The coolant explosion reminded me of what happened after we just finished our
5-cylinder conversion last week and ran the van for a while.  When we shut
down the engine (as it was overheating) the expansion tank went pop
(overflowed out of the cap).  It was due to an airpocket in the system
(apparantly we didn't bleed the system enough).
After we bled the system then no more problems.
However on the Vanagon I have heard that the tank can develop hair cracks
around the sensor hole and this can cause leaks, and possibly the ejection of
the sensor.  New tanks with caps are not that expensive and also be sure to
get a new sensor and o-ring.
Have fun and I hope you don't get a repeat.
Ken Wilford
Van-Again
John 3:16
PS Bleeding the 5-cylinder Vanagon setup is so much easier than the
waterboxer.  I used a A2 Jetta expansion tank that just slides into place.
When you go to bleed the system you just fill the tank, crack the bleed on
the rad and pick the tank up until it is above the radiator (the tank becomes
the high spot).  Do this a couple of times and the system is bled.  No
jacking, no running at 2000 rpms, no nothin'.