Low oil pressure - ideas?

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citygent111

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Hi all

Managed our first trip of the season out today, all went well except I noticed the oil pressure gauge had dropped to just above the Low indicator. Continued a bit further on and it definitely reduced a bit more.

I checked the bilge and there was a very small amount of oil on the floor under the engine.

It's a 2009 Merc 4.3mpi. I checked the dipstick and it was right on the "good" level, so when we stopped to be on the safe side, I threw in another 100ml or so. After a break of about 30 minutes we set off again, and the gauge had increased nicely, but did continue to very slowly drop.

Got back to the yard and the oil in the bilge had hardly increased, and I checked the dipstick again which was well above good.

What am I missing? What could be causing the pressure to drop?

For clarity, we've only done about 25 hours since it was last serviced, but that was about 12 months ago (yes, 2024 was THAT bad!)

Thanks in advance 👍🏻
 
"oil in the bilge had hardly increased" - it shouldn't increase at all, unless you have an oil leak, in which case that's your problem! I'd start by completely drying and cleaning the bilge and working out if you have an oil leak or not.

If you don't have an oil leak, always worth throwing a pressure tester on instead of the sender that connects to the dash gauge - the senders do fail and give erroneous/spurious/intermittent readings/issues, so worth checking with a good old fashioned gauge before digging deeper.
 
I’m not sure a very minor oil leak will cause low oil pressure unless your oil level is below minimum. What were the engine temps like? Pressure will drop with high engine temps. How old is the oil? Is the pressure ok at idle?
 
It is easy to "mis-remember" temp and pressure readings on your Gauges especially after a decent gap since last use.
Oil pressures will drop when oil gets hot to some extent.
Oil pressures will drop due to wear but this does not happen overnight.

On my last trip back the oil pressure on one engine dropped to Zero. :eek:
This either meant a knackered engine ££££££££ or a problem with the oil pressure sender unit or associated wiring £.
A quick test with a borrowed oil pressure test gauge proved the oil pump was performing as it should and fiddle with the connector on the sensor sorted things. You can buy one for not much money. Get oil hot and check at tick over and briefly at WOT if you are brave enough.
There is also a low pressure alarm switch which did not go off.

The oil pressure gauges on Yanmars are notorious for frightening the lives out of skippers for showing not much pressure on hot tick over, this is purely down to rubbish calibration between sender and the pressure scale printed on the gauge.
Some just accept the readings , others feel the need to replace both senders and guages for peace of mind.
 
oldgit said:
It is easy to "mis-remember" temp and pressure readings on your Gauges especially after a decent gap since last use.
Oil pressures will drop when oil gets hot to some extent.
Oil pressures will drop due to wear but this does not happen overnight.

On my last trip back the oil pressure on one engine dropped to Zero. :eek:
This either meant a knackered engine ££££££££ or a problem with the oil pressure sender unit or associated wiring £.
A quick test with a borrowed oil pressure test gauge proved the oil pump was performing as it should and fiddle with the connector on the sensor sorted things. You can buy one for not much money. Get oil hot and check at tick over and briefly at WOT if you are brave enough.
There is also a low pressure alarm switch which did not go off.

The oil pressure gauges on Yanmars are notorious for frightening the lives out of skippers for showing not much pressure on hot tick over, this is purely down to rubbish calibration between sender and the pressure scale printed on the gauge.
Some just accept the readings , others feel the need to replace both senders and guages for peace of mind.
Click to expand...
Wise words
 
oldgit said:
On my last trip back the oil pressure on one engine dropped to Zero. :eek:
This either meant a knackered engine ££££££££ or a problem with the oil pressure sender unit or associated wiring £.
A quick test with a borrowed oil pressure test gauge proved the oil pump was performing as it should and fiddle with the connector on the sensor sorted things. You can buy one for not much money. Get oil hot and check at tick over and briefly at WOT if you are brave enough.
There is also a low pressure alarm switch which did not go off.

The oil pressure gauges on Yanmars are notorious for frightening the lives out of skippers for showing not much pressure on hot tick over, this is purely down to rubbish calibration between sender and the pressure scale printed on the gauge.
Some just accept the readings , others feel the need to replace both senders and guages for peace of mind.
Click to expand...
Snap, on my previous offshore RIB with a Yanmar 6LP I was cruising past Swanage one day and suddenly noticed oil pressure dropping to zero and panicked! No oil alarm, level was fine and no oil in the bilge so I continued on to Poole absolutely fine, connected a manual oil gauge and no issues at all. Yanmar senders are incredibly expensive and seem to use some custom resistance range so I couldn't find a generic spare that worked, ended up with a spare from a friend who'd changed all his senders/gauges to generic ones.
 
rafiki_ said:
I’m not sure a very minor oil leak will cause low oil pressure unless your oil level is below minimum. What were the engine temps like? Pressure will drop with high engine temps. How old is the oil? Is the pressure ok at idle?
Click to expand...
Thanks for the reply - Temp was going up as pressure was coming down, it was just above normal on the gauge.

I'd definitely describe it as minor -, after 2 hours at 3000rpm there was probably a couple of tablespoons of oil in the bilge.

The oil is just 12 months old, it was changed in May last year and we've only done about 20 hours in that time.

Finding the source of the leak is my next challenge - I think I'll get the guys at the yard to sort it out but I didn't want to paying lots of man hours to identify the problem!

So yes, low pressure reading, slightly higher than normal engine temp, but no warning beeps, and a splash of oil in the bilge 🙈
 
Focus points

0: Is the right oil viscosity used?
1: Oil filter. Change if not done already - to the specified model (valve etc.)
2: Oil pressure sender. Might be defective.
3: Wiring from sender to instrument.
4: Leaks. Should have a zero tolerance as engines are not designed to sweat oil. If you see any oil on the engine or in the bilge, do a thorough bilge and engine wash down to get a starting point for hunting for traces.
5: Engine temp seems to be within range. A notable increase combined with a too low oil viscosity might trigger the low pressure, but would be rare. Very rare. In any case, check the cooling and thermostat. If sea water cooled, build-up (crud, salt etc.) in the water jacket & system might cause reduced cooling, hence higher temperature. Do not forget to check impeller replacement history.
 
ChromeDome said:
Focus points
. . .
4: Leaks. Should have a zero tolerance as engines are not designed to sweat oil. If you see any oil on the engine or in the bilge, do a thorough bilge and engine wash down to get a starting point for hunting for traces.
. . .
Click to expand...

Obviously not someone brought up with British motorbikes, and their patent anti-rust feature!
 
LittleSister said:
Obviously not someone brought up with British motorbikes, and their patent anti-rust feature!
Click to expand...
For a City Gent it would be a Triumph for him to find it. Or he could go on line, and use Norton antivirus but being an educated man he could use his BsA Honours Degree to solve the problem.
 
paulbrown22 said:
Snap, on my previous offshore RIB with a Yanmar 6LP I was cruising past Swanage one day and suddenly noticed oil pressure dropping to zero and panicked! No oil alarm, level was fine and no oil in the bilge so I continued on to Poole absolutely fine, connected a manual oil gauge and no issues at all. Yanmar senders are incredibly expensive and seem to use some custom resistance range so I couldn't find a generic spare that worked, ended up with a spare from a friend who'd changed all his senders/gauges to generic ones.
Click to expand...
Yanmar retail £240.00
Yanmar dealer in IOM £150.00
New boxed on ebay £75.00 or £100.
Searched long and hard .There are no generic chinese clone senders around.
 
citygent111 said:
Finding the source of the leak is my next challenge - I think I'll get the guys at the yard to sort it out but I didn't want to paying lots of man hours to identify the problem!
🙈
Click to expand...
Wouldn't be my priority, but I dont particularly like a challenge
I'd be prioritising trying to establish what my actual oil pressures were, because that could be of some importance.
I dont know this engine, but if I needed to I would read up on how the oil pump works, and particularly on how its driven.
 
Many things can alter the oil pressure so me methodical and begin at the beginning, is the oil grade and viscosity correct for the engine, does it have an oil grade sensor as many engines do.
Is your gauge correct or over and under reading, I prefer to put in a Tee piece and attach a known calibrated gauge so you have a correct pressure reading and a direct comparison between gauges as electronic type gauges are known to be particularly fickle at times. What other evidence of low pressure do you have such as other unusual engine noises.
Is your oil filter clogged or restricted and is the sump oil pipe pick up scren blocked or gloggedEstablish the leak, how much it is leaking and where from as an oil pick up sucking the smallest amount of air will reduce oil pressure.
How long has it been since the engine was run as some aftermarket filters non return valves sieze pr partially sieze which again restricts oil flow.
Being a Mercedes engine throws up some potential issues and one is bore rusting and bore wear and sticking piston rings and if you have a borescope it may be worth looking inside the cylinders with it.
 
A pedants view.

On all my ancient Volvo and Perkins engines in both Princess and Broom craft ,the engine wiring appeared to be of better quality, heavier gauge and remained fairly flexible even after several decades.

The wiring on my Yanmars in a much newer boat appears to be thinner and seems to have become rather rigid.
Had to remake several wiring to ring terminal/ lucar joints.
Assuming the "plasticizers" have leached out due to engine heat, rendering the cable to being more vunerable to vibration, the result of cost reductions
 
LittleSister said:
Obviously not someone brought up with British motorbikes, and their patent anti-rust feature!
Click to expand...
Had a BSA Lightning back in day, at the time when Japanese bikes entered the European market.

Tested a Yamaha 250 and was amazed by the difference in build quality. Nimble little thing that made BSA look old and cumbersome in every way. Not least electrically.

Then came all the bamboo rockets like 3-cyl Kawasakis 500 and 750. Left the British behind in many ways.
 
Ah ok , now i see it in the initial posting .

LIke other mentioned i would first hook up a mechanical gauge to confirm the pressure really drops .

If yes there are basicly two possibilities : worn pump or worn bearings . Oil with different viscosity may alter oil pressure a bit but not that severe .

You oil leak needs to be investigated but imho its unrelated to the pressure loss .

Oil filter is the correct one ?
 
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