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Spark Plugs and Coils Replacement
 
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Pelican Technical Article:

Spark Plugs and Coils Replacement

Steve Vernon

Time:

2 hours2 hrs

Tab:

$60 to $400

Talent:

**

Tools:

T30 Torx driver, 5/16th spark plug thin wall socket extensions and universal joints, small flathead screwdriver

Applicable Models:

Mercedes-Benz C350 (2008-14)
Mercedes-Benz GLK350 (2010-14)

Parts Required:

Spark plugs, coils if needed

Hot Tip:

Work on a cold engine

Performance Gain:

Better mileage

Complementary Modification:

New air filters

Replacing the spark plugs on your Mercedes-Benz C350 is recommended routine maintenance though be it at much longer intervals than before. Mercedes recommends replacing the spark plugs on C350 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 you do not know the last time the plugs in your motor were changed replace them right away. I do NOT recommend putting anti seize on the plugs. The outer threaded part of the plug is used as a ground, and anti-seize can affect the ground connection.

You will need to remove the front (red arrow) and rear (yellow arrow) engine covers along with the air ducts (green arrows).
Figure 1

You will need to remove the front (red arrow) and rear (yellow arrow) engine covers along with the air ducts (green arrows). Please see our article on engine cover removal for additional assistance.

With the covers removed you can see the six coils (red arrows), three on each cylinder head.
Figure 2

With the covers removed you can see the six coils (red arrows), three on each cylinder head.

On the Mercedes 350 engine the coils sit on top of the spark plugs.
Figure 3

On the Mercedes 350 engine the coils sit on top of the spark plugs. It is a good idea to number each coil for reinstalling. While the coils are interchangeable, if you need to trouble shoot an error code you can swap coils to determine whether it is a bad coil or another problem with the cylinder or plug.

To remove the coil, begin by using a small flathead screwdriver and sliding back the gray clip in the connection (red arrow).
Figure 4

To remove the coil, begin by using a small flathead screwdriver and sliding back the gray clip in the connection (red arrow).

Next, use the gray clip to press gently down and slide the electrical connection back off the coil (red arrow).
Figure 5

Next, use the gray clip to press gently down and slide the electrical connection back off the coil (red arrow).

Use a T30 Torx and remove the two Torx bolts holding each coil in place (red arrows).
Figure 6

Use a T30 Torx and remove the two Torx bolts holding each coil in place (red arrows). The coils on the rear of each cylinder head are a tight fit, so be prepared with some extensions and universal joints

Pull the coil straight up and out from the valve cover.
Figure 7

Pull the coil straight up and out from the valve cover.

The electrical connection from the coil to the plug is inside a rubber sheath (red arrow).
Figure 8

The electrical connection from the coil to the plug is inside a rubber sheath (red arrow). This helps protect the connection from the elements and electrical interference. Make sure you line these up when reinstalling.

You will need a thin walled 5/8 spark plug socket.
Figure 9

You will need a thin walled 5/8 spark plug socket. A regular spark plug socket will not fit in the valve cover.

Use your thin walled socket and an extension and remove the plugs.
Figure 10

Use your thin walled socket and an extension and remove the plugs. The plugs should not be torqued in very tight. If you are having real difficulty removing them there may be another issue you are dealing with. The plugs at the rear of each cylinder head are difficult to access, so make sure you take your time and that everything is lined up correctly.

The plugs from our project car motor were fine.
Figure 11

The plugs from our project car motor were fine. You want to inspect the electrode for wear and to get a general idea of the health of the engine. The plugs should have a burnt grayish brown look; any signs of oil, carbon build up, white ash are all signs of something else wrong with the motor. You can place a little di-electric grease on the electrical connection end of each plug before installing. Take your time, especially when installing the new plugs, as you do not want to cross thread the openings in the cylinder heads. Remember to work on a cold engine. A hot engine causes the metal to expand, making the removal of the plugs difficult. It also causes the metal to become "softer" making cross threading easier. Installation is the reverse of removal.


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Comments and Suggestions:
Alex Comments: What is the part number for T30 Torx bolt that holds the coil in place? Where can I get a replacement for lost one? Thanks!
September 17, 2016
  Followup from the Pelican Staff: I’m not the best with part numbers.

Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 

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