Erosion. Look up cylinder cavitation erosion. I also know of some Hondas that have suffered from bad casting porosity causing oil leaks.

Dennis,
From my phone.
________________________________
From: Al Knoll<mailto:anasasi@GMAIL.COM>
Sent: ‎12/‎31/‎2013 9:19 PM
To: vanagon@GERRY.VANAGON.COM<mailto:vanagon@GERRY.VANAGON.COM>
Subject: Re: Overheating

Porosity as in corrosion?  Or porosity as in casting flaws?

Inquiring minds want some snow.

Pensioner.

On Tue, Dec 31, 2013 at 2:54 PM, Dennis Haynes <d23haynes57@hotmail.com>wrote:

> These blocks are also getting porosity (aka Ford Power Stroke failure) and
> cracks in the cylinder walls.
>
> Dennis,
> From my phone.
> ________________________________
> From: Jim Felder<mailto:jim.felder@gmail.com>
> Sent: 12/31/2013 5:48 PM
> To: Dennis Haynes<mailto:d23haynes57@hotmail.com>; Vanagon mailing
> list<mailto:vanagon@gerry.vanagon.com>
> Subject: Re: Overheating
>
> Yes, in my haste to answer I left out the obvious. Warped or cracked head
> equals a blown gasket as far as the car is concerned. My last round with
> such a problem was only solved, after trying several heads, by sending the
> block out to be resurfaced. The block tends to pull up "hills" around the
> head bolt holes which leaves "valleys" between the studs, in the very
> places that do the water sealing between the head and block.
>
> Jim
>
>
> On Tue, Dec 31, 2013 at 4:18 PM, Dennis Haynes <d23haynes57@hotmail.com
> >wrote:
>
> > Could also be a cracked head or block.
> >
> > Dennis,
> > From my phone.
> > ________________________________
> > From: Steve<mailto:loudmouth_70@YAHOO.COM>
> > Sent: 12/31/2013 3:57 PM
> > To: vanagon@GERRY.VANAGON.COM<mailto:vanagon@GERRY.VANAGON.COM>
> > Subject: Overheating
> >
> > Vanagon Listers:
> >
> > I've got an 82 diesel, she's overheating.  On 5 hour road trip, after
> hour
> > 4 started blowing cold air then back to hot, then cold again.  Soon the
> > needle raised up and light flashed.  Had extra coolant with me, while
> > topping off expansion saw that the cloth wrapped hose to fill tank had
> come
> > off.  Replaced and gingerly made it to destination.  Stopped three times
> to
> > let cool.  I figured I'd gotten air into the system and needed to bleed
> it
> > out.  I've done that today--with little success.  Got it up to temp,
> parked
> > on a ridiculous incline to get the air up to the radiator.  Not much
> > hissing.  After 8 attempts only once did coolant bleed out.  Air
> continued
> > to blow mostly cold with heat cranked, occasionally while driving around
> it
> > warmed a little.  Needle would raise at low RPM's then return closer to
> the
> > middle when I revved it up.  Even at slower speeds with engine revved it
> > would cool--that confuses me a bit as little air is moving over
> >  the radiator--makes me think it'd be hotter.
> >
> > I'm wondering about two things: first what's likely to be the cause of
> > this behavior?  Secondly, any tricks for getting me home before starting
> > these projects?
> >
> > I'll be leaving Friday, I'm at Lake Chelan in WA, little town of Manson
> to
> > be exact.  My gut tells me Radiator for sure, pretty sure it's the
> > original.  Not sure about thermostat or heater core.
> >
> > Happy New Year, appreciate the advice.  Nothing like bringing in the new
> > year with a little repair work.
> >
> > steve
> >
>