Plastic Welding is virtually worthless on old stressed out plastic of
any formulation and cross linked polyethylene would be even harder to
weld, if not impossible.  Linear, you might have a shot.  Telling the
difference between the two is a real trip too.  Rescue tape might not
save you here either.

DM&FSAt 05:55 PM 2/21/2011, David Beierl wrote:
>I recently pulled the coolant pressure tank on my '89 2.1l WBX and
>found three significant stress cracks that looked ready to
>fail.  Photo of one here:
>https://picasaweb.google.com/dbeierl/Vanagon1989PressureBottleStressCracking#
>- it looks worse in person than in that still-installed flash picture.
>
>I would suggest draining and removing the tank and examining it with
>a strong light shining through it from behind and above.  The
>likeliest areas for stress cracks are at the areas immediately above
>and below the joint running around the circumference; the third crack
>on mine was on the neck, in line between the pressure cap and
>low-coolant sensor.  The first two (one above, one below) were long
>and clearly formed by repeated flexing of the tank.  I estimate that
>patching them on the road would be very difficult except by plastic
>welding, because of the proximity of the joint.  The third was much
>shorter, gaped open in the middle and was of a different character
>generally.  If I learn anything of significance about it I'll say so.
>
>Yours,
>David