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Mercruiser 350 MAG MPI - Oil pressure issue

M

MasterLX

New member
Hi!

I am new here - hello everyone! - immediately with a strange issue on my Mercruiser V8 engine:

It starts and runs without any issue, oil pressure at start and cold oil close to 40psi at idle rpm, dropping down to roughly 20psi when oil is at temperature.. everything as expected.
When then driving at normal speed of about 22knots at approx 3800-4000rpm oil pressure immediate increases again with rpm to approx 40-60psi (as expected) and everything looks fine.

But after some minutes at steady rpm around 4000rpm, oil pressure suddenly decreases within maybe 10-30sec down to approx 25-30 psi?!
This stays there independent from rpm between 1500-4500rpm..
It only recovers when going down to idle for some seconds.. then it is ok at around 40psi again when increasing rpm above 3000rpm again... For some minutes till it drops again?

Complete oil pump has been replaced (so spring of over pressure bypass seems was not the issue), all filters and oil (original Mercruiser/Quicksilver) has been changed several times.. even special additives have been tried out.. no change.

Anyone figured out some similar issue and can help me out?

Thanks!
Alex
 
Welcome
Guessing your using the gauge at the helm.
Have you tried connecting a mechanical Gauge?

The electrical gauge can have issues with connections, so bad connection or ground can cause it to change
 
AllDodge said:
Welcome
Guessing your using the gauge at the helm.
Have you tried connecting a mechanical Gauge?

The electrical gauge can have issues with connections, so bad connection or ground can cause it to change
Click to expand...
Hi!

Thanks for the quick reply!

Yes - these pressure values are read from the gauge at the helm.
And yes, we've also had an issue with oil pressure at idle already before winter - there the alarm started some times at idle, when entering gear and idle rpm was at lowest (~700rpm) that oil pressure was too low. The instrument in cockpit was also a little wobbling, therefore we also replaced the oil pressure sender --> then metering was very stable, but still too low when oil temperature was reached.
To ensure gauge in helm is accurate, we also applied a mechanical gauge directly at the engine (where oil pressure sender is mounted) and checked the oil pressure for several rpm's in marina (no boating)--> they where identical to gauge at the helm.
--> therefore we decided to take out the engine and replace the oil pump. --> oil pressure at low rpm is now ok, no alarm anymore, just the issue during boating, so suddenly oil pressure decrease is not solved.

But we didn't check the behavior during boating with mechanical gauge - so this is something we could try out again - thanks!

But would a bad connection or ground issue fit to the deterministic behavior and that it always and only recovers when going to idle rpm (or switching off the engine)?

Thanks!
 
Verified with Mech gauge than your reading is accurate

Unless your using some kind of weird oil which I'm sure your not, there is an internal issue.

The GM block uses bypass valves between oil filter and galleys, but even with that it should not drop the pressure. Without any more info I would say you have a bearing issue and block needs rebuilt

You could check pressure at the cam connection directly below the distributor and if it shows same pressures than there is nothing else left
 
AllDodge said:
Verified with Mech gauge than your reading is accurate

Unless your using some kind of weird oil which I'm sure your not, there is an internal issue.

The GM block uses bypass valves between oil filter and galleys, but even with that it should not drop the pressure. Without any more info I would say you have a bearing issue and block needs rebuilt

You could check pressure at the cam connection directly below the distributor and if it shows same pressures than there is nothing else left
Click to expand...

Yes, check in marina with mechanical gauge showed reading should be fine. If there could be some GND issue only during boating.. we might have to check.

Oil we've also tried different blends (quality marine oils like Liqui Moly Marine...) and with additives as mentioned - but actually it is the original Quicksilver 25W40 Synth Blend motor oil that is recommended for this engine.

Yes - somewhere there must be a leak or bypass in oil system? But it hast to be somewhere within the oil circuit, as it does not loose a drop of oil even after some hours of operation?
All the connection rod bearings to crankshaft have also been exchanged (no real worn out visible.. standard for this age and operating hours), main bearing at least on one has been checked - same, looks ok. --> and also there is no noise - would expect some noise and knocking in case a bearing is so worn out, that oil pressure decreases? And why only after some minutes at load? (of course it could be temperature of the oil under load.. but really so quickly then? and why immediately recover? ..I don't know..)

To check pressure at the cam is something new to me? - here I have to check where and how this can be done?
But is a good idea! Thx!


PS: Maybe some extra information to the engines history:
- we had an issue already in summer last year, a broken head gasket let water in (into cylinder 6) --> was recognized first by reduced power, then an issue at start of engine (water in cylinder, blocked engine during start, dependent on location of cylinder.. so sometimes it started without issue.. sometimes it didn't start at all --> looked like starter was too weak to turn engine --> but water was the issue, which was recognized after new starter build in.. and with starting engine without ignition plugs in, showed water spraying out of this cylinder.
--> therefore head gasket was replaced, also lifters have been replaced, as well as new exhaust manifold, raisers, elbows have been mounted.
After flushing engine oil several times (with additives to clean), engine runs fine again --> except this issue with decreasing oil pressure...
 
Need to watch pressure with Mechanical gauge when running since it only happens when running, could still be bad connection.

Cam area port: look below the distributor at rear of motor and you should find a sensor or a plug in the rear deck of the block. The gauge can be connected there

If running and pressure still fails it has to be inside the motor.
With motor out and all bearing surfaces are clean and dry you can use plastic gauge
 
AllDodge said:
Need to watch pressure with Mechanical gauge when running since it only happens when running, could still be bad connection.

Cam area port: look below the distributor at rear of motor and you should find a sensor or a plug in the rear deck of the block. The gauge can be connected there

If running and pressure still fails it has to be inside the motor.
With motor out and all bearing surfaces are clean and dry you can use plastic gauge
Click to expand...
Yes, already ordered additional accessories to easier connect the mechanical gauge and read it also when running - to check for bad connection.

Ok - will check to find the sensor or plug and connect then also the gauge there.
But if it is already failing at oil pressure sender connection (pressure decreases) - I guess it has to be the same at upper connection at the cam?

If it still fails - engine will need to be taken out again yes --> then also the main bearings can be checked again (also for tolerances with plastic gauge) - and maybe also camshafts to be checked.. maybe there is somewhere a probllem, so oil is leaking?

Thanks!
 
It should be same at cam port, only thing that could cause a change is something inside the oil galley causing a restriction which is a far reach for me also
 
Maybe have the engine oil analyzed for metal shavings, etc. from a place like, https://www.blackstone-labs.com/

therefore head gasket was replaced, also lifters have been replaced, as well as new exhaust manifold, raisers, elbows have been mounted.
After flushing engine oil several times (with additives to clean), engine runs fine again --> except this issue with decreasing oil pressure...
Wonder if any type of debris fell down into the engine while cleaning up the surface areas affecting oil flow readings now.

Also, how many quarts of oil are you putting in?
As another short-term test, you could try adding and subtracting a quart of oil of your specification amount just to see if symptoms get better...adding a quart sometimes have helped determine oil pump issues.

From a service manual reminder... Overfilled Crankcase: Oil An overfilled crankcase (oil level being too high) can cause a fluctuation or drop in oil pressure and rocker arm clatter on Mercury MerCruiser engines. This condition results in the engine crankshaft splashing and agitating the oil, causing it to foam (become aerated). The aerated oil causes the hydraulic valve lifters to bleed down. This, in turn, results in rocker arm clatter and loss of engine performance, due to the valves not opening properly. Care must be taken when checking the engine oil level. The oil level must be maintained between the ADD mark and the FULL or OK RANGE mark on the dipstick. To ensure that you are not getting a false reading, ensure the following: • Boat at rest in the water, or• If the boat is on a trailer, raise or lower the bow until the boat is setting at the approximate angle that it would be if setting at rest in the water.• Allow sufficient time for the oil to drain into the crankcase if the engine has just been operated or oil has just been added.

IMPORTANT: Oil filter by-pass valve and block adapter assembly should be inspected whenever engine is disassembled for major repair or whenever inadequate oil filtration is suspected… Also consider a stuck oil valve where the filter goes up against.
 
Fun Times said:
Maybe have the engine oil analyzed for metal shavings, etc. from a place like, https://www.blackstone-labs.com/

therefore head gasket was replaced, also lifters have been replaced, as well as new exhaust manifold, raisers, elbows have been mounted.
After flushing engine oil several times (with additives to clean), engine runs fine again --> except this issue with decreasing oil pressure...
Wonder if any type of debris fell down into the engine while cleaning up the surface areas affecting oil flow readings now.

Also, how many quarts of oil are you putting in?
As another short-term test, you could try adding and subtracting a quart of oil of your specification amount just to see if symptoms get better...adding a quart sometimes have helped determine oil pump issues.

From a service manual reminder... Overfilled Crankcase: Oil An overfilled crankcase (oil level being too high) can cause a fluctuation or drop in oil pressure and rocker arm clatter on Mercury MerCruiser engines. This condition results in the engine crankshaft splashing and agitating the oil, causing it to foam (become aerated). The aerated oil causes the hydraulic valve lifters to bleed down. This, in turn, results in rocker arm clatter and loss of engine performance, due to the valves not opening properly. Care must be taken when checking the engine oil level. The oil level must be maintained between the ADD mark and the FULL or OK RANGE mark on the dipstick. To ensure that you are not getting a false reading, ensure the following: • Boat at rest in the water, or• If the boat is on a trailer, raise or lower the bow until the boat is setting at the approximate angle that it would be if setting at rest in the water.• Allow sufficient time for the oil to drain into the crankcase if the engine has just been operated or oil has just been added.

IMPORTANT: Oil filter by-pass valve and block adapter assembly should be inspected whenever engine is disassembled for major repair or whenever inadequate oil filtration is suspected… Also consider a stuck oil valve where the filter goes up against.
Click to expand...
Well yes - oil analysis would be no bad idea. But now after approx. 5-6 times oil change (due to issues and maintenance work) within maybe last 10-15h operation time.. might not show too much anymore?
Debris or other residuals could have been an issue --> when we had seen this issue end of last season (after exchanging head gaskets etc.) --> therefore we did first engine flush several times.. and as issue not got better, opened also oil pan, cleaned out engine bottom and replaced oil pump and connection rod bearings. --> so would not think within engine still debris is an issue (oil is as clear as fresh out of the bottle).

Oil amount is a little strange - also after removal of oil pan - only ~3.5 quarts (+filter) are needed to be at max on oil stick?
Here I'm not sure.. but on prior boat with similar engine it was more around ~5 quarts... but at this boat/engine at first initial service (after we've bought the boat) the oil pan was exchanged (corrosion) --> maybe this is a little smaller one? But nevertheless we were boating with this oil pan already almost 3 seasons without issues - so the issue has to be a new one..

Yes, this idea of changing oil amount I also got yesterday - maybe it is a little too less in, so air gets sucked from the oil pump.. or while boating and boat is a little heeled it could be too much, so it could foam up and then oil pressure decreases..!? --> this is also on my to do list for next time on the boat - Thanks! (y)

At the oil filter we didn't do too much yet --> we have mounted the remote oil filter set, therefore the two longer hoses to engine top mounted filter --> here I really need to check, as I didn't know yet, that there is also a bypass valve available - Thanks!
 
Last edited:
Oil amount is a little strange - also after removal of oil pan - only ~3.5 quarts (+filter) are needed to be at max on oil stick?
Click to expand...
This could explain everything your seeing
The 5.7/350 uses 5 quarts with filter replacement at minimum. If it has a deep oil pan this can go to 6 and maybe a bit more

If it only took 3.5 quarts the first time that is a problem
If it took 3.5 quarts each time after the first than that may not be an issue if oil is being sucked out and not from the drain plug
 
What year and/or engine serial number is your engine?

Add a quart of oil and go run the boat then maybe add another half and see while on the water… it best to check oil levels while at rest on the water for level checking… engine off of course.

Edit: when you put the dipstick tube into the block or pan? Did it seem to bottom out all the way down and left there or was the tube pulled up just slightly?
Also was there silicone adhesive around the tube at the insert area so oil won’t flow out under pressure? If silicone was added, be sure not to add too much pushed down as it “could” leak down and start to block the oil pump… I’ve seen that a few times in the past so just worth sharing as a reminder is all.😔
 
Last edited:
AllDodge said:
This could explain everything your seeing
The 5.7/350 uses 5 quarts with filter replacement at minimum. If it has a deep oil pan this can go to 6 and maybe a bit more

If it only took 3.5 quarts the first time that is a problem
If it took 3.5 quarts each time after the first than that may not be an issue if oil is being sucked out and not from the drain plug
Click to expand...
To add to this,
A mercruiser dip stick tube goes almost to bottom of oil pan so when oil is sucked out thru dip stick tube it gets almost all the oil, may leave 1/2 qrt or less in pan.

If the dip tube was replaced with a non merc OEM dip stick tube it may only go so deep thus the need for so little oil needed to top off.

I have never heard/seen a 5.7 use anything but 5 qrts (+ based on remote oil filter.)

If you dont have the correct dip stick and or tube, you actually have no idea how much oil is in the pan. That being said, if you have added say 7 qrts unknowingly then severe foaming may be occurring thus the loss of oil pressure.

Some deeper truobleshooting is required here to get facts on what i mentioned unless you have that info already
 
AllDodge said:
This could explain everything your seeing
The 5.7/350 uses 5 quarts with filter replacement at minimum. If it has a deep oil pan this can go to 6 and maybe a bit more

If it only took 3.5 quarts the first time that is a problem
If it took 3.5 quarts each time after the first than that may not be an issue if oil is being sucked out and not from the drain plug
Click to expand...

Yes, I see it already the same way --> checking for different oil pans, I couldn't find any hint (also not in cars) that there could be one model taking less than ~4.5 quarts of oil --> so I'm also quite sure here we have a problem.

We also thought ok, we're only able to suck out ~3,5 to 4 quarts.. the rest we can't get out of the oil dipstick..
But while flushing engine (after first repair) several times.. we only got out maybe ~2 quarts from oil dipstick.. and thought our pump has a problem.. but also from drain plug it only added up to this ~3.5 quarts...

So there has to be happened something to the oil dipstick while first repair of head gaskets (maybe it got hit from the head and pushed in further?) , therefore not showing correct fill value anymore --> will travel to boat today and try out next 2 days to play a little with extra oil.. (y)

Thanks!!
 
Fun Times said:
What year and/or engine serial number is your engine?

Add a quart of oil and go run the boat then maybe add another half and see while on the water… it best to check oil levels while at rest on the water for level checking… engine off of course.

Edit: when you put the dipstick tube into the block or pan? Did it seem to bottom out all the way down and left there or was the tube pulled up just slightly?
Also was there silicone adhesive around the tube at the insert area so oil won’t flow out under pressure? If silicone was added, be sure not to add too much pushed down as it “could” leak down and start to block the oil pump… I’ve seen that a few times in the past so just worth sharing as a reminder is all.😔
Click to expand...

Engine should be from 2001, Boat 2002 - 0M04xxxx something - so from "350 MAG MPI ALPHA/BRAVO 0L331599 THRU 0M299999"

Yes - will try this weekend to add oil.

The dipstick tube has not been removed during repair --> it was/is somehow glued in, so we were not able to remove it (without destroying it), therefore we kept it in --> should still be the original tube, has not been replaced.
But yes - I also think it has maybe been pushed down further during repair, therefore at max level with only ~3.5 quarts of oil..
 
kghost said:
To add to this,
A mercruiser dip stick tube goes almost to bottom of oil pan so when oil is sucked out thru dip stick tube it gets almost all the oil, may leave 1/2 qrt or less in pan.

If the dip tube was replaced with a non merc OEM dip stick tube it may only go so deep thus the need for so little oil needed to top off.

I have never heard/seen a 5.7 use anything but 5 qrts (+ based on remote oil filter.)

If you dont have the correct dip stick and or tube, you actually have no idea how much oil is in the pan. That being said, if you have added say 7 qrts unknowingly then severe foaming may be occurring thus the loss of oil pressure.

Some deeper truobleshooting is required here to get facts on what i mentioned unless you have that info already
Click to expand...

Dip stick as mentioned should be still the original one.. but seems not fitting properly anymore.

I've also checked some pictures of the replacement of the oil pump --> there it is visible, that the dip stick tube goes down even a little beyond the oil pan filter! --> so it really should go almost down to the bottom of oil pan, as filter is only ~0.2 inch above the bottom of oil pan (if I remember correctly).
I just don't know how far the dipstick goes down inside the tube?
..but yes.. again.. will try to fill in additional oil, so that I try at least with ~5 quarts if the issue gets better..

Thanks!
 
The dip stick tube has a flange to stop it at the block hole it goes into that only allows it to go so far into pan. Designed that way. It is a tight slip fit into hole in block and should easily be removed. If you pull it all the way out, slide dip stick in and see how far it goes in. If the dip stick goes pasr end of tube then that may be the issue. when reinstalling add a small amount of rtv at flange.
 
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