Low oil pressure

[single]

I motored for 16 hrs yesterday non stop at 2000rpm. When i throttled back at the end the oil pressure dropped and the oil warning was trying to beep.It went back up to normal when i increased the revs slightly and was ok if i decreased to idle slowly but is this normal after a long passage? Oil quantity is fine.

 

[rob2]

Not that unusual in my experience in these somewhat elevated temperatures - that oil was damned hot and subsequently thins out a bit!

Rob.

 

[MASH]

Does a thermostatically controlled watercooled engine really respond to air temp? Anyway, it's only 10 degrees warmer than "normal" and that's hardly going to thin the oil much even if it did affect the temp...modern oils are supposed to maintain their viscosity after all...

While thin oil could give the symptoms described I think you'd want to confirm it was running considerably hotter than normal before discarding the usual suspects, ie low oil pump delivery pressure or loss of pressure in the engine due to wear.

 

[Bilgediver]

I motored for 16 hrs yesterday non stop at 2000rpm. When i throttled back at the end the oil pressure dropped and the oil warning was trying to beep.It went back up to normal when i increased the revs slightly and was ok if i decreased to idle slowly but is this normal after a long passage? Oil quantity is fine.

Try and find a small pressure gauge that can fit the sensor switch hole in the engine and see what is really happening as even faulty sensor switches are not unknown.

You forgot to mention what engine. Some engines such as Bukh and early volvo's have their fuel pumps mounted within the sump and so leakage at these items goes to the sump oil and dilutes(thins) it. When did you last change your oil?

 

[Malish]

Two questions -
1) how old is the engine / how many hours on the clock.
2) What grade is the oil - 15W 40?

If the engine is reasonably mature then I wouldn't be surprised if the oil pressure dropped at tick over, amplified if a modern thin oil has been used.

 

[Trundlebug]

Does a thermostatically controlled watercooled engine really respond to air temp? Anyway, it's only 10 degrees warmer than "normal" and that's hardly going to thin the oil much even if it did affect the temp...modern oils are supposed to maintain their viscosity after all...

While thin oil could give the symptoms described I think you'd want to confirm it was running considerably hotter than normal before discarding the usual suspects, ie low oil pump delivery pressure or loss of pressure in the engine due to wear.

It's not only the air temp that's elevated - the raw water temp is increased too, which will have some effect on the overall cooling of the engine.
Net result being it will run slightly hotter, especially when the ambient air temp in the engine bay is also considered.
After a long run in hot temperatures what happened isn't necessarily a big surprise, nor is it necessarily anything to be unduly alarmed about, as said by Rob2.

2 things also worth mentioning:
1) Engine health isn't all about oil pressure - oil flow is also important. There may not have been much pressure at the end of this long run, but there was clearly oil flow, it had just run for 16 hours. It's not that unhealthy. It can be quite common for older engines (petrol and diesel) to have low oil pressure at idle; it doesn't mean they're on their last legs.
2) Sometimes with older engines the oil pressure sender unit gets a bit tired and can result in bleep alarms at lower pressures than are actually alarm-worthy. Before you go replacing expensive items or ripping the engine apart to replace pumps, bearings, relief valve etc one of the first things to change is the sender. Only a fiver or so from the right sources. If you are concerned about what happened I'd start with this before doing anything else.

 

[TQA]

With some engines this would be an indication of crankshaft bearing wear, on others it would be normal.

So what engine is it?

Do you know what pressure your warning buzzer cuts in. There are some switches that sound the alarm at 15 psi., some at 3 to 5 psi..

 

[Lakesailor]

Is it back to normal now it's cooled down? Panic over.

 

[No Regrets]

Worry not!

My last Perkins 4108 which was very very healthy did the same after a good hard run, but stopped when it cooled.

The current Volvo TAMD40b's do it too, if hammered for ages.

Only on a low idle when very hot. Otherwise fine! Oil pressure will zing back up with the tiniest amount of revs...

 

[single]

Its a thornycroft t95 (mitsubishi) , 700 hrs on it. New 8 years agil changed about 50hrs ago.

 

[MASH]

Well penny to a pinch of sh!t it ain't got nothin' to do with air temperature. That's for certain. Watercooled engine says that.
I also don't believe it's got anything to do with water temperature as the engine has a thermostat to deal with that. If that were the case everyone's engines would run stone cold in winter and overheat in the caribbean and we know this just doesn't happen.

Sorry, there's a more practical reason. Low oil pressure can only be down to three causes - overheating (which I think we've dealt with), low delivery pressure or loss of pressure in the engine due to wear.

Sure, there are subtleties like sticky prv's and such, but the above pretty much covers it i think. Sorry.

However, low oil pressure at idle isn't necessarily a problem as long as it comes up to scratch at working revs which is where it matters.

 

[single]

How about if the filter was a bit blocked?

 

[cryan]

Almost certainly down to the oil temperature. Check your oil cooler for blockages and poor heat transfer. This is easily done without stripping it down by aiming a laser thermometer at inlet and outlet of the cooler and checking the temperature change. A sea water temp rise of a few degrees can really effect your coolers if they are not clean. If the temperature difference is not good enough (At least a ten degree drop) Then the first thing to check is the water flow by feeling the hoses(are they pressurised? Can you feel a flow?). If your jacket water is flowing well and the jackets are not over heating (or sea water is flowing well if SW cooled) then the next suspect is a dirty cooler. This will mean stripping the cooler down and manually cleaning the dirty oil deposits out of the tube nest (soak in degreaser then rod each tube with a bottle brush or similar tube cleaning implement).
It could very well be pumps etc as mentioned above but I have had this happen several times before and it has always been a cooling issue. If it was a pump etc then it would happen all the time not just after an oil cooking 16 hour long passage. The oil has thinned due to high temperature, you slow the engine which slows the pump which means the flow drops and, due to the lower viscosity of the oil due to heat, the pressure drops just below the alarm level and sounds the alarm.

 

[single]

Engine water temp was perfect for the whole time. I think i'll do another oil change and see what happens.Right at the end at idle the top end sounded quite tappety too.

 

[cryan]

Engine water temp was perfect for the whole time. I think i'll do another oil change and see what happens.Right at the end at idle the top end sounded quite tappety too.

If the engine temp was fine then it sounds like a dirty cooler. It is common for it to sound, "Tappety," if the oil has thinned out a bit after a prolonged run.

 

[single]

So where is the oil cooler? Its not like the car one so is it part of the heat exchanger?

 

[vyv_cox]

I cannot help thinking that people are inventing extremely complex solutions for this problem. The temperatures you are currently enjoying in UK may be warm but are nowhere near those we are seeing in the Med and no doubt in other parts of the world. A 15w40 oil is perfectly capable of retaining adequate viscosities in considerably higher ambients.

I don't know your engine but I would be quite surprised if it had an oil cooler, unusual on small marine conversion engines. Even supposing that it did, it would have engine coolant on one side and oil on the other, so what will foul it?

The first rule in diagnosing this type of problem is to check the instrumentation. Either buy a new sender or fit a mechanical gauge, quite cheap on Ebay.

 

[single]

Here is the instrument panel.All my gauges are in the marked black areas on the bezel under normal conditions. The oil pres. is in bigger black area (and was all the time on that long passage) then after maybe 6 hrs dropped to the lower end of the big black then down to about 1 at idle at end of the trip.View attachment 33764

 

[vyv_cox]

Is it a mechanical or electrical pressure gauge. Mechanical will have a copper tube connecting it to the engine. If it is mechanical then it could be a genuine reading as this type rarely goes wrong. If electrical - is there a warning light or alarm, and if so did it sound? Then you need to check the sender, sometimes a single one will have two connections, one for alarm and one for gauge. If indicated pressure fell but no alarm sounded (if installed) then definitely the sender. If no alarms fitted I would change the sender anyway, I assume with an automotive conversion this would not be prohibitively expensive.

 

[single]

Alarm was trying to beep when it went down to 1ish.Its electrical type.