[CaptainGizmo]

Steve,
Sometimes a small crack can cause an intake of air rather than a noisy leak. It depends on the flow and scavenging effect. The area between the DPF and SCR is insulated by design because of the need to keep the exhaust gasses hot. Too bad there is no sensor here to monitor the temp. Maybe a probe could be placed against the pipe, with the insulation on top. My gut still tells me the problem is here somewhere.

I would be suprised if the ECM by itself is causing heavy overdosing. Maybe something is telling the ECM that the emissions are off somehow. Maybe something to check would be the EGR. Maybe a restriction or cracked EGR itself. I don't think you had a check engine light but there may be codes that show sensor feedback that would tell the ECM to add more DEF.

In any event, the pipe between the DPF and SCR needs to be clear of DEF crystals or there will be a tendancy for additional crystals to form.

You are a super capable guy and you'll soon fix the problem you've been "blessed" with.

Gee, Bob...thank you for your confidence in me. Actually, ya know...I'm probably more stubborn than capable. LOL But, yuh bedda believe I'm gonna prevail; one way or nuther!

You've got some more great concepts, and YES...I've already pulled that cross-pipe off to make sure it was clear. Not only was it as clean as a whistle, but SO WAS THE FACE OF THE SCR!! Myself and 2 other mechanics inspected it and were happy to see the ceramic element was in great shape.

Stay tuned!  ;-)