Hi,

I thought I'd post a rundown of the steps I needed to replace the sparkplugs on a 1999 ML 320.

Difficulty: moderate

(only because of the sheer amazing amount of force needed to remove the spark plugs - which gave me fits of self doubt: "WAIT, I didn't miss some advisory about them using left-handed threads on this car...". You almost have to put aside your [perceived] knowledge about fastening torque and aluminum heads)

Also, I removed the left tire and fender liner to get better access - others have reported not needing to do so.

Tools:
* floor jack
* jack stand
* tire iron
* T-30 Torx bit
* 10 mm socket
* 5/8 deep-well sparkplug socket
* ratchet (swivel-head preferable) and extension
* torque wrench
* phillips screwdriver
* 17mm wrench (I did NOT use the Hazet wrench others have referred to, just a standard open wrench)
* masking tape

Steps:
--------
1) jack left front tire up and place jack stand under car (being sure rear wheels are chocked, transmission in park, etc.), and remove left front tire

2) unscrew 6 nuts holding the left fender liner. If you have fender moldings installed, remove the 4 screws holding the molding, and remove the attached clips. Now remove the fender liner - no need to remove the fender molding (if applicable)

3) unscrew 3 10mm nuts holding the heat shield above the manifold in place, and remove the heat shield

4) remove plastic engine cover from top of engine

5) remove engine air filter box by unclipping air intake tube (to manifold), and box cover, then removing air filter, and finally the bottom part of the box. The bottom of the box is NOT held in by screws, so you need to give it a firm tug

6) now pry off the spark plug boots with the 17mm wrench, using the manifolds on the left and right as a fulcrum - others have reported using a piece of wood to get additional leverage. Be careful not to damage the boots, in particular note that the metal part of the boot has a ridge running along it. Note that the 2 wires for each respective cylinder are labelled A and B. Although both the coils and the head have clear labelling, I prefered an additional visual hint and wrapped the bottom of each "A" boot with a piece of masking tape. For the passenger side, I was able pry the boots off from the top, whereas for the driver's side, I accessed the boots from the left wheel well.

7) Unbolt the 6 T-30 Torx screws holding the 6 coil packs (3 left, 3 right) in place and move the coil packs and attached plug wires out of the way - this facilitates access to the plugs

8) Unbolt each spark plug using the deep-well socket and ratchet - NOTE: the factory-installed plugs are AMAZINGLY tight - I'd like to hear from others (DIYs and Pros) whether this is common. I changed the plugs with the engine cold - perhaps that's part of the problem?

9) NOTE: I do not intend to start a flame war - on the installation I do some things that may or may not be commonly accepted. I am always happy to hear differing opinions, but FWIW, this is how I do it.

Check plug gap and gap new spark plugs to 1.0mm (if you are using the Bosch plugs sold on FastLane, they come pre-gapped at 1.0 mm)

Lube threads of new spark plugs with antiseize compound, and lube ceramic body of new spark plug with dielectric grease

Screw each plug in finger-tight, then tighten with torque wrench to 25Nm

10) reattach coil packs, and put a dab of dielectric grease inside each spark plug boot

11) reconnect boot to respective plug (remembering "A" vs. "B"), making sure the boot "snaps" on - don't forget to remove the masking tape from the boots if you used that to keep track of "A" vs. "B"

12) reinstall air filer box and reconnect intake hose

13) reattach heat shield

14) reattach fender liner (and fender molding if applicable)

15) snap plastic engine cover in place

16) reattach left front wheel, being sure to torque lug bolts to proper specs

Done.

I think it is in keeping with the spirit of this forum, if I add the disclaimer that although the above worked for me, I make no representations whatsoever that this is the complete, correct, factory-approved procedure and will not cause damage to your vehicle and / or bodily injury to yourself. Always take all necessary safetly precautions before performing any work on your car.

Hope this helps.

__________________
1997 E420