If a Valve steam seal fails can intake boost get in to the crankcase?

[mossietheno1]

Hi all,

My Mazda 3 1.6 diesel (DV6C). It has always had a bit of oil leak on start up, I have put this down to higher mileage valve stem seals (245000km ~153000miles). There is no noticeable movement on the dip stick and only last for the normal 10-20 sec on start up and then clear as day. There is a throttle but is fully open unless the EGR is engaged and that only happens under low boost condition not under no boost. This mean the only time there is vacuum(less then 12psi or less the atmospheric) is on start up.

My question is that is the stem seal leak oil down into the intake runner when under vacuum then can boost get into the crankcase and pressure the crankcase. Looking up how to make a rig to test crankcase pressure.

My max boost is 22psi normally about 14psi on a gtv1544v. Crankcase pressure should be about 0-6psi but I'm used to petrol so that might be different in diesel land.

 

[Trav]

Diesels and forced induction engines produce no real vacuum without help from a vacuum pump for scavenging crankcase vapors and creating negative crankcase pressure.
Diesels take all the air they can ingest all the time and regulate fuel delivery only for throttle, add pressure in the intake manifold and it can add some pressure through the intake valve guides through the open bottom of seals when the valve opens, the guides would probably have some wear for this to happen to any significant degree. This will not happen with the exhaust valves, in fact years ago some engines didn't even use exhaust valve seals.

Different manufacturers have different ways of using vacuum pumps so this is just generic info. If you have higher than normal crankcase pressure check the pump and lines/passages.

 

[mossietheno1]

Hi Trav, so It is possible for this to happen, I'm asking if the valve stem seals are the seal that stop intake pressure going into the cam area (OHC) or is there another seal that stops that.

That's odd you think there is exhaust pressure and you don't want them gases coming into the oil, but i suppose that that stuff is getting into the oil anyway bit by bit via the cylinder walls. As well thinking about it crankcase is a bit higher then outside/atmospheric so i cant see too much taking that route.

Ive have seen a throttle sensor on every diesel I've worked on (only worked on 4 so take that with grain of salt) but they all had trottles I not sure but In think its to generate a restriction to allow more EGR flow under light loads.

I don't really know how a vacuum pump is used. I have basic understanding of how it works from understand how the intake manifold is used as the vacuum pump in a petrol. how would the increase crankcase pressure have a interaction with the pump. Is it because the vacuum pump alsos pull the crankcase gases out like a petrol along with a PCV stopping it generating a negative crankcase pressure?

 

[Trav]

Sure they have a throttle but it is different from a gasoline engine, a gasoline engine regulates the engine rpm through the throttle body or car and in FI systems controlling the timing of the injectors but that is getting away from the basics. Modern diesels use electronic engine controls but the principal is the same as it always was, the fuel delivery determines rpm not the throttle plate in a throttle body.

Some vacuum pumps are engine mounted with internal passages with takes offs for brake boosters and other vacuum controls, others are belt driven external units but all can used in the scavenging of crankcase gasses, PCV valves are not really nessecery with a diesel although a fixed oriface my be used to maintain pump vacuum if one is used.
The effects of negative pressure in all engines is negated by use of a crankcase breather, basically vacuum in one end and fresh air (usually through a hose or pipe connected to the air induction system post air filter).

With so many engine controls on modern diesels its best to get a FSM and look at the systems.

 

[mossietheno1]

That all makes sense.
Trying to find a manual atm, hard to find, there must be some where you can pay for that info from Mazda.

 

[Trav]

You will probably have better luck with that search in the UK where more diesels were sold.