Has any one cut one of these heater core's apart to see what's going on inside? Are they leaking at the braze joints (or where?)? What's your water treatment look like?
Some possibilities could include:
1) pH totally out of whack. High ph eats aluminum. Guess what the thinnest aluminum part is? It doesn't mean it will eat a hole, all that's necessary is eating around a braze joint. Are they reusing your old coolant every time? Ask them to do a pH test. That should solve this question. Remember, coolant contains components to help reduce corrosivity, even if the pH measured is slightly outside of the 'safe' range. pH strips are terrible in accuracy, which is what most shops use. A good digital meter is much better...but must be properly calibrated or checked against a known set of pH buffers, of different known pH levels. Then you know your meter is reading properly. See charts below.
2) Bad engine ground and somehow the heater core has become the anode?
3) Too much pressure (Per Burla above). But to add ....it high pressure can be caused by a bad headgasket too. A headgasket can be 'semi-bad' for quite a while prior to a full-on white-out! LOL. How does the upper radiator hose feel when the truck is warm? Do you hear gurgling in the AM when you go to drive the truck? Do you smell combustion through the radiator? Bubbles in the coolant at higher rpm's.
4) 4 bad heater cores in a row. Unlikely, but certainly a possibility. Maybe there was a bad run of H/C's where braze joints are weak. Or the way they are packaged, they take too much abuse via shipping which weakens the inlet pipe braze joint.
5) Incompetent dealer tech getting 'rough' with the h/c during install.
These two charts say a lot: Aluminum is low nobility ...meaning it pretty freely gives up it's electrons compared to many metals (like Copper). And 2) Too high of a pH is good for iron, but not for Aluminum. Meaning a high pH will cause AL corrosion. In a mixed Iron/Aluminum system, a pH of aroud (or just under) 9.0 is within the narrow window for both metals. Copper core heater cores (like back in the old days), copper doesn't readily give up it's electrons (as easily). It could withstand coolant neglect a lot more.
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