Hi everyone.

Back when I got my 2005 S-Type a couple years ago, I was searching for reasons why water was collecting under the spare tire in the trunk. There were many theories about taillights, but after examining the way they're mounted, the theories didn't seem plausible, because the tail light mounts just didn't seem to share a space with the trunk.
Yet, every time it rained a bit more, there would be water in the trunk.

I tore out some of the trunk lining after one rainstorm and saw that the water was dripping from near the latch in a clear stream. I came to the conclusion that the water must be wicking itself under the aged trunk lid seal gasket (weather stripping?), over the pinch joint that the gasket is mounted on and onto the inner side of it. This was supported by removal of the gasket and finding a fair amount of moisture inside it. It probably wicks up near the highest parts of the trunk opening (the hinge side) and then in the longitudinal parts some of the water is sucked by capillary action onto the other side. From there it flows inside the rubber channel onto the lowest part, where it pools and leaks into the trunk.

A brand new trunk seal gasket will probably fix this, but what I did was that I thoroughly cleaned the pinch joint where the gasket mounts, then I cleaned the mating surfaces of the gasket. I then sprayed silicone oil onto the inside of the gasket and reinstalled it.
This worked for a couple of months, but the leaking returned. I figured that the oil wasn't a very permanent solution, so I instead took a tube of CRC Silicone Grease and pretty much filled the gasket with it before putting it back in its place.
This seems to have finally solved the issue of water leaks into the trunk. I've seen very minor amounts of water in the trunk after some heavy rain, but it's been completely negligible and something that will evaporate in no time.

If you're one of the many S-Type owners who suffer from water leaks into the trunk, try it out. Silicone grease is cheap and it's good for rubber anyway.

 

Most of the trunk/boot leaks are from two areas. 1. the gasket seal and 2. The deck plate is misaligned or broken or both. Be sure the gasket/seal is the right height all the way around. The deck plate can be cracked,misaligned from use, or the seal isn't closed or the screws/or holes are worn. Under no circumstance should a drop of water leak through under normal use.

 

I always just punch a small hole in the floor near the battery just in case the water collects in the boot.
Small hole is good insurance.

 

It was the same for me as for this thread starter ArtoR, that is water was wicking UNDER the rubber seal.
Before I discovered that I tried everything to work out where it was coming from. All the seals around the tail lights were in excellent condition and so was the rubber boot seal. No water would come in after I would hose around it.
I realised the lack of cleaning around that area was the cause. An accumulation of dirt and dust only in certain areas (I suspect where the different metal body pressings overlap or are joined) at the very base of the rubber seal that looked like dried mud would form a type of wick when it rained.
I pulled the seal off completely gave it and the steel boot lip a thorough clean.
To avoid it happening again I placed some regular electrical insulation tape over the steel boot lip/ridge first and then fitted the cleaned rubber seal over.
I now pay more attention to the area and clean around the bottom of the rubber seal with a very soft bristle paint brush when I am washing the car and occasionally with the same brush and a bucket of water
That seal is now 19 years old but still in excellent condition.
It has never leaked again since.

 

yja, thanks for the info. I will pull the seal and clean it this weekend. Did you put the electrical tape along the entire width of the trunk lip?

 

Scotlad, I placed the tape nearly around the whole perimeter, as per one of the photos attached below which I have outlined.
I have also attached three of Gus' photos from his 2011 post. He had issues I believe with the seals around his tail lights, hence his commentary in his photos. The seals around his lights are definitely different to mine and as I have mentioned my lights have never leaked. I suspect his photos show an X200, the X202 must have got a different tail light seal.
I have included these photos as they do show evidence of dirt build build up which looked exactly like mine, which caused the problem.

Blue highlight in the first of Gus' photo shows the same spot where the dirt would also accumulate in my car. The black circles (just visible) showing obvious dirt patches. I think this car must be an X200. In my X202 the tail light seals are different and appear to be more effective than the ones shown here.

White highlighted circle showing wick forming dirt patch.

Surprisingly dirt eventually works its way right under and well within the boot seal if left uncleaned for a long enough period. Must have something to do with the air pressure in this area forcing the dirt and water 'up and over'.

White line showing where I placed the insulation tape. The tape must must stop the dirt wicking up.