2008 S550 (W221)

Vehicle has a developed a persistent water entry issue resulting in water accumulating in the left rear passenger footwell following heavy storms.

Displacement of the headliner reveals water entering from the left rear drain catch basin of the sunroof frame.
The metal sunroof frame of this vehicle is equipped with a plastic water catch basin on each side of the frame, at the rear. Each basin contains a plastic discharge nipple that is in turn connected to a black rubber drain hose.

The issue at hand is although the nipple conducts water out of the frame via the drain hose, water can also be seen leaking from the juncture of the plastic basin and the top and bottom metal panels of the SR frame that encases the basin. The water subsequently flows down the inside of the headliner, down the C pillar and fills the rear footwell.

The plastic catch basin has no visible part number and Mercedes advises that it is not available as service part. It is included in a new sunroof frame assembly, though.

Having never seen a freestanding sunroof frame I can not determine whether this plastic drain basin is a removable item. Close up photos suggest a pressure release tab and the item appears to just snap in place. Mercedes advises the piece is glued in place but I have no confirmation of that. If it is glued in place I don't know if it could be removed intact.

So, how to stop this leak?

Attempt removal of the intact existing catch basin? And then re-glue into place?

Or attempt to seal the juncture of the basin with the metal frame of the frame? Varying sized gaps are present here between the metal and plastic; some the size of a pencil point, others the width of a 26 gauge hypodermic. Slather the whole thing with epoxy or perhaps use a pliable sealant?

Mercedes advice is to quit wasting time and just install a new sunroof frame... which is expensive ($1400 for the frame) and labor intensive (headliner and SR frame removal/reinstallation). The frame is manufactured for Mercedes by a third party German manufacturer but we are now two generations removed since the w221 was produced.

Anyone ever seen an intact catch basis for the W221 sunroof frame? Is it a simple snap-in component? Held in place with glue?

Any recommendations as to regluing or alternate repair procedures?

I believe I had the same problem in the front driver's side of my '07 w219 CLS 550 last year. I dropped the corner headliner to discover the drain was intact. I figured out where the stains were originating from, and surmised it must have been a deterioration of some type of adhesive or sealant between the sunroof basin and frame--it sounds just like your situation.

Rather than go through the likelihood of further damage to the headliner by even "professionals" taking it down/reinstalling, and replacement of the entire cassette assembly for a sealant area we won't see anyway, I had success in using both Hondabond HT I had on hand, plus butyl/mastic strip by Eastwood (basically whichever was easier to apply under nooks and crannies). I was concerned that applying it externally under the aluminum sunroof rails against the plastic basin wouldn't hold (stick to the plastic properly), but over a year now and extremes of weather changes, thank God the former wet areas are always now dry as a bone.

Sunroof frame removal was not required. Successfully eliminated the water leak by the repeated application of Capt. Tolley's liquid sealant, followed by the use of Gorilla waterproof patch and seal tape. Capt. Tolley's was applied with the use of a small diameter vessel dilator (0.89 mm) and 5 cc syringe. The sealant was feed directly into the gap between the drain catch basis (white plastic) and the sun roof frame (black metal). The gap was closed as the sealant dried.

The existing CAN faults were remedied by renewal of the wet X connectors; the fuel pump control unit had to be replaced as well. Rear seat removal was required and the rear carpet had to be elevated but the front seats remained in place, as did the front carpet panels.