View Full Version : Small diesel engine blowby

14-01-10, 04:37
G'day People,

I have a small old 8hp Bukh DV8 diesel inboard in my small yacht.

Its been running faultlessly until last week, when it sprayed about half its oil a full 360 degrees around the engine cowl !.

It still ran ok, and an engineer says its from blowby/high crankcase pressure but unsure why.

My questions are:

1) Apart from rebuilding the whole thing, how can i determine if its valve or ring or piston related ?

2) It now seems to be running ok - and oil consumption has stabilised - is it ok to continue running the engine with high crankcase pressure ? (Will I be doing any further damage ??)

3) How do you measure compression in such an engine ? (with no glow plugs)

4) Is there an acceptable amount of blowby that can be tolerated in these engines ?

Any info would be greatly appreciated before it starts costing mega dollars !!


14-01-10, 10:41
Welcome to the forum!

You don't say where you are located, but your greeting suggests a long way away!

However, the experts on Bukh are the UK agents based in Poole www.Bukh.co.uk They are very helpful so drop them an email

14-01-10, 10:46
I spoke to them shortly before Xmas and was told they don't do emails. However, they are very helpful on the phone. Number on the website.

14-01-10, 12:24
G'day Craig and welcome aboard the YBW forums.

First step would to do a compression test, just remove the injector and leave the decompression lever in the run position, spin her over and note the reading.

Now put enough engine oil into the cylinder to cover the top of the piston, not easy I know, then spin her up again and note threading.

Compare the readings;
If the second reading is say more than 10% higher your valves are probably ok but you may have a problem with the piston or rings.

If you do decide to pull the engine down, DO NOT pull the governor apart, it's a mass of interconnected cones connected to a spring as the bottom of the engine, and be resembled in several different ways, known only by the Druids.

Parts are very expensive.

14-01-10, 12:34
I don't know your engine but it sounds from your description as though the crankcase breather may have blocked, pressurising the crankcase and blowing the oil out of one of the crankshaft seals. This is a recognised problem on the DV20. On that engine the breather is a small hole that emerges into the air inlet on the cylinder head and it must be cleaned (although I cannot find any mention of it in the very poor workshop manual) I suggest you have a good look around and find where it is on your engine.

All engines generate some crankcase pressure and it may well be that if you clean your breather out it will be fine.

15-01-10, 07:31
Thanks very much for all your great info !

Assuming its no longer loosing oil, am i doing any further damage by running an enging with high crank case pressure ?? (It was suggested that the oil may only have come from the breather whilst heeling when motor-sailing).

Thanks again for all your help - your knowledge is a great resource !!

15-01-10, 08:19
two things come to mind : 1. was the engine overfilled ? this could increase c/case pressure . 2. fit a bigger breather , did this to a v/penta engine and it cured the blowby ,it was not fully opened out and restricted easily . good luck

15-01-10, 09:14
Assuming its no longer loosing oil, am i doing any further damage by running an enging with high crank case pressure ?? (It was suggested that the oil may only have come from the breather whilst heeling when motor-sailing).

The breather must be clear. Under normal operation there is almost no pressure in the crankcase. Running it with pressure risks various seal problems that would be expensive and difficult to repair.

The suggestion cannot be true. The breather comes from high on the engine and the boat would almost need to be inverted for oil to run from it.

16-01-10, 16:49
A bit puzzled by the engineers diagnosis: unless you had used the engine at much higher revs than usual, blowby from rings/bore wear would have built up over time. Wear does not happen suddenly! Nor would it have stabilised again as you describe. Also why did it spray all round the engine compartment? Where did it come from? It obviously got on to a rotating part - probably the flywheel - to get the 360 degree spray. I would want to do a bit more checking round the oil pump and pipes which are down the front of the engine IIRC before deciding doing a full strip.

I would check the breather system also, as suggested.

A quick test for crankcase pressure is to open the oil filler cap. If there is pressure it will relieve itself that way rather than through the breather! Same test as opening the oil filler on an old banger when it is running and warm. Be ready for a mess! If you can block the breather system, hold a rag firmly sealing the oil filler and run at fast tickover for 8 - 10 seconds. If there is a 'whoosh' when you release the pressure - then yes you have a big problem! DOnt let it build too much, as you can damage or blow out the seals, but the engine would need stripping anyway if there are bore troubles and seals would need replacing anyway on rebuild.

Generally speaking, a worn engine will start blowing oil out of its breather pipe, which either goes back to the inlet manifold or dribbles all over the engine so that it gets in a mess. In the manifold it can build enough for the engine to start burning it as fuel, causing it to run uncontrollably for a few moments - I had an old Sherpa that used to do it - spectacular and causes a change of underwear each time, but it always survived to run another few hundred miles before the MOT man finally did for it!

16-01-10, 18:33
Just my 2pennorth, but I had a similar problem with oil sprayed all round 360 deg at the front of the engine, awful mess.

It was apparent that it had come out behind the flywheel which had then flung it everywhere.

Cause was a worn crankshaft seal which leaked and threw oil onto the flywheel.

Cured by replacing the seal.

Could this be the problem?