Oil pressure loss 50 Mercruiser

"Hello, I have a 1992 Bayline

"Hello, I have a 1992 Bayliner 3055 powered by twin Mercruiser 5.0's (carbs). On the conclusion of a 70 mile trip I noticed a Clacking sound from the port engine when docking, the oil pressure was good, but I was at idle docking. Oil level full, Temp ok. I decided it was my imagination. Yesterday took the boat out and made it 7 miles out and noticed an RPM drop on the port engine that gradually became larger (600 to 800). Slowed to an idle and noticed the clacking, only louder, and the engine seemed to be making a louder exhaust noise. Then remembered to look at the oil pressure, there was none. Shut down the engine and returned to dock. I restarted after it cooled down. The clacking noise was gone but the louder exhaust noise was still there. The oil pressure was back also (35 psi).

Why would the pressure be good at a cool idle? Did I damage the engine? What should I do next?

Thank you"

"I guess the most obvious ques

"I guess the most obvious question is how is your oil level? If it's up to par, about the only reason you would lose oil pressure is if the oil is foaming (not too likely) or your oil pump is getting flakey for some reason (again, not likely). Or perhaps your oil pickup is partly plugged off.
The clacking sound could be a dry lifter, which could happen from lack of oil. Or it could be shot connecting rod bearings.
I am thinking you are going to need to R&R the engine and have it torn down for inspection and repair. How many hours on it? Do you change oil regularly? What oil are you running? Are the engines equipped with oil coolers?
I also think you need to get the opinions of some of the other experienced contributors on this site.
Rod"

"A simple test for internal en

"A simple test for internal engine damage is to drain the oil through a large medical gauze bandage which will trap metal particles and other crud. Pull the oil filter and drain it upside down and punch a few holes in the bottom with a nail so it drains faster. It can be cut open for inspection of the filter media for fine metal particals.

If you can drop the oil pan w/o pulling the engine, it would reveal a plethora of the engine's condition showing how much sludge has accumulated and any metal chunks it contains over 15 years from less than perfect engine oil service.

The exhaust noise could be a perforated exhaust manifold and the clacking could be from, as Rod says, dry lifters from "0" oil pressure or just a collapsed lifter.

Fuel from a leaking carb would thin the oil and reduce oil pressure somewhat until it vaporizes from hot oil. Thinned oil will not lubricate properly under a load and may reduce oil pressure.

A loose flapper that was jammed would cause back pressure, reducing RPMs, and if it finally broke loose and is no longer blocking the exhaust RPMs would improve.

Change the oil filter and the oil thru the pan drain plug if possible and use the gauze. Check the oil for fuel contamination. If you see metal fragments or shavings, you will need to R&R the engine for teardown per Rod.

Check the flapper and inspect the exhaust manifold for cracks or perforations from rust. Pull the oil sending unit and replace with a test gauge. Old sender may be plugged with sludge or failing. Fire it up and see what you get."

Yikes. Sounds like a rod knock

Yikes. Sounds like a rod knock. Rod noise becomes more pronounced as the motor warms up and the oil viscosity thins out. Dry lifters would still make noise after cooling off. All good advice. Let us know what you find in the oil.

Guy & David;
Thanks for j

Guy & David;
Thanks for jumping in here. Those are all valid points and suggestions in my humble opinion.
Ken; you have some work to do.
Best of luck and let us know what you find.
Rod