8.1 Mercruiser Low Oil Pressue with Alarm

[lookintahook]

Hello,
I have a 2006 340 Sportsman, it has twin 8.1 Mercruiser (inboard) motors. I noticed the port side motor had developed a decrease in oil pressure. Typically, both motors are at ~50 psi @ 3200 RPM's and then drop to about 20-25 PSI at Idle. The port motor started indicating ~20 PSI at 3200 RPM's and then below 5 PSI at idle. The alarm will then sound until I throttle up a bit to get it out of the alarm low pressure range. It will continue to indicate 10-15 PSI lower than the STBD motor at idle after that but with no alarm. I changed the oil and filter and that seemed to solve the issue for a few trips out but it is now doing the same thing.
Any help will be appreciated

Thanks
Harry

 

[Seapig]

I’d start with a check with a mechanical gauge or swap the senders between engines.

Any deep knocking noise on a cold start?

 

[lookintahook]

I’d start with a check with a mechanical gauge or swap the senders between engines.

Any deep knocking noise on a cold start?

Thank you for the reply,
No, the motor seems to run perfectly, just the excessive drop in pressure (indicated) and alarm. I will check the pressure on each motor and follow up on this thread.

Thanks again

Harry

 

[lookintahook]

Hello again,
I just purchased an oil pressure mechanical gauge/tester, I believe that my motors have an oil pressure sending unit and an oil pressure sensor. Does someone know if I remove and connect the direct pressure gauge to either of these or is there a different connection location? Also, if the pressure indicates good, which of the sensors controls the indicator display/alarm?
Any help is appreciated
Thanks

Harry

 

[Seapig]

While I'm not 100% sure, I believe that era will have only 1 sensor and it'll be atop the motor toward the rear of the engine, just behind the intake manifold or distributor. If I'm wrong, it (or another sender) will be above the oil filter adapter on the bottom of the engine. (follow the oil lines from the remote filter location to the adapter)

You should test the pressure with the engine at full operating temp at idle. I'd get worried if I saw 10PSI or less. Keep in mind that the real operating pressure when the engine has been "working" and the oil is hot with much lower viscosity will be lower than what you'll see when you test it after simply running it without a load.

 

[Spark600]

When I was playing with race cars the guy that built my motors said he is happy with 10 PSI for every 1000 RPMs.
Not sure if it matters being a race motor that is being built to different specs

 

[P22]

Did you ever correct this problem? I have this exact issue right now.

 

[lookintahook]

Yes,
I first changed my oil and replaced the filter with the same size filter that was removed (Sierra 18-7944). This size filter was recommended to me because it was said that a larger/taller filter would interfere with the hatch closure. This worked for a few outings but the oil pressure then significantly decreased (~20 psi). I did some research and replaced the filter with the long version (Sierra 18-7945) (no clearance issue) and it has been running perfect (~50 psi) for this entire season. So the "short" answer is: If you have the smaller filters, try replacing them with the larger/long filters.