Why has my engine suddenly stopped using up so much oil?

[Babylon]

Engine is a 28 year old Bukh DV20, rebuilt about twelve years ago.

So far this season, having done lots of short motoring (mostly 0.5hr to 2.0hrs at a time over a total of 33 hours) the oil had twice dropped to the bottom mark on the dipstick and had to be topped up again; ie on average one full top-up every 16hrs. In previous years, I'd expect to top-up the oil only once every 30 or so hours.

Last weekend, prior to a cross-channel trip, I topped up for the second time, and in the event we motor-sailed a good part of the way at a steady 3/4revs for a total of 9.5 hours (5hrs longest continuous engine run). Then, checking the oil level prior to departing on the return crossing, I was surprised to see that the level had hardly dropped much at all - it was still quite close to the top-mark on the dipstick!

Why is this? Does this indicate that - in contrast to lots of short uneven engine hours including lots of slowing and speeding up and going astern when berthing or maneovering in the river - a steady load for several hours has the effect of heating everything up nicely and bedding (if that's the right expression) all the moving parts down nicely?

And, if I do lots more continuous long motoring of this sort, will this reverse the previous trend - or does this indicate that the engine might be due for a second rebuild?

Babs

 

[VicS]

It might indicate that fuel is leaking into the crankcase.

Not familiar with the engine but I guess it has the normal diaphragm type of lift pump. A punctured diaphragm would not be the first.

More seriously injector pump seals could be the problem but as I say I am not familiar with the engine so do not know if that is likely or even possible.

I don't appear to have a workshop manual.

 

[William_H]

Oil consumption

Either the engine has stopped burning oil or something is being added to the sump replacing the burnt oil.
As Vic says it may be fuel. You should however make sure it is not water by drain a bit of oil from the bottom of the sump. Let it settle for a while it will soon become obvious if it is water. Not so easy with fuel of course. good luck olewill

 

[oldsaltoz]

On the brighter side, it just might be that the longer run has freed up a bottom (oil scraper) ring and you are not burning engine oil any more.

Lots of short runs will cause all sorts of problems for small diesel engines, give them a good long work out so they reach full operating temperatures and add a bit of hard work, they love it.

 

[sjclewes]

Forte engine flush

This stuff might help we used it in a merc sprinter van, that used about a half a litre every thousand miles from new, according to merc this was within limits, anyway the forte flush worked wonders it hardly used any after the flush, might be worth a try as long a your lifter pump hasn’t gone.

 

[Finbar]

Just a thought, make sure there is no water in your oil. Worn seals in the water pump a very common cause of this problem in the DV20. Take off the rocker box cover and check for signs of grey sludge, which is caused by water emulsified in oil.

 

[Babylon]

The Bukh engineer's suggestions

Thanks peeps for the advice. Now spoken to the Bukh engineer who services the engine, who has given me a few clues to follow up, as well as advising me that on Bukh DV20s, which don't have a large oil capacity (3l), one shouldn't really need to top up the oil at all, let alone every 40 hrs as I've been doing for the last two seasons.

The first question is where is the oil going? Its either leaking out the engine (check oil pressure sender, also check front end) in which case there'll be evidence of it in bilges, else its being burnt and going out the exhaust, in which case there'll be some black smoke in the exhaust gasses (but maybe not enough to really notice?).

He did suggest one other area to check: the vent valve under the rocker-cover could have been sticking (but maybe the steady hard run across the Channel has now un-stuck it?). The test is to slacken off the oil filler cap on top of the rocker cover, then - with the engine running - unscrew it altogether and slide a piece of paper over the top of the filler-hole. If the vent valve is working properly, then the paper should be held lightly on due to a slight vacuum over the hole. If this test doesn't show any vacuum, then I need to take the rocker cover off and inspect the vent-valve (there is a possibility that the hose and or jubilee-clips holding it on have failed).

Time to get my hands oily!

 

[oldharry]

Thanks peeps for the advice. Now spoken to the Bukh engineer who services the engine, who has given me a few clues to follow up, as well as advising me that on Bukh DV20s, which don't have a large oil capacity (3l), one shouldn't really need to top up the oil at all, let alone every 40 hrs as I've been doing for the last two seasons.

The first question is where is the oil going? Its either leaking out the engine (check oil pressure sender, also check front end) in which case there'll be evidence of it in bilges, else its being burnt and going out the exhaust, in which case there'll be some black smoke in the exhaust gasses (but maybe not enough to really notice?).

He is right about the oil consumption. The DV20 I had in my previous boat never used any oil.

If its burning oil the smoke will be blue, not black, like the blue haze from an old car or 2 stroke bike. Black smoke is caused by fuel problems, or overloading. I agree with your engineer, there is nothing better for a diesel than a good long run at crusing speed - solves all sorts of 'gumming up' problems, and this sounds to be what could have happened in your case.

 

[Fire99]

All marine engines i've had, from little 2hp 2-stroke OB's to 200hp V8 Petrols, have run significantly better after long runs rather than lots of short start/stop trips.

Significant amounts of water in your oil would be pretty obvious with a grey emulsion appearing on the dipstick / oil filler cap etc.

Diesel leaking into the engine is possible but i'd bet 50p the engine has been enjoying it's long consistent run.

If there are no visual signs of anything bad and it runs and sounds perfect, i'd doubt anything sinnister is going on.

 

[Boo2]

else its being burnt and going out the exhaust, in which case there'll be some black smoke in the exhaust gasses

I thought it was black smoke for excess fueling, white smoke for water (ie head gasket) and grey smoke for oil burning ?

Boo2