After a few too many years of Vanagontude, I can't be the only one with an 
array of heater cores ever at the ready to be pressed into service on a 
moments notice.  Now, the rear heater core poses no real dilemma; if a 
replacement leaks, pinch off the hoses and try again.  But, the front core is 
a different beast because, as we all know, if it chooses to piss coolant it 
is a pain to change out.

So what do we do to proof a heater core enough to risk buttoning up the 
dashboard?  Hook up the hoses and run the car until the core is good and hot, 
then close up everything if no leaks are found?  I did that once with a new 
core from an aftermarket vendor (Hey dude, it's made by the same manufacturer 
who supplies them to VW.  Sure).  It must have taken a few thermal cycle to 
open the crack enough to be palpable, for within a week the core was filling 
the air with that sweet smell of failure.  A little irksome to say the least.

I have two ideas of how to test a front heater core before installation and I 
would like to elicit any thoughts or experiences, especially experiences, 
regarding this matter.

1/ Cap off one of the heater core stubs and connect low pressure air set at 
little more than the coolant system relief pressure to the other.  Then, 
immerse the subject of the test in a tub of boiling water and watch for 
bubbles.

2/ I have two spare valved taps in my hydronic heating lines at home.  The 
system operates at 180 �F with about a 30 ft. head.  Properly guard the core 
under test, plumb it to the spare taps and let it run for a week in February.

If anyone says that this is so anal that they want to slap some sense into 
me, let it be the one who has installed a new front heater core, then has had 
to rip it out almost immediately in order to replace it with a junkyard 
jewel.  That experience can justify almost any amount of paranoia.

George