We just changed all 6 GPs on my son's W210 turbo diesel and the dreaded happened on one of the glow plugs.

Here's what seemed to not work: 1/4" dewalt impact driver (think deck screws, construction). This did not have the power to get them out. I've also seen a video of a guy using a 1/2" air impact wrench. This got them out but also broke some. Also, muscling the GP out does NOT work.

Here's what did work: a 3/8" breaker bar and light tapping clockwise and counter clockwise. Do not muscle the GP out or it will break. Just swing the long breaker bar back and forth and let it sort of "impact" the GP loose a little at a time. Tip: Mix acetone and transmission fluid and spray glow plugs before and during removal. After spraying turn them back in a little. Back and forth is the trick; not only counter clockwise. Also, warm up the engine to running temp (we ran it without the intake, but of course do this at your own risk) before beginning the GP removal process. Using this method none of them broke. Some came out easy and some were challenging. If you hear creaking, that is one to be VERY careful with!

The one that did break was before this method and we got it out by breaking off the remaining electrode, welding a nut on what was left inside and turning that counter clockwise. Now we had an empty GP housing to work with. We then tapped the housing of the GP to M8x1.25, but before actually tapping we drilled it slightly bigger than M8 in the section where the GP threads were at (since this will be drilled out). After it was tapped to M8 to the depth of about half of the GP body (not counting the threaded section since that will be gone), we drilled the GP slightly smaller than the actual GP's threads. I think we bored the factory threaded section of the GP to 3/8" diameter. 0.400" was what we were shooting for (just smaller than the minor diameter of the stock threads) but we ended up off center so 0.375" diameter was as big as we could go. In a way it worked out well that it was off center because we almost were through on one side. This allowed the GP metal to fail here when pulled and squish as it pulled out (as opposed to being rigid and holding its threads into the head).

We then inserted a grade 8 M8x1.25 threaded rod by double nutting it. I would assume we had 8-12 threads worth of engagement into the GP (probably 1/2 to 5/8" of thread grab). We removed the double nuts that were used to install the rod, put a 13mm socket over the GP, put a washer and grade 8 nut over that and pulled it out. It came out with some torque but not enough to break anything. I could tell it was pulling the GP or I would have stopped cranking on the nut.

In an ideal world, the 3/8" drill bit we used to clear out the threaded area of the GP would have been more centered and we could have drilled out a little bigger still (without touching the head's thread for the GP). But in the end it worked. We did not damage the heads threads. AND NOTE: we only drilled to 3/8" up to the end of the GP threads (mark the bit for depth). Otherwise, there will be no M8 threads to pull the GP out with!

The idea came from DieselPumpUK (YouTube video from 7 years or so back) except he used a slide hammer. The threaded rod and socket puller worked pretty well and probably was a little easier to use in the engine bay than his huge slide hammer (which worked great on an engine out of the car). Here is Luke's very useful video:
https://youtu.be/U9AO_cDdwS8?t=40


NOTE: to turn the nut, no I did not use the socket wrench. I used a 13mm wrench (the box end) with another wrench hooked onto the 13mm to add leverage. Could have also used a through socket (would have worked even better). I would estimate the torque needed to be at 30-50 ft lbs.

Other tips: use a short handle tap wrench since there isn't much room. And if uncomfortable with a tap, find someone who is. Too much torque on a tap or tilting it will lead to the tap breaking off in the GP body. This is an even bigger problem. If in doubt, have someone else do the tapping.

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