Spark plug change on a Mercedes Benz C320 (W203)

Posted: 2011-02-04 in diy, mercedes
Tags: , ,
Battery Cover

Fig 1: Battery Cover

Let me just preface things by stating that I’m not a professional mechanic. However, changing spark plugs and wires is something I believe anyone with even a modicum of ability should be able to do without resorting to paying an exorbitant amount to a dealership. Having said that, the author takes no responsibility in the event the reader disables his or her car. Attempt this work only if, like me, you have steelballsafety, so to speak.

Still with me? Very well. Find a comfortable place to work with your car. You should have a clean area that is free of clutter. The first thing we need to do is disconnect the power if we’re going to be replacing the spark plugs. You knew that the spark plugs produce their sparks because of electrical power fed to them, right? Open the hood. On the passenger side of the car, you’ll see the battery cover as shown in Figure 1. Unclip the 3 fasteners you see at the rear of the cover and you should be able to remove the cover entirely from the car. Set it aside someplace safe and and admire your battery. It should look like something in Figure 2. Using a socket wrench with a 10mm socket attachment, loosen the nut clamping the negative (black) terminal in place. You should then be able to slip the terminal off the battery’s post. It may be a bit difficult to do this because it might not have moved in years (or at all). Have no fear. Muscle it out of there but do be careful not to damage the battery post sticking up out of the battery. Make sure you don’t contact anything metallic with the exposed terminal. What I did to ensure no contact was immediately cover the lead with a plastic bag and tied it around the terminal. Now that you’ve disconnected the battery, your car does not have any electrical power. You’ll need to reset things like the clock once we’re done and perhaps the security code as well.

Fig 2: Battery

Fig 2: The Battery

Fig 3: Engine Cover

Fig 3: Engine Cover

Alright, now let’s have a look at the engine. Toward the front of car and the engine cover, you should see a panel with the Mercedes logo. Grab a hold of that panel. That’s the front engine cover. There are actually two panels connected to each other. We’re removing the front panel first. Grab it and lift up, then pull it forward toward the front of the car. You may need to unscrew the engine oil cap to ensure it comes completely free. Once it’s free, only the rear engine  cover should be in place. The overall picture should look something like what you see in Figure 3, but perhaps a bit cleaner as I had not cleaned the engine in quite some time. Take note of the two clip holders on both sides of the capital “M” of Mercedes on the long gray bar. The front engine cover clips into and out of those holder. Now you’ll notice that there are two large plastic tubes connected to the sides of the remaining engine cover at the front. These are air intakes and need to be disconnected. With gentle twisting motions, disconnect those tubes. Figures 4 & 5 illustrate what the intakes look like after they have been disconnected. After the intakes have been disconnected, grab the front of the remaining rear engine cover by the rectangular tabs pointing to the front of the car and lift it off. The engine cover is held in place via tension with special rubber tipped knobs, so it should just pop off without too much trouble. For now, put the rear engine cover aside where you put your battery cover. We should now have access to the engine as in Figure 6.

Passenger side air intake

Fig 4: Passenger side intake

Driver side intake

Fig 5: Driver side intake

Exposed Engine

Fig 6: Exposed Engine

Now the fun begins. On each side of the engine, you’ll see 3 black square shaped boxes with cables connected to them on the top and bottom. They are each attached to the engine by a Torx #30 screw. These are electrical components that need to be unplugged and detached from the engine. Figures 7 & 8 show these in detail and how they are situated to the rear of the air intakes that we previously disconnected.

Passenger side coils

Fig 7: Passenger side coils

Driver side coils

Fig 8: Driver side coils

Closeup of coils

Fig 9: Closeup of coils

Have a good look at these for a moment, then start unclipping the electrical components from the top of each coil box. Be careful when doing this and don’t apply too much pressure or you might damage the clips. Take your time here. Figures 10 & 11 demonstrate how the clips disengage. I recommend that you do the passenger side first, as it has a bit more room to work in than the driver side. Remove all the clips from the top of the boxes and pay special attention to which clip goes with with which box. We’ll need to restore these in exactly the same order when we’re finished replacing the plugs and wires.

Unclipping the coil connector

Fig 10: Unclipping the Coil Connector

Coil Connector Unclipped

Fig 11: Unclipped Coil Connector

Once all the clips are removed from the 3 passenger side boxes, use your Torx #30 to remove the screws that are holding the boxes to the engine. After each is screw is removed, keep it in a safe place so you don’t lose it. I suggest depositing these small parts and screws into a cup or saucer that you keep nearby. As each screw is removed, you’ll want to remove the two wires that connect to the spark plugs beneath. This is a bit tricky because it will require a bit of strength and dexterity. Man up here now. This wasn’t a job for pansies and you’re not a pansy are you? Good. Get yourself a 17mm spanning wrench to detach and remove the plug wires from the spark plugs that are screwed into the engine. Your local auto parts store has these. Try to get the longest one you can, but it should be 17mm as this is the perfect size for unplugging the spark plug wires from the heads of the plugs. Make no attempt to unplug the wires from the coils, as that is unnecessary. You should only need to do that if you’re replacing the wires as well. DO NOT attempt to unplug the wires by pulling them out by grasping the wires. Yanking on the flexible parts of the wires will almost certainly damage them. If you’re replacing the wires, this is obviously less of a concern, but if you’re retaining them, make sure you use the open end of the 17mm wrench to pry them out by the hard metal collar near the plugs. Use the engine block as a fulcrum and apply firm even pressure until you feel the wire harness pop off. Don’t worry about damaging the engine this way. It’s stronger than a herd of buffalo and you’re unlikely to harm it unless you’re some sort of titan. Figure 12 illustrates the process of extracting the wires with the spanning wrench. Figure 13 shows what the engine looks like after 2 coils have been removed. Figure 14 shows the passenger side coils after they have been completely removed from the engine.

Plug wire extraction

Fig 12: Plug wire extraction

2 coils removed

Fig 13: After 2 coils removed

Now that the coils from the passenger side have been removed, we’re finally able to unscrew the old spark plugs. Personally, this is my favorite part. Get your spark plug socket wrench ready for action. Hopefully, you have one with an articulating ball and socket joint for greater flexibility for greater access, but others should work fine as well. Working from the front of the car to the rear, reach down below the engine with the wrench and insert the socket over the tips of the plugs. It will be difficult to actually see what you’re doing, but you should be able to tell, by feel, when the socket is securely over the plug. Once in place, ratchet the plugs out. You’ll be able to notice when the plug is loose enough to simply slide out. Figure 15 shows what your old spark plugs may look like. After all the old old passenger side plugs are removed, insert and tighten your replacement plugs. Most of the tightening is done with the same socket wrench you used for removal, but once you feel the tightening start, use a torque wrench to finish it off. Set the torque wrench to 25 foot-lbs. When it “clicks”, you’ll know you’ve achieve the proper amount of tightness. Don’t worry about “gapping” the plugs. The plugs for these cars are pre-gapped, so that’s one less thing to worry about. You also don’t need to treat the threads of the plugs with any chemical products. Those are likely to throw off the accuracy of the torque wrench as well. I also recommend that you use Bosch platinum plugs. Mercedes uses Bosch themselves at the factory, so I think you should too. Consult your local auto parts store when making the purchase of the plugs and/or wires.

Removed coils

Fig 14: Passenger side coils removed

Passenger side plugs removed

Fig 15: Passenger plugs removed

Now that the passenger side replacement plugs are in, screw the coils back onto the engine with the Torx #30 and the screws that you carefully saved. Once the 3 coils are back on, in the same precise locations (no musical chairs here), use your hands to insert the ends of the wires over the new plugs. There should be an audible “click” and you should feel it when they’ve been seated properly. Again, apply firm even pressure STRAIGHT onto the plugs until the wires click into place.

Now, the reason I asked you to completely do the passenger side first, is because at this point we can connect the battery again and see if the car turns on and runs properly. If there is something wrong, it’s far easier to isolate the problem because you haven’t touched the driver’s side at all. Isn’t that clever? Hopefully, you won’t have any trouble. Let the car run for a couple minutes. If everything seems fine, take it for a spin around the block. Still OK? Congratulations! Proceed to the driver’s side. If you do have a problem, I think the most likely problem is that 1 or more wires isn’t properly seated onto a plug, or perhaps one was damaged. Work through the problem. If you can’t solve it, do not attempt to go on to the driver’s side. Take it to the shop and explain the problem. It’ll be embarrassing, but you may get some respect for having tried. Again, part of all this is to learn and develop confidence.

If all is well with the car at this point, disconnect the battery again, and repeat the process with the driver’s side. It’s going to be a bit more difficult because there isn’t as much room there. Keep at it. You can do it. When you’ve finished, reconnect everything and see if the car starts and still behaves by running it for a few minutes. Listen to it. It should sound just as it did before. In fact, it may sound better. If you feel or hear any hesitation, or stuttering, you may have a plug firing timing problem. Make sure you’ve connected everything properly. When you’re satisfied, put the engine covers back again and reconnect the battery again. You may have to enter your radio security code. Also, your car may give you a BAS/ESP error message on your dash. This is a side-effect of the loss of electrical power. Turn the wheel all the way to the left (counter-clockwise) and then all the way to the right. That should clear the message and your car’s electrical systems should be restored to normal operation.

That’s it. Happy motoring!

  1. pgpdoug says:

    Thank you for the info
    put 4 in without a 17mm – very difficult
    again thanks for the time you put in

  2. Ink says:

    Thanks for the info, I will definitely give it a try. I really didn’t feel like paying e savage 750$ bill that I was quoted at the dealership.

  3. wogga says:

    Thanks – very interesting.

    On disconnecting the battery: can you explain how the coil/spark wires could have current when key is not in the car?

    • gitsum says:

      I don’t know. Not having a complete understanding of how the car’s electrical system works, I decided to play it safe and disconnected the only source of electrical power (that I was aware of) that could provide power while the engine was off. It’s probably correct that if the car is off and the key is removed, that no power is going to the coils, but I didn’t know for sure and didn’t feel like testing…

  4. W203_Toronto says:

    Thank you for a tutorial! This is very informative!!!

  5. Thank you for some other informative website. The place
    else could I am getting that kind of info written in such
    an ideal method? I have a project that I’m just now running on, and I’ve been on
    the look out for such information.

  6. gregchambers787800639 says:

    Well done! That last plug on the driver’s side was a knuckle buster! Thank you for posting.

  7. danny says:

    very good diy, enjoyed doing it, also if you got the socket take the time to re-torque the valve covers, mine was leaking a little oil so nip them up and all good ever since thanks again

  8. fathi says:

    عرض جيد و ميسر بالشرح و الصور – و شكراَ

  9. flyingwater says:

    Thanks, very useful and informative.
    I’m having trouble removing the spark plug boot (tried the door panel remover but didn’t work). The boot can rotate easily but just wouldn’t come off. I’ll try the 17mm wrench today, where do you think is a good place to leverage? or maybe I should ask a guy to help…

    • gitsum says:

      I leverage against the aluminum colored engine block itself. Apply pressure and it’ll pop off very suddenly as the boots are generally attached very firmly!

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s