94 Shadow (GF car)
2.5 TBI
Auto

The original heater core failed at about 160k, and now I have put 3 different cores in over the last year and a half. ( I have this job down to about 4 hours now ) They last about 3 months and then one day you start the car and get a whiff of antifreeze and within a few weeks, the car will start loosing enough coolant to get close to overheating. The 1st replacement core was from Autozone, I went to Rock Auto after that and the one in the car is from NAPA and just started leaking last week. I cant for the life of me figure out why these are failing. They are plumbed with stock hose routing, I eliminated the intake heating loop after the last one in case that might have caused an issue. I have put a pressure tester on the radiator and have no indications of leakdown, and I ran the car up to temperature with the gauge installed and the pressure peaked below 20 psi (if I remember right). Nothing to indicate a pressure spike or abnormal conditions, the car runs cool even with the A/C on and 113 OAT.

Anyone else have a problem similar to this or got any suggestions on where to start?