[tcb] Re: Speaking of Valves
- From: atx <atx_bus@xxxxxxxxx>
- To: "tcb@xxxxxxxxxxxxx" <tcb@xxxxxxxxxxxxx>
- Date: Mon, 25 Jan 2010 19:15:00 -0800 (PST)
Good stuff. How do you vent the covers?
Jeff
sent via iPhone
On Jan 25, 2010, at 9:02 PM, "w.wood" <evil.scientist.boo@xxxxxxxxx> wrote:
it's heat and pressure related. even when an engine has very little leakdown
(piston rings, valve guides etc.) there's still a lot of pressure that builds
up in the crankcase. In a stock ACVW the 3/4 side also get a lot of oil slosh
(windage) into the valvetrain area which further causes some pull when it
drains back in. This means that the air inside those covers is also less dense
than the air outside the valve covers. all it takes is enough pressure
differential and heat and you'll pull a gasket.
I used to think it was just bad gaskets or warped covers but venting those same
covers and alleviating the crankcase pressure helps solve the problem. If you
don't want to vent the covers then a little permatex works wonders. Just be
prepared to do a little scraping between gasket changes.
On Mon, Jan 25, 2010 at 12:49 PM, sammie smith <bugcollections@xxxxxxxxxxxxx>
wrote:
Here is a good tech question for you Will or any other ingenious VW
engineer/mechanics that want to chime in.
What causes the valve cover gaskets to "suck" in, i.e., sometimes work loose on
the bottom and move up into the rocker chamber, which of course causes
tremendous oil leakage. Happened to a VW mechanic friend of mine a while back
on his daughters car and she ran it out of oil with all of the resulting burned
up engine problems. Valve cover gasket had "sucked" resulting in loosing all
of the oil in the engine.
I know how to prevent it: have talked about that before; but what causes it.
Got into an academic discussion with another VW mechanic friend about how is it
possible to "suck" a valve cover gasket. Normally there is positive crankcase
pressure and the pressure through the push rod tubes should keep a positive
pressure inside the valve cover/rocker assembly resulting in the pressure to
attempt to "push" the valve cover gasket outwards, which of course it can't do
because of the lip around the valve cover. Have talked to one old VW dirt
track racer who ran big stroker motors on the dirt tracks where there was a lot
of acceleration/deceleration of the engine and it he says it was a common
problem which they usually cured by welding some tabs around the inner side of
the gasket channel. Have had it happen to me and it is a rather common
occurence in the VW engine.
But what causes it? For the engine to "suck" a valve cover gasket there has
got to be at least some time when there is a negative pressure (vacuum) in that
chamber. With my knowledge of the flat-four, 4-cycle engine, I don't know how
that is possible.
The only thing I have been able to deduce is that it might be possible under
extreme engine deceleration for the piston on the down-side compression stroke
(since it has no positive pressure on that stroke under deceleration) to have a
vacuum so severe that it sucks pressure through the valve guides and creates a
vacuum in the valve cover chamber. But I don't really think that is possible.
Comments?
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