Kubota D600 high crankcase pressure.

[davierobb]

I had an exhaust leak last season and tracked it down to a crack in the exhaust manifold. Having sorted this I now find that exhaust gas is also coming from the crankcase vent that vents into the engine bay.

Is there any easy way to determine whether the problem is caused by worn valve/stems or piston rings. There are visible exhaust gases coming out the breather pipe, would this suggest that the fault lies with the valve/stems as there is enough smoke to be clearly seen. I am assuming that worn piston rings would cause venting into the sump and that the gas would be cleaned of visible smoke before it left the breather pipe, or is this assumption just, bo11ocks.

Any other ideas on the cause would be appreciated.

The engine is in a 60’s snappy so I would prefer to make repairs as opposed to initialling a new engine. Any views or opinions the later course of action.

The engine seems to be developing good power. I have just fitted a new prop (14x8) to replace old prop (14x12) and it pushes us along at max 6.5 knts. As an aside, the new prop has cured the horrendous propwalk/propwash that the old prop created and has added about 0.5 to 1.0 knts to our max speed.

 

[boguing]

A compression test would give you a pointer towards the rings.

But if it's the rings then you're going to be taking the head off anyway. So why not take the head off, check the stem guides and seals, grind the valves and have a wobble of the piston crowns while you're in there?

 

[Topcat47]

Rings are the most likely culprit......sorry. If it was guides, then the exhaust would find it's way into the rocker cover and be vented into the inlet via the breather (probably). It really is very unlikely that exhaust gasses would find their way into the crank case from the head.

I'm not sure the news would be any better if it was burnt lub oil smoking out of the vent....

 

[davierobb]

Thanks for the reply. I think that in my lack of mechanical understanding that I have incorrectly described the problem. The exhaust gas is coming from a vent pipe in the rocker cover, I had assumed that pressure in the crankcase would eventually emerge in the rocker cover area via the oil drains from the head to the sump. Is this a correct assumption?

If my description of crankcase pressure was incorrect then is it likely to be the valves/guides?

 

[Topcat47]

Do you have a crankcase breather on your engine? I'm not familiar with the Kubota.

Engines do have small drains in the head to all the small amount of oil lubricating the valve gear to return to the sump. If you have exhaust gasses in the sump and this is the only place for them to escape, then I'd expect lub oil to back-up in the head. This may not be hugely noticeable in a slow speed oil engine. The breather in the rocker cover is usually coping with a little lub-oil smoke caused by the higher temperatures here than in the rest of the engine. Oil build-up in the head will produce more smoke and anyone with any experience of BMC A-Series petrol engines will attest add a litle condensate to the mixture and you'll get white mush in the rocker box, which may block the breather.

I am surprised the rocker box breather doesn't go the the back of the air filter, but as I said, I'm unfamiliar with the Kubota.

If it is exhaust fumes, rather than oil smoke a broken piston ring might show up as a pulse in the smoke, coinciding with the firing stroke. The smell should give you a clue,

If it's lub oil smoke, there may be some sludge blocking (even partially) the head drain hole. It's often worth flushing the system with flushing oil and replacing the filters and oil.

A trick worth trying is remove the dipstick with the engine running, if you do have exhaust gasses in the sump, this'll be the easiest place for them to escape and you can see them clearly.