Apparent low oil pressure after coming off plane

[helix757]

I have a new to me boat that has a 6.2L MX MPI Mercruiser engine. Upon a cold start, the oil pressure gauge shows 35-38PSI. Once warmed up, it settles at about 20PSI. I cruise at about 3500RPM, and while cruising, oil pressure reflects 30-35PSI. However, I noticed when I get off plane when entering a channel, the oil pressure gauge drops to what looks like 6-8PSI. It slowly comes back up as I assume the oil cools. Temps at all times never go above 165, and the engine sounds fine -- no ticking or anything. Also, no alarms. Just the normal two beep test when key is turned to the on position for the system self-test.

Engine was professionally serviced including an oil change with a new filter.

Is this behavior normal?

I tested the gauge by tapping the signal stud to ground, and the needle does go to the max pressure. I am thinking about replacing the oil pressure sender as it does not look OEM. Also, I cannot seem to find the oil pressure switch for the alarm. It is not a "tee" fitting with the sender unit.

Any insight would be greatly appreciated!

 

[Chris1956]

Any idea what oil was put into the engine?

The oil pressure switch grounds when there is no oil pressure. Look for a "Peppermint Patty"-like device with a single wire on the block.

 

[helix757]

Any idea what oil was put into the engine?

The oil pressure switch grounds when there is no oil pressure. Look for a "Peppermint Patty"-like device with a single wire on the block.

Good question, texting my mechanic now. What oil SHOULD have been used?

I will take a trip down there to try to look for the pressure switch again.

 

[Scott06]

Good question, texting my mechanic now. What oil SHOULD have been used?

I will take a trip down there to try to look for the pressure switch again.

25W-40 quicksilver or equivalent. Not to turn this into an oil thread - you just dont want 5w-30 or any of the thinner oils typical of modern automotive use.

Given you are not getting an alarm- I would measure the OP with a mechanical gauge to vet the dash gauge. Would not be the first time a sender or gauge wasn't accurate.

Would also check the oil for signs of fuel contamination (smell)

 

[Scott06]

Good question, texting my mechanic now. What oil SHOULD have been used?

I will take a trip down there to try to look for the pressure switch again.

if it is the pre 2015 ish chevy based 6.2 the oil pressure switch will be by the distributor and as Chris stated single wire ground it and you should hear the alarm- not a bad idea to test

 

[helix757]

if it is the pre 2015 ish chevy based 6.2 the oil pressure switch will be by the distributor and as Chris stated single wire ground it and you should hear the alarm- not a bad idea to test

It is a an early 2000's for sure, probably the first generation to be honest.

Looking at the engine head on, the oil pressure sender is located on the back right, under the exhaust manifold.

Would the oil pressure switch be on the bottom side of the block too, or on the top side?

 

[Scott06]

It is a an early 2000's for sure, probably the first generation to be honest.

Looking at the engine head on, the oil pressure sender is located on the back right, under the exhaust manifold.

Would the oil pressure switch be on the bottom side of the block too, or on the top side?

on my 5.0 and 4.3s the sender and oil pressure switch(2wires- for the electric fuel pump) are in a T above the oil filter housing on block, the alarm is a seperate switch with one wire on the block just next to dist.

 

[helix757]

on my 5.0 and 4.3s the sender and oil pressure switch(2wires- for the electric fuel pump) are in a T above the oil filter housing on block, the alarm is a seperate switch with one wire on the block just next to dist.

Got it.

So on your engine, one switch cuts the fuel pump, the other switch, in another location, operates the alarm.

 

[helix757]

Also, wanted to show the dipstick. Is it overfilled?

 

[Chris1956]

Is the boat engine level? A list can affect the dipstick level.

 

[helix757]

Is the boat engine level? A list can affect the dipstick level.

This was in the water, me standing in the engine bay -- same side as the dipstick.

 

[helix757]

on my 5.0 and 4.3s the sender and oil pressure switch(2wires- for the electric fuel pump) are in a T above the oil filter housing on block, the alarm is a seperate switch with one wire on the block just next to dist.

Curious, what color wire do you have going to your pressure switch by the distributor?

 

[Fun Times]

Curious, what color wire do you have going to your pressure switch by the distributor?

Post number 9, yes it appears the level could be roughly a quart and a half over full which could also affect oil pressure readings…. Did this oil issue start before or after the oil change?

Engine serial number is always nice to have to confirm correct engine parts to be used/shown but item number 7 is the oil switch to sound the warning horn when pressure get s to low at anything under 4psi,

https://www.marineengine.com/parts/mercruiser-sterndrive-parts/mx-62l-mpi-bravo/0l680003-thru-0m299999/senders-and-sensors

The location for the oil switch is often up higher, rear port side of engine to the left of the distributor assembly. Port side is the left side when facing the bow of the boat… if you turn around and face the front of the engine, now it’s on your right side towards the rear of the engine. Found an image of one…. This mentioned, this item in question shouldn’t be your fluctuating issue.

Boat Tech Help - Mercruiser, Volvo Penta, Outboards, Engines Q&A | 1999 Mercruiser 5.7l efi: | Facebook

1999 Mercruiser 5.7l efi: Can someone guide me where to find the oil pressure limit switch? Edit: I think I got it, behind the distributor cap.

www.facebook.com

 

[Lou C]

I would:
correct the high oil level, it is possible for the crankshaft to hit the oil in the pan when it is too high and that can mix air bubbles in the oil which could lower pressure...
hook up a manual gauge to your oil pressure port on the engine to see if the readings on the manual gauge are the same or different...
Your readings appear to be low even with a cold engine.
What I've always gotten was 45-55 on a cold start, fast idle, 40 after coming off fast idle, 50-60 when running on plane, and after coming off plane and idling, 25 psi. This is with an original 1988 short block....

 

[helix757]

Post number 9, yes it appears the level could be roughly a quart and a half over full which could also affect oil pressure readings…. Did this oil issue start before or after the oil change?

Engine serial number is always nice to have to confirm correct engine parts to be used/shown but item number 7 is the oil switch to sound the warning horn when pressure get s to low at anything under 4psi,

https://www.marineengine.com/parts/mercruiser-sterndrive-parts/mx-62l-mpi-bravo/0l680003-thru-0m299999/senders-and-sensors

The location for the oil switch is often up higher, rear port side of engine to the left of the distributor assembly. Port side is the left side when facing the bow of the boat… if you turn around and face the front of the engine, now it’s on your right side towards the rear of the engine. Found an image of one…. This mentioned, this item in question shouldn’t be your fluctuating issue.

Boat Tech Help - Mercruiser, Volvo Penta, Outboards, Engines Q&A | 1999 Mercruiser 5.7l efi: | Facebook

1999 Mercruiser 5.7l efi: Can someone guide me where to find the oil pressure limit switch? Edit: I think I got it, behind the distributor cap.

www.facebook.com

Thanks for the response. I was asking because I want to confirm the sensor/buzzer is actually functioning.

I will check and test

 

[helix757]

I would:
correct the high oil level, it is possible for the crankshaft to hit the oil in the pan when it is too high and that can mix air bubbles in the oil which could lower pressure...
hook up a manual gauge to your oil pressure port on the engine to see if the readings on the manual gauge are the same or different...
Your readings appear to be low even with a cold engine.
What I've always gotten was 45-55 on a cold start, fast idle, 40 after coming off fast idle, 50-60 when running on plane, and after coming off plane and idling, 25 psi. This is with an original 1988 short block....

So I (think) I found the issue a few weeks ago.
One of the spark plugs ceramic was cracked causing a spark grounding to the block — so zero ignition in that cylinder. I believe the unburnt fuel from that contaminated the oil, reducing its viscosity and thus pressure. Also, explains why the oil level was reading high.

I have since done an oil change using the garden hose threads on the dipstick. However, this appeared to leave about a quart of (contaminated) oil still in the pan. I am planning on doing another oil change using the equipped pan drain hose to get it all out this time.

Its important to note even with doing the oil change via the dipstick tube, my oil pressures has improved, and stay higher even at operating temps/rpms for extended periods of time.