I just managed to split the first plug. Pulled the stem from the body. Still have threads sticking out. Deciding what to do next. It's a clean shot to drill it out from the top. I don't think my easy out's are long enough to reach nor strong enough. Is it me or is the head aluminum? Put in steel with no anti seize & this is what you get.
Lets back track-How many miles on your Sprinter? Is separation/split of the glow plug the reason you could not pull it out initially? How many are affected (broken)? How many are turning but unbroken and won't come out?
Glow plugs are not very different from spark plugs when it comes to extraction difficulties.
First, the glow plug threads are not quite "steel" against the aluminum head threads but are nickel plated to help prevent seizure.
I have seen anti-seize treated, new, nickel plated spark plugs, correctly torqued, seize in an aluminum head-then break when removal was attempted. Anti-seize can help but it is not the "cure". Exercising or replacing spark plugs periodically is critical to helping prevent seizure-glow plugs may require similar attention.
The cylinder head must be at operating temp when first attempting to loosen glow plugs. If a glow plug won't break loose try again at the next heat cycling of the cyl head- patience is extremely important as is lack of brute force.
Liquid penetrate will not go where you need it if the plug is still seated-threads must be partially exposed for penetrate to help. Doktor A