Replacing the spark plugs on your Mercedes-Benz C320 is recommended routine maintenance. Mercedes recommends replacing the spark plugs on C320 models every 100,000 miles. With all the major engine design changes over the years, spark plugs now last up to three times as long as they did in years past. This is good and bad. It means you save money and time not having to service them so frequently, but run the risk of a spark plug seizing in the cylinder head. I suggest replacing your spark plugs every three years regardless of mileage. If the last time they were serviced is unknown, replace them right away. I don NOT recommend putting anti seize on the plugs.
You are going to have to remove the engine air ducts and covers to get access to them.
Remove each duct by compressing them towards the engine and slipping them off the air inlet.
With the ducts off remove the front engine cover. It pulls up and away from the engine. The cover is held on by five clips and will easily come off with hand pressure.
Next, you will need to remove the main engine cover that also houses the air filters. It is held on by four pressure clips. You can see the two at the front of the engine, to remove the housing lift it at the front of the engine and once the front clips release the rears (which you can not see) will release as well.
We are going to begin with the right side covers. You will be able to see the valve covers on top of the heads, they hold the coils.
Disconnect the individual wiring harnesses going to each coil, there are three of them. They simply squeeze in and pull out of the coil.
Use a T-30 Torx and remove the bolts holding the coil packs in place. With the bolts gone you can just swing the coils out of the way.
You do not need to remove the wires and boots from the plugs but now is a good time to take them off and clean them. The boots can be very tricky to remove and will take a little patience, especially the ones at the rear of the motor. The boots are on quite snug and you will need to use a 17mm open end wrench to place over the end of the boot and using the valve cover gently pry the boots off. If you want to make the job very easy Pelican sells the Hazet tool that is basically a specially bend open ended 17mm wrench that makes getting the boots off a ease and you can use them to help seat the boots when you reinstall them. Note: NEVER try and remove the boots by pulling on the wires, you will only end up ripping the wires from the boots!
Remember the while the coils are interchangeable the wires from each coil must go to the appropriate plug. With the coil mounted on the cover the "A" wire always goes to the forward plug. Carefully inspect the coils and wires for any fraying, cracking, drying or other damage.
There are twelve plugs on the C320 and some of the ones at the rear of the engine are more difficult to access, just get yourself a 5/16 thin walled socket along with a series of extensions and a universal joint and remove each plug. Take you time especially when installing the new plugs as you do not want to cross thread the openings in the cylinder heads.
Installation is the reverse of removal.
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Time: 4
Tab: $72
Tools: T-30 Torx driver:www.pelicanparts.com/Mercedes..., 5/16th spark plug thin wall socket extensions and universal joints:www.pelicanparts.com/Mercedes...
Applicable Models:
Mercedes-Benz ML320 (1998-03)
Mercedes-Benz W203 (2001-07):www.pelicanparts.com/catalog/...
Parts Required: Spark Plugs:www.pelicanparts.com/cgi-bin/...
Hot Tip: Work on a cold engine
Performance Gain: Better mileage
Complementary Modification: New air filter