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Thread: HOW-TO: Replacing M104 coil pack & spark plug boot

  1. #1
    E500E Enthusiast gerryvz's Avatar
    Join Date
    Nov 2008
    The Woodlands, Texas, USA

    HOW-TO: Replacing M104 coil pack & spark plug boot

    Recently, my wife's 1995 E320 wagon, which has 187K miles on it and runs perfectly, starting having a few problems:

    • occasional difficult (stumbling) starting, particularly when not driven for 2-3 days
    • one day, extremely rough running and rotten-egg smell coming from the exhaust

    The rotten-egg smell indicated the presence of significant amounts of unburnt fuel in the exhaust. The rough running characterized itself as a very rough idle and acceleration. The car was clearly not running on all six of its cylinders.

    Research and some diagnosis indicated that a bad coil pack was likely the culprit. Use of the home-made "blink-code-tester" to test the HFM system indicated a code 22, which showed that the ignition coil powering cylinders #2 and #5 was defective and/or non-operational. This is actually quite a common failure on M104 engines. Remember that the M104, as of model year 1993 in the W124 chassis, utilizes the HFM fuel injection system, which eliminates the EZL and distributor(s) used in earlier systems (including the M119's LH system) and integrates engine management into the mix. Instead of dual coils, as found in the M119 with LH injection, the M104 with HFM utilizes three "coil packs" -- each of which provides the spark for two of the M104's six cylinders.

    After some research, an order was placed via for a new Bosch-branded coil pack. Not knowing the condition of the rubber boot that connects the coil and the plug directly below it, I ordered a new Karlyn rubber spark plug connector boot from AutohausAZ as well. These boots are often quite fragile and fossilized with age, and can be defective, resulting in a poor connection between coil and plug.

    The current MB part number for the coil pack is: 000 158 75 03 (Bosch part number 00119) with MB list price of $120.00, available for $86.40 from had the Bosch version of the part for $60.57. Once the part was received, I made arrangements to replace the offending coil pack.

    Here are the steps required to replace a single (or multiple, if required) coil pack:

    1) Remove the air pipe that crosses over the top of the engine top cover. This is clamped at each end (one end to the MAF, the other to the plastic intake manifold. A long Phillips or flat-blade screwdriver is required to fit underneath the curved intake manifold runners to get at the horizontal ring clamp that holds the end of the intake pipe into the intake manifold. One ring clamp at the MAF can be loosened with a screwdriver and the rubber boot removed. The intake pipe also is bolted to the top of the engine cover in the rear by two 10mm nuts, which are easily removed with a 1/4" or 3/8" ratchet and extension. Remove the air temp sensor from the side of the intake air crossover pipe. Remove the round wire/connector from the side of the MAF by twisting the locking ring and pulling the plug straight off of the side of the MAF.

    2) Remove the six bolts that hold the top cover of the engine (which seals up the valley in the center that contains the coils and wires), which are attached via Allen bolts. Lift off top cover from the engine. The coil packs and wires are now exposed.

    3) Disconnect the wires from the offending coil pack(s). Each coil pack has three connections: one connection straight down, with a rubber boot attached, onto the top of the spark plug below it; via a spark plug wire about 12 inches long to another reddish plug connector elsewhere (for example, cylinder 2 coil pack also connects via plug wire to cylinder 5); and the high-tension lead that distributes the signal between the coil packs. Carefully remove the plug wire and high-test lead by pulling them off of the coil pack.

    4) After both wires are removed from the coil pack, reach with your fingers and pull the coil pack directly UPWARD so that its rubber boot comes off of the top of the spark plug below it. Remove the coil pack from the vehicle. Pull the rubber boot straight off of the bottom of the coil pack. Examine it for condition -- is the rubber still pliable, or is it hard and brittle? Do the connections look or smell burned? If not, then the connector can likely be re-used; however they are cheap enough (around $6 apiece) that it is a good idea to replace them when the coil pack is replaced. The current MB number for the plug connector (3 are required) is 000 159 35 42.

    5) Attach the spark plug connector to the bottom of the new coil in the same way -- press it straight onto the bottom of the coil.

    6) Slowly lower the coil down onto the top of the engine, making sure that the rubber spark plug connector boot makes apoppropriate contact with the top of the spark plug down in the cylinder head.

    7) Press the coil down all the way, ensure that the two pins at opposite corners of the coil pack frame are in the holes and the coil pack is not moving around; it must be firmly sitting on top of its mounting point.

    8) Reconnect the high-test and spark plug wires to the appropriate connectors on the coil pack. Be sure to carefully route the wires in the appropriate places inside the compartment.

    9) Replace the top cover and tighten down the six Allen bolts that hold it to the cylinder head of the car.

    10) Replace the crossover air pipe by placing it down on the top of the engine cover, making sure that the two studs sticking up from the top of the engine cover are engaging in the holes. Align & press-fit the MAF/rubber boot to one end of the air intake pipe, and insert the other end of the air pipe into the hole in the intake manifold. Tighten the ring clamps with the screwdriver in the same manner that you loosened them. Put the 10mm nuts that hold the air pipe to the engine cover on the studs, and tighten down snugly (not too tight).

    11) Replace the air temp sensor in the air pipe by pressing it into the side the pipe until you hear/feel it "click". Replace the wire into the MAF and snug it down by twisting the locking ring. Ensure that the wires for each sensor are correctly routed and are not rubbing or touching anything inappropriately.

    12) Grab an ice-cold beer, preferably a Shiner Bock. Move your tools out of the engine compartment and start up the engine. Test everything carefully and enjoy a freely running and smooth M104 experience.


    Example of a Bosch coil pack

    EPC screen shot of the coil & rubber connector boot (part numbers 5 & 8 in the diagram)

    Attached Images Attached Images

  2. #2
    Site Admin
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    Glen's Avatar
    Join Date
    Jul 2009
    Carlsbad, CA USA

    Re: HOW-TO: Replacing M104 coil pack & spark plug boot

    Nice write-up, Gerry. I've had to replace three coil packs over the last 11 years in my wife's '95 wagon as well.
    '94 E500 (744) | '94 E500 (199)         Misc. snapshots

  3. #3
    Join Date
    Sep 2011
    Ibiza and UK

    Re: HOW-TO: Replacing M104 coil pack & spark plug boot

    Shiner Bock, appropriate Texas/German combo.


  4. #4
    E500E Enthusiast gerryvz's Avatar
    Join Date
    Nov 2008
    The Woodlands, Texas, USA

    Re: HOW-TO: Replacing M104 coil pack & spark plug boot

    It appears that the original coil (and the other still on the car) are original units. Can't argue with 187+K miles really. Expect that the other two will be going out in the coming months.

    Philnor - indeed Shiner celebrates Texas' German heritage. They just came out with a new brew called Shiner "Wild Hare" which is a pale ale -- it's really quite good. Shiner gives nice tours of their brewery (it's about half-way between Houston and San Antonio, about 30 miles off I-10 in the middle of nowhere).


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