Again, many thanks to all for their help in my previous post(s). After putting everything together, tightening fittings thourougly, and replacing the expansion tank cap, my new radiator (cross fingers) seems not to be leaking. The final say will be tomorrow, after it's sat overnight and everything's allowed to cool down.

If everything checks out, then it would appear that the cap was at fault, allowing my previous radiator (Nissens, approx. 1.5 years old) to catastrophically fail when the pressure became too high. This would correspond with the fact that the plastic rad. tank cracked shortly after I shut off the car in the parking lot, since this is the point at which the pressure in the system is highest - no moving air or cooling fan, and turbo still releasing a LOT of heat into the system, I imagine.

When I switched to the new cap, it was immediately apparent that it was doing its job to relieve the pressure - when I opened it, even after driving for a while at full temperature, the level of the coolant only inched up a little. With the old cap, even if I opened it VERY carefully, I would have an incredible upsurge of pressure (even after just 20 seconds or so!), indicating that it had allowed the system to pressurize beyond its capacity.

For everyone on this board, ESPECIALLY for those 940t owners who don't have access to all-metal radiators and have to go with the plastic tanks - let this be a lesson that a silly little $9 part can save a LOT of headache and hundreds of $$$. There's quite a bit of info that the expansion tank cap can leak and cause the system to leak itself - but not much about it doing the opposite and compromising the system with excess pressure. For those with a radiator with a slight flaw or weakness like mine, this can spell disaster in no time.

Personally, I'm going to invest the $9 once a year, and replace this sucker yearly as a high wear item!

All the best to everyone, and many thanks,

Ben