* My intention with this post is to share my experience with the EGR pipe replacement job. It may not save someone else the hassle of doing the job, but it will at least clarify, before hand, what's involved.
* These days I mostly work on '95 and older cars, but I do have several early W210 customers. One such 2/96 W210.055 w/ HFM 104.995, came in with the familiar P0400 code. After replacing the common broken plastic vacuum line near the switch-over valve, and the EGR valve (per TSB on sticking valves), I thought I was done. The car came back with yet another P0400 code.
* I read most everything on the forum related to the EGR malfunction fault and realized that the EGR pipe needed to be checked. I could hear the compressed air, introduced at the EGR-valve-end of the pipe, leaking into the intake manifold. So the EGR pipe wasn't totally blocked, possibly just restricted. I didn't have another W210, with a clear pipe, to compare to. Had read that the carbon restriction usually occured at the intake manifold end. Detaching that end and trying to examine the pipe end, in situ, is difficult and not that conclusive. Nevertheless, I decided to replace the pipe.
* The posts that I read indicated that this was ~3 hours work and didn't require removing the intake manifold. If that fact is true for some models (and I seriously doubt that) it isn't true for the 96 W210/M104 above.
* This job required removing the intake manifold (upper section of two-part manifold) and the injector rail w/ injectors. A reasonable time quote for the second or third pipe is 6 hours. If it's your first pipe, you'll probably take longer.
* You'll probably also want to replace the int manifold seals and the two o-rings on each injector. Don't forget to get a new ferrule, nut, and gasket with the new pipe. To speed up reassembly, turn the clamps on the rear hose of the two intake hose sections so that the clamps can be tightened from below. If you decide to take the resonance chamber off the intake manifold before removing the manifold, there are 4 allen bolts, 2 of which are hidden under rubber plugs.
* The only dimensional difference between my new pipe and my old pipe was a slightly larger OD (+.008in) on the new pipe. The fit is very precise, so bending/reshaping the pipe to remove/install it is definitely not an option. But with the int manifold out of the way, the new pipe fits like a glove.

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

Did you try the speedo cable cleaning trick first?

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Prost!

* 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

I recently pulled my EGR valve and cleaned the pipe out. The pipe was not totally blocked, but it was restricted.
It is not necessary to remove the intake manifold on a 1998 E320.
I did install a new EGR gasket.
My CEL went out 3 weeks ago.
Happy,happy,happy!