I finally got into a spark plug change on an ML320 and thought the experience might be worth adding to the large group of comments on that subject. This was the FIRST plug change on this car, and I found that the factory had installed MIXED spark plugs, all Bosch -- but some were +2's and others were regular single post Platinum?!?!

To summarize the job experience:

I used all the tools mentioned in other threads and then some. These included the normal wrenches and sockets and other attachments, PLUS
a pair of curved needle nosed pliers and a short length of pipe to use as an extension of the ratchet handle. That became necessary on 4 or 5 of the plugs -- which were firmly seized in the block, after 90K miles, and took tremendous leverage to start them out of the "socket" in the block. The short length of pipe provided that leverage handily.

I used the curved needle noses to help pull off the metal plug boots, which were very tight. By carefully inserting one side of the needle nose pliers into the outer end of the boot, it was possible to exert significant pulling force on them in some cases and remove them. I also used the 17mm open end wrench and a wire removal tool with padded jaws -- that wasn't too sturdy as it turned out.

I did not remove fender liners or the bottom of the air filter module, etc.etc.
I simply took off the top engine cover and the top of the air filter module, moving the air intake up out of the way and tying it up slightly with nylon cord.

I then simply disconnected the coil modules from the harness and removed the coil module bolts; then pulled the metal boots off the plugs, placing the coil modules with wires attached near the car in their respective sequence for reinstallation. The driver's side was more difficult in terms of access to the boots hemselves (less room to get at them). Do it first and the job remainder will be easier!!

All in all, it was not an extremely difficult DIY job, with the exception of those cantankerous metal boots -- which were very tightly ensconced around the plugs. The rest of the job, tight spark plugs included, was really fairly straight forward.

I installed Bosch +4's, putting a little dielectric paste in each boot and some anti-seize on the plug threads.

I hope this helps others who plan to do this job and recommend the curved needle nose pliers for careful use on the metal boots, and a short piece of "leverage pipe" for the ratchet handle, in order to break lose the stubborn plugs you may encounter

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Ben Harrison Carter
1999 Mercedes ML320 87K
1992 Mercedes 300CE 89K
1995 Corvette 29K -- Sold Dec 09
1989 Mercedes 420SEL 99K -- (Sold 4/08)
1968 Mercedes 230S (106K) (Sold 9/06))
1976 Mercedes 450SEL 130K (Just sold - 06)
1961 Mercedes 220Sb (sold years ago)