OIL PRESSURE DROP AFTER RUNNING HARD / PERIOD OF TIME
500EFI’s ~ 100hrs since rebuild
308 Dart Iron Eagle heads, 741 cam, AZM Throttle Body, Stainless Marine Exh. Manifolds, No Oil T-Stats, 140* T-Stats. Mobil 15w50 Synthetic oil. Cleaned and Checked Oil Coolers

I have an oil pressure difference of 25 psi from one engine compared to the other after running hard for a period of time. Only after 3500 RPM, does the oil pressure gauge slowly drop over time. However, it comes right back after coming off plane and idling for a few minutes. Only the Starboard motor has been doing this since I got the boat 3 years ago and I can’t seem to figure this out.

Hoping someone has seen this before and can shed some light and help me solve this.


STAR ENGINE 515CI

OIL
RPM PSI TIME FROM STARTING OUT
---------------------------------
2500 58 1 minutes
3000 58 2 minutes
3500 58 3 minutes
3600 48 3 minutes
3600 45 4 minutes
3600 45 6 minutes
3900 45 11 minutes
4000 40 12 minutes
4000 40 13 minutes
4500 40 14 minutes
5000 33 15 minutes - then back to idle
1000 20 1 minute
1000 25 1 minute
1000 30 2 minutes
1000 35 2 minutes
1000 40 3 minutes

PORT ENGINE 509CI

OIL
RPM PSI TIME FROM STARTING OUT
---------------------------------
2500 58 1 minutes
3000 58 2 minutes
3500 58 3 minutes
3600 58 3 minutes
3600 58 4 minutes
3600 58 6 minutes
3900 58 11 minutes
4000 58 12 minutes
4000 58 13 minutes
4500 55 14 minutes
5000 50 15 minutes - then back to idle
1000 40 1 minute

You`re aerating the oil for some reason.
How deep are the oil pans, is the pickup shrouded enough?

Stock 500EFI 8 qt pans and coolers on both motors

Did you remove the stock oil T-stats???
You need to measure your oil temps. Possibly the oil is bypassing the oil cooler.

For sure start with oil temps... that tells a lot...
then use mechanical gauge, could be a simple sender.
new engines? or have they always been this way? if new cut oil filter open also, just to be sure.
Tuned? I know my oil temps changed when engines were running much leaner, maybe one is off..

Griff, Full Force - I did remove the T-Stat from the trouble motor (star) during the rebuild. The other motor already had them removed by the previous owner. The oil temps range seem ok (160-190*) from the gauges on my dash. However, the Star motor gauge always shows about 20 degrees hotter after a hard run.

Im going to try connecting a mechanical gauge as suggested and see if the sensor is fooling me . Thanks guys.

I had an electric sender do that. Put a mechanical gauge on, the pressure was fine.

That would cause the change in pressure you are seeing.

Are you running the stock 500efi program?

Debris in cooler tubes?