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Oil pressure drops then backfirevalve tap etc

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backfire101

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"Have a 2005 Volvo Penta OSE-3

"Have a 2005 Volvo Penta OSE-3 with 180 hours. When running, the oil pressure gradually drops from 60 to 25 over an hour or so. If running hard (4000 RPM for extended period of time), eventually get a backfire - easing back seems to fix it but recently when coming off plane the pressure dropped to below 20 and heard valve tapping and engine ran rough - started every 15 minutes after when docked and pressure gradually climbed back. What gives?"
 
"What grade of oil are you cur

"What grade of oil are you currently using, and how is the condition of the oil in the engine?

And out of curiosity, what kind of engine are we talking about?"
 
"Tom, if the oil pressure goes

"Tom, if the oil pressure goes down and the gauge is working well, I would look first at the oil condition (fuel or water dilution), and make sure the oil grade is correct for the engine (I believe yours is the correct one, 30W synthetic).

Traditionally, V6 & V8 marine gasoline engines in Mercruiser applications were running SAE25W-40 mineral oil, and I am currently using SAE20W-50 in my 5.7 because I removed the oil cooler long time ago and the oil pressure was dropping to below 20 psi at idle after a good hard run and the valves did not sound too good. Going to a higher grade of oil solved my problem.

If you run the engine hard (especially in hot weather), chances are the oil will get hot and the viscosity will decrease (and so will the oil pressure).

If it was my engine, and unless the manual said otherwise, I would probably try 20W-50 or 15W-50 synthetic and see if that makes any difference."
 
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