boxster engine fault

[panelman]

Boxster Porsche engine was running then cut out with the world supply of smoke coming out of the exhaust and wouldn’t turn over on the starter. Also had a pool of oil on the floor.

Turns out it is hydrolocked with oil. Dealer says that engine breather failed allowing oil into inlet manifold and hence to cylinders.

I’m struggling to see how a failed breather could dump enough liquid oil into the inlet fast enough to cause this (but then electrical is my field)

Anyone got any ideas?

 

[mattsooty]

I'm not really a mechanical sort of bloke but I don't think there should be enough oil in the engine to do that???

MS

 

[GregLocock]

You only need to do it to one cylinder, and 50 cc is enough to do it.


Cheers

Greg Locock

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[patprimmer]

If the Boxter is dry summped like all other Porsche flat 6s I have seen, there certainly is enough oil to do that, but how it would all get into the inlet manifold that quickly is a puzzle. I would need to look at it in detail to figure it out.

If you have a locked engine and a pool of oil, you have a very serious and expensive problem no matter how the condition occurred.

If it did in fact hydraulic lock from oil flowing in a breather, I would be looking to increase the size of the breather to atmosphere, and reducing the size of the breather to the manifold if I repaired it.

I would also look to ensure the oil level could never get anywhere near as high as the breather in the tank.

Regards

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[panelman]

Greg……That’s what I’d worked out too on the basis of 550cc per cylinder and 11:1 CR. The mid two pots were most badly affected.
Pat…….This is a standard production Boxster which is not dry sumped but does have a scavenge pump in each cylinder head which pumps oil back to the crank case.
My thoughts & understanding…..
The sump has bulk liquid oil in the bottom, then an air gap filled with oily air (windage?) then at the top the breather connection. The breather should only ever see oily air, never bulk liquid. Breather is a container, bigger than connections and filled with baffles/gauze. Air flow expands into it and so slows down so big oil droplets settle on baffles/gauze and gravity takes them back to the sump. Less oily air gets sucked into inlet and burnt.
What I’m struggling with is how liquid oil could get into the inlet manifold at all let alone fast enough to cause the problem. If say 30cc got in and didn’t burn then surely most of it would get pushed out past the exhaust valves on the next exhaust stroke so for 50cc to get in one go the flow rate must have been very high, to my mind much higher than could be obtained using inlet manifold vacuum to suck oil uphill .

 

[mattsooty]

Sorry, the brevity of my alcohol fuelled post didnt really explain what I meant (a pattern seen in most of my saturday night posts)

Greg, as you correctly say, 50cc would be ample to hydraulic lock one cylinder but surely far more would have to be supplied to that one cylinder to actually trap that 50cc - on account of the valve overlap that this, a high perfomance engine, is likely to have?

Pat, my thinking was that there should never be enough oil inside the sump when the engine is running to ever even get close to the breather pick up.

This engine will have been tilt rig tested ad nauseum as well as abuse tested on the track to ensure that a breather system failure like this could never occur, probably even tested with an over filled sump.

Bearing in mind that Porsche engines are generally known for their teutonic excellence I would be most surprised if there is not another, underying, failure which has caused this.

MS

 

[kevindurette]

I'm not familiar with your engine specifically, but aren't a lot of Porsche engines oil cooled?

I'm sure the vacuum isn't constant between fluids (air and oil) because it's essentially a positive-displacement pump at that point. The air will compress or expand to accommodate its own inertia and compressibility, but I wouldn't be surprised if you could pull nearly a whole atmosphere on the oil.

 

[patprimmer]

Kevin

You should only be able to pull the nearly an atmosphere on the air in the sump, well above the oil level.

It would need to be grossly overfilled with oil to get even close to the breather, like 15 litres of oil instead of say 5. That's why I asked about a dry sump as dry sump tanks can run say 3/4 full so a bit extra could get the oil near the breather.

Regards

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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[panelman]

The boxster engine departs from the Porsche tradition in that it is water cooled and wet sump.

As far as “teutonic excellence” goes the engine in question has only done 45k miles, been serviced above requirements & gently treated all it’s life and whilst Porsche have set up assistance programs (as in free new engine) for 5 (yes five) different engine destroying design faults unfortunately this fault with this is not one of them.

Whilst I’m not automotive or even mechanical by training I just can’t see how a vacuum operated breather system could dump enough oil into the inlet manifold quickly enough to cause this.

One thought that did occur to me….. as previously mentioned the heads have scavenge pumps to pump the oil back to the crankcase. They also have a connection into the breather above the crankcase connection but below the expansion/separator chamber. If a scavenge pump failed then the high flow/high pressure oil feed to the cams could/would flow through the breather pipe but I would have thought that once the oil hit the breather system proper then gravity would take over and send the oil back to the sump.