Date:         Thu, 02 Jan 1997 21:18:31 -0700
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Will Wood <wkwood@burgoyne.com>
Subject:      Re: how engine oil degrades (was: sysnthetic oils)

At 02:17 PM 1/2/97 -0600, cbennett@kpmg.com wrote:
>Hi there,
>
>I have experience problems with my oil light and buzzer coming on while
driving
>my 86 Vanagon Syncro, and I've concluded that it was because I was using too
>thin oil for the temperature outside.  When I use 20-40 it rarely happens.
>
>But I am still wondering why are there two oil sensors, which I am told
measure
>too high as well as too low oil pressure on the engine. I can understand
too low
>pressure, but why would they want to warn the driver if the oil pressure
is too
>high?  I never understood that.  Can anyone explain it to me?
>
>Thanks,
>
>Charles
>
>

An important thing to note is that when an engine wears, the clearances
between the
moving parts increases.  Thus main and rod bearings allow more oil to
escape, this reduces 
the effective oil pressure within the engine.  When you're going at speed,
the oil pump is
providing enough pressure to keep the light/buzzer quiet.  But if you were
to use a gauge, 
you'd find that the pressure is really too low.  Changing the oil provides
a new viscosity 
level, which with oil age decreases and you have pressure problems again.
Going to a
higher viscosity oil, like 20-50 helps, but indicates that there is an age
problem or other
mechanical issue.  One thing to note is that VW uses (since 71) a dual
pressure system in all of
the flat fours.  I have seen the pressure pistons sieze up.

When this happens, they can be stuck in the "relief" position which allows
most of the pressurized oil to flow back into the crankcase instead of the
main journals.

Although this is rare, I have seen it happen which is why it's one of the
good reasons to change your oil regularly.

As to the too high question, there is a condition known as a blowout.  This
happens
when the pressure entering the oil filter literally blows the filter up.
I've seen it on
Type IVs as well as Type I & IIs using a full flow system.  The pressure
coming off of a 30MM
oil pump can be as much as 300PSI cold, which is why the dual relief system
exists.