Indeed, Engatwork's suggestion above was exactly the process I used successfully. Literally just got the last of the three stubborn glow plugs removed. Once I was able to turn the plug, I just kept at it, back and forth by degrees initially, then more pronounced as it began to move a little freer. Each of the three plugs continually chattered and popped loudly as they were turned and didn't move freely until the very end.

A couple of thoughts, now that this fraught procedure is over, from a Mercedes and diesel newbie, so take this lengthy recap for what it's worth. First is that the glow plugs were stuck because of a buildup of carbon and tar on the shaft that extends below the threads--not due to the tip or corrosion seizing the threads. The tips were all straight, though some showed a little buildup of carbon. This accumulation on the metal shell, not the heating tube, made it difficult to withdraw the plug since it was in contact with the carbon on the sides of the pre-chamber. Even when the plug was turning relatively fairly freely, there was enough friction so that it could not be withdrawn, even with vice grips. See photo of plug. This vehicle just turned 135K, and there was no indication of the plugs ever having been replaced, so that's not surprising.

Once the plug was entirely unthreaded from the head but still unwilling to be withdrawn, I sprayed copious amounts of 2+2, a carb cleaner that works on carbon, around the threaded hole and just continued to turn the plug back and forth. This was a drawn-out process that took probably five hours for the three plugs. One plug eventually backed itself out as if it were threaded all the way to the heating tube. The other two weren't so cooperative.

With the plug rotating relatively freely and without the scary noises, I got a screwdriver under the hexagonal shoulder above the threads and levered on it while turning the ratchet. This eventually allowed to the plug to be extracted slowly, along with frequent sprays of 2+2. The damage to the threads in the glow plug picture was caused by the levering action of the driver.

What made this removal somewhat easier was the use of a Snap-on socket, FSMS12. This is a stubby, semi-deep socket a tad over 1.5 inches long. With my normal-sized deep socket, the ratchet would require several extensions to get it past the metal hoses, vacuum lines and wires to give it some room to turn. The Snap-on socket was short enough so that I could get a 3/8" breaker bar on the end of it and fit it between the metal fuel lines and other fittings. See photo. This gave me a surer control of the breaker bar than with a long extension and also eliminated the possibility of losing control of it and breaking something else. The socket was short enough to allow me to fit it over all of the glow plugs without removing anything. It also allowed me to slip it from the plug shoulder and move it to a new gripping point without pulling it totally off the plug.

Now to buy a reamer and some anti-seize compound and do the installation of the new plugs. A satisfying end to a somewhat nerve-wracking DIY project. Then, onward to the engine harness replacement.

Regards,
Tom

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