Depending on which plug it is, you may be able to drill it out. Here is my experience.
Len
It was about a month ago that my glow plug saga started and now that it is nearing an end, I wanted to update you, especially those with a 210 chassis diesel.
My glow plug light and check engine lights came on on my instrument cluster. Had the code read and it indicated a bad glow plug(s). Turned out that #2 plug was bad. In order to replace any plug but possibly #1, the manifold has to come off. The general feeling is that while the manifold is off, you may as well replace them all to avoid having to go through the repair again anytime soon. So that was my plan.
Many thanks to Lightman and Rick from MercedesShop.com who did a great write up about glow plug replacement. See it at
http://www.mercedesshop.com/shopforu...&postid=580948 The removal of the manifold is not that big of a deal, so I wouldn't hesitate to tackle that again.
Plugs 2 - 6 came out with no major headaches. #1 would not budge. Looking closely at #1, I noticed that there were a lot more threads showing sticking out of the head than the others. It turns out that #1 plug was replaced by the dealer about 18 months ago. It was done without removing the head. It is my theory that it was a little hard to reach and see and it wasn't screwed in all the way and thus did not seal, allowing carbon to enter and seal that sucker up solidly in the prechamber.
So I had to either leave it alone or get it out. I chose to try to remove it since at sometime someone, most likely me would have to do it anyway. How hard could it be I figured? It was in so tightly that my socket eventually rounded off the hex head. So then I went to a Snap-on socket that grabs the flats instead of the corners. Same thing - it wouldn't grab it either and just clicked around the glow plug. Next came a twist socket that grabs the hex head and digs into it while applying force. It did its job so well that it tore off the tip of the glow plug just above the threads. So now where do I go? After reading many posts on many different lists, I found out that this was not a terribly unusual occurrence and that everyone, but one person had to have the head removed to the tune of about $2600 to get this thing fixed. That one person was Alan McReynolds who was able to get his broken off plug out by drilling and tapping and pulling. BTW Mercedes has come up with a new procedure, P-B-8.20/134, and a tool, p/n 611 589 00 99 00, for dealing with this situation. I thought that maybe I'd go that route and save the expense of having to remove the head. Well, it turns out that the tool/kit lists for $1300 at Mercedes to do this job. I'm not sure exactly what the procedure is, but it wouldn't be all that much cheaper than removing the head once all is said and done.
So with Alan's guidance, I tired to do it myself. Actually, not all of it myself. After fooling with it for almost a month,my son came to visit over Thanksgiving and took it as a personal challenge to fix this. Did all of the following - drilled out the center heating element as deep as I could without drilling all the way through the end of the plug - used a drill bit a few 64ths smaller than what was required for the M8 tap. Drilled out the glow plug's threads in an attempt to free them up from the head - the shaft of the glow plug measures about 0.41 inches and a 25/64ths is just a hair under that. Tapped the inside of the glow plug shaft with first an M8 and then a 3/8th's tap. Tried threading a bolt into the glow plug's fresh threads and pulling it out. Took a blue wrench and freeze spray to the head and the glow plug. Nothing would budge it. We were at the point where we were about ready to give up and go the head removal route. My son said that as a last resort, let's try to drill the glow plug shaft out a little bigger in steps and see if that does anything. He took the first drill bit to it and in about five seconds he had the remnants of the glow plug in his hand. You cannot imagine how happy I was to see that. The next hurdle was to see if there were any threads left on the head to allow the new glow plug to screw in. We had done some serious drilling in the area of those plugs and had come very close, too close we suspected, to damaging them. Well, we tapped the hole (it's an M12 x 1.25)in the head, screwed the glow plug in and it torqued down just fine. BTW, I bought a reamer from Baum Tools,
www.baumtools.com, for this job and used it to ream out the glow plug holes. #1 definitely had more carbon built up in it that any of the others.
So everything is back together now and the car runs fine and the warning lights are out. I'm not completely done with the job, however. I had to remove the fuel preheater that is immersed in coolant and am waiting for a new gasket for that. When that comes in, I'll remove the manifold again , check for fuel leaks and make sure everything looks okay before installing the new manifold gasket and buttoning it all up and not have to touch it for a long time I hope.
BTW, total cost of the job was probably not much more than $300. That includes six glow plugs at $20 each and the reamer for $54. The rest of the costs were for various taps that weren't in my set and other miscellaneous bits and pieces.
If it happens again, I think I would proceed a lot more quickly to the drilling it out stage,
I can't thank Alan and my son enough for their guidance and help. Same goes to Lightman and Rick. It is really great when fellow Mercedes owners share their wealth of knowledge and their experiences and insight.
Len
'99 E300TD 71,500 miles
Glow plugs are no fun - help! 
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