Perkin 4108 (4-108) - 1981 Oil alarm.

[icarusbop]

Hello:

I have recently bought a boat with a Perkins 4108 inboard engine. Whick I have had some issues with.
I have replaced both the fuel filters and bled the fuel system and now the engine runs quite well (better than it did -the fuel filters were blocked.)

If I leave the engine idling for a while, the oil alarm starts buzzing intermittantly and the oil lamp flcikers (looks like it is on the edge of on / off and flipping between the two) if I increase the revs, the alarm clears.

If I let the engine cool then start it again, no problem at first, then alarm after a while.
Looks like the alarm is only happenng after the engine / oil is warmed up.
Can anybody please advise on what could cause this?

Thanks

 

[Rossynant]

Many possible reasons, but start with obvious.
Simply speaking: at idle pump works on lowest of it's efficiency, and oil pressure will be at lowest, so to say at the threshold, warm oil will have less pressure then cold. This is normal, question here - is the oil pressure too low, or still inside a limit. Probably not much problem there, as only at idle.

As you have only warning light, no display of actual pressure, it's hard to say. It may be, even, that he sender for this light is not precise... Checking the pressure exactly would be best, but first:
Most obvious reasons:
- revs for idle too low - don't remember for sure, and there were slightly different versions of engine, somewhere about 620-650 rpm; but 700-800 rpm is also OK for any diesel so if you have lower, may just increase a bit. Perkins will go even at 500 rpm (mine did), but no reason for keeping so low. Make sure it's not lower then 600 rpm.
- oil not dense enough, or old, or too little of it ( should be API CF or lower , no synthetics, SAE 40 when hot so 15/40, 20/40 or so)
- pump worn, or pressure relief valve, or such,
- engine worn.

So first increase the idling rpm a bit, if light goes off then you got a peace of mind. Put correct oil to correct level next time.

I cannot check more detail, as I don't have manual at hand now. For better answer get yourself Workshop Manual, can be downloaded from the net, just google it. There is a table for fault finding.

 

[davidej]

Try to get or borrow a proper oil pressure gauge and check the reading with the engine hot.

Many worn engines have very low pressure at tick over, but as long as it comes up when the engine is on load, it will probably soldier along for some time.

 

[billskip]

If the fuel filters were that bad then lack of general maintenance is a clue.

Ok old 4108's get a bit slappy,but as said above check the idle rev's.

Also do a complete filter and oil change, but if anything like my old one it drunk oil faster than its service requirement,from your description it does seem likely it could be the oil itself

 

[nimbusgb]

+1 for the two comments already. My 1985 engine was the same. Would do exactly the same after the engine was good and warm. Replaced oil with a heavier, traditional ( non synthetic ) oil, fitted a simple oil cooler and an electronic gauge so that i could keep an eye on the real pressure.

That was 2 seasons ago and about 100 hours of run time.( a lot of engine time the last 2 seasons)

 

[PCUK]

Yes, it could be the oil. You need old-fashioned 20-50 or HD 30. Never use modern oils in old engines as they are too thin when warm.

 

[Bodach na mara]

I second all of what has been said, especially the suggestion to fit a pressure gauge. I have had similar problems with boat (and car) engines and the problem has never been low oil pressure, always a fault in the warning light circuit. The sensors seem to age and I have even had one leaking oil, presumably through a diaphragm.

I have always found to be getting the fittings is the problem in fitting a gauge. You need to identify the correct taper thread for the sensor, get a T-piece with one end male and the other female of that hread and the side arm of a size to take the gauge. I usually needed an adaptor for the side arm, but then I use a gauge rescued from a scrap car about 45 years ago.

 

[icarusbop]

Thanks to everyone for your replies.
I have done some more work on the engine and replaced the oil pressure switch with a new one.
I have aslo fitted a new thermostat as the engine was not warming up properly.
Since then, the engine is getting to it's proper run temp and idles nice and smooth with no oil alarms going off, so it looks OK now. suspect oil pressure switch or engine not getting warm enough, causing the issue.
I have oil and a new filter to do an oil change next time I am on board, I ran out of daylight when I was there last weekend.
hopefully new oil and filter will not bring back the issue (15W40 mineral - it was recommended by a diesel engine guy), but by the looks of the old oil I think it has not been changed in some time, so needs to be done.
If the alarm/light come back with the nebw oil I'll try and get some thinker oil and change it out again.