So in the process of trying the "wrap the hole in stainless steel wire" technique, and the "make a stainless steel collar out of a hose clamp" technique, I not only didn't fix the original problem, but managed to completely destroy the pipe.
Drumroll, please
:
Luckily the newer pipe still on the van, while barely hanging on by a thread:
(note that the crack goes *all* the way around the pipe...)
...was much more amenable to fixing with the Muggyweld SSF-6 and a propane torch instead of the oxyacetylene that had gotten me in so much trouble before.
I clamped everything into place to make sure the gaps would be as hairline as possible, then started above the crack and chased the SSF-6 down to the flange join. The final result looks really solid, no light, no extra flux caking things up, and it passes a 25 PSI pressure test.
I put the pipe back on the van, and for the moment it's holding. However, I think it will break again, and here's why.
Originally I though that I had stripped one of the bottom screws going into the EGR cooler the first time I did the repair. We'll remember that this is the second time I've fixed this pipe since owning the van. I didn't understand that when using thread grease one had to completely re-calculate recommended torques, and one of the screws stripped upon re-installation into the soft aluminum of the EGR cooler. My solution, since the threading is much deeper than the screw was long, was to find a longer screw and put in 5 exhaust-grade lock washers to take up the slack. Naturally I was supernaturally careful the second time with the longer screw not to overtighten. However, the second pipe failed in the same way, and I think I know why. I think the EGR cooler screw was stripped by whoever installed the last pipe before I got the vehicle. Because that side wasn't bolted down as well, it would probably flex a little when the pipe got hot. The other side was tighter, so it stayed in place. Over time, this differential stress eventually caused the pipe to spring a hole in the far side, as well as start to pull away from the flange.
Once I did the second repair, I had a verified-stripped hole. I compensated with the longer screw that went past the bad threads with lock washers on it. Even if this was as tight as the other side (which it probably wasn't because of my paranoia about stripping it more), it's my belief that the lock washers probably provide enough extra thermal expansion to cause exactly the same problem to happen again. Also, when I installed the second pipe, again being super careful about the stripped hole, I think it may have stripped *again*.
Moral of the story, I think that hole was stripped in the first place, and the lack of solid contact with that side of the flange caused both pipes to fail. What I *should* have done this time was drill out not one but *both* EGR cooler threads and install a heliocoil or similar thread replacement, then gone back to two identical factory torx bolts, using a torque wrench to guarantee identical tightening. Unfortunately there comes a point in every repair where getting it done become a higher priority than getting it done right, and I was already way past that point by the time I got the second pipe fixed. I also categorically refuse to remove the EGR cooler to re-do the threads, meaning it will all have to be done over my head.
If and when the new pipe starts to leak, I will re-tap both holes, or execute my Other Crazy Scheme to more permanently eliminate the problem. Since I have the GDE tune, my EGR valve never opens, so all of this plumbing exists for no other reason that to provide leaks, making all this work even more frustrating and time-wasting.