Low oil pressure

[vyv_cox]

With luck it's just the sender. Changing that would be my first action. Assuming there is no problem with the wiring, connections good, no chafing of insulation, etc. 1 psi is so low that I'm not sure there would be enough to squirt up at the bores, suggesting that it is a spurious reading.

 

[single]

With luck it's just the sender. Changing that would be my first action. Assuming there is no problem with the wiring, connections good, no chafing of insulation, etc. 1 psi is so low that I'm not sure there would be enough to squirt up at the bores, suggesting that it is a spurious reading.

Sorry, the 1 i refered to is 1 Kp/CM2 so thats about 15psi.

 

[Lakesailor]

You haven't told us if the pressure is back to normal now. This is the most important bit of information.
Those who are doom-mongering and claiming that the cooling system will cope with are missing the fact that after a prolonged run the heat soak within the motor and the engine room will have raised the demands on both the cooling water and the oil. Oil is a major component in the cooling system of any engine.

Like Vyv says. Check with a different sender/gauge. I prefer capillary gauges anyway.
Flow is the important factor. The oil pressure is merely a measure of how hard the pump is finding it to circulate the oil. If the oil has thinned out due to working hard for a prolonged period there will be less resistance and a lower pressure will show. The same amount of oil (or in fact more) will be passing through the important bearing interfaces. It is not pressure that protects the bearings and journals but the oil's presence.

 

[cryan]

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.

The issue is not in my mind the ambient air temp or even really the sea water temp it is the 16 hour run time on passage.
After a prolonged run like that on a small diesel I would expect the oil to be hot and of less viscosity due to the heat. If it was a instrument problem then I would have expected the alarm to sound at anytime not just as the revs were brought back and the flow of oil reduced.
I don't know if the OP's engine has an oil cooler but a bit of googling suggests that at least some were fitted with a Bowan 3365 oil cooler? Oil coolers get dirty on the oil side from the engine deposits. The same stuff that you filter out and that turns the oil black. As the oil passes through the tube nest it leaves some of these deposits on the tube walls. These build up and reduce efficiency. If the OP does not have an oil cooler then in my mind that places an even greater possibility that the cause of the low pressure at reduced revs after a prolonged run is due to hot thin oil. Fortunately if the OP does not have an oil cooler he won't have the hassle of cleaning one.
I certainly wouldn't worry about it. If the OP has a cooler cleaning it will reduce the problem if not then as long as know it will happen your OK. On a workboat I used to be employed to maintain that suffered exactly the same issue i advised the skipper that reducing the revs slowly on the way back to the pontoon would reduce the alarm rate. I would say the same here.

 

[cliff]

How about if the filter was a bit blocked?

No problem - the internal PRV opens and bypasses the filter element but then you are pumping unfiltered oil round your engine

 

[single]

No oil cooler fitted. It seemed ok after i left it at about 1300rpm for a bit then to idle.I think the pressure switch is fine, it beeps before start up anyway.I'll do oil and filter change next time i'm there but i reckon its just the length of time it was running.Longest non stop i have run it.