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A-Class W176 interior water leak repair

Discussion in 'DIY: Bodywork, Wheels & Trim' started by Uncle Benz, Feb 1, 2019.

  1. Uncle Benz

    Uncle Benz Senior Member

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    This is a very common problem. Manifests itself as a puddle in front and rear foot wells, most commonly on just one side of the car to start with. For quite a while Mercedes were fixing these for free, but that seems to be getting rarer now, and in fact a lot of cars are coming in for their second time. Air vents behind the rear bumper are the root cause of this fault. Mercedes have superseded the original part number, but to be honest I cannot see any difference in the part, so it will likely rear its ugly head again in time.
    First step is rear bumper off...

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    You can see the two vents just below the black bumper reinforcers. This is with the vents removed...

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    This is the vent itself. You can see the yellow/cream soft rubber seal. This is the problem. It stops sealing and rainwater sneaks in between the body and the seal. From there it tracks along behind the inner arch trim and sluices down under the rear seat onto the floor.

    [​IMG]

    The design is not that clever. The rear lamp seals to the body with a triangular foam, and rainwater drains behind the body of the light itself, through the back of the bumper reinforcer and straight onto the yellow/cream seal of the vent.

    [​IMG]

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    To fix the problem for the longer term I fit some new vents, but clean the mounting area thoroughly with some degreaser...

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    And apply a bead of silicone sealer to the yellow/cream edge of the new vent...

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    With the new vent clicked into place a satisfying splodge of silicone oozes around the edge. It matters not how it looks, because the bumper hides it all...

    [​IMG]

    All that's left to do is refit the bumper.
     
  2. OP
    Uncle Benz

    Uncle Benz Senior Member

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    Some points to note if you fancy a go yourself. The hardest part is rear bumper removal. First off remove the rear lamps. They are held with a plastic wing-nut type affair...

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    Then the centre plastic trim that covers the boot striker needs to come out. Remove the two luggage hooks, the centre bolt and then it levers up off of six plastic clips...

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    With that out the way you can remove two grommets to reveal a couple of 10mm nuts run down very long guide pins. You need a rediculously deep socket, but I use a regular deep socket and a croc nose mole grip...

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    Then you have four acme type bolts, one each side where the bumper meets the rear quarter panel, then yank the bumper outwards from plastic guide it sits in.

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    And two underneath near the exhaust tailpipes...

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    Six push pin rivets hold the arch liner to the bumper, three each side

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    And then probably the trickiest bit. The bumper cover is clipped into the black bumper reinforcer each side, just below where the rear lamp fits. It needs teasing out. I have a nice wedge trim tool thing that does the trick...

    [​IMG]

    Don't forget to unplug the parktronic sensors if fitted. Lay out an old blanket for the bumper to save marking the paint.
     
  3. OP
    Uncle Benz

    Uncle Benz Senior Member

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    To add something I failed to mention in the original post, one of the dead giveaways of this fault is visible condensation within the rear lamp. If you can see any condensation inside the top of the rear lamp from outside then you have water getting in through the body vent. Often you can see only one side affected, but if you are having the bumper off anyway you'd be foolish not to do both sides.
     
  4. Blobcat

    Blobcat Moderator

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    If you wish to discuss this thread, please post your replies here
     

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