha... a lump of snot in the oil passage... it all makes sense." or something like that, you have to keep going until it does make sense. Otherwise, you end up wasting a lot of time and money and be no better off."
 
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