* Hirnbeiss, no, I didn't use a speedo cable. I used a larger OD flexible cable with fewer turns per inch. Needless to say, it didn't make it past all the bends and never got to the last 2 inches of the pipe, where all the caked carbon was located.
* But I didn't know if or where the carbon was until I had the pipe out. The old pipe was restricted but not blocked. And since I'd never replaced a pipe before, under these circumstances, or know how air flow through it should sound, it was a process-of-elimination guess as the source of the recurring P0400 code. The owner liked the more permanent solution of replacing the pipe with a new one, so I didn't try to find a cable that would make all the pipe bends.
* I'll keep an eye out, though, for a speedo cable to try on the next pipe. It seems to me, though, that it's still a fairly subjective process; that is, you judge the air flow through the pipe by the sound of the flow, not by some measured test. A clear examination of the manifold-end of the pipe isn't as possible or definitive as the post(s) imply. In short, it'll be a guess that decides when you're done (and the customer may be back in two months for the same problem).
* I also got in trouble by quoting the labor time I'd read on the forum. The intake manifold, and related parts, DID have to come off to replace the pipe. And this doubled the time quote. With my errors and new-job pace, my time was more like three times the forum quote. (Now that I've done one, I plan to carefully time the second one. I suspect that 6 hours will be a fair time to charge).
* But the job turned out very well in every way anyway. The new pipe fit like a glove. I learned some useful lessons, not the least of which were caveats for forum posts. The customer is wise, patient, and kind and wasn't unduly upset by the overrun. (It also helps that I charge $40/hr labor in my semi-retirement shop at home. I would've had more trouble in the full-size shop I sold 10 years ago).

__________________
ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536