Hi,

the 928 is sleeping outside for some days now and it has rained.
Now i noticed a lot of moist on the inside of the windscreen.

Carpets are dry except in the trunk. Think I have a leak in the trunk but the rubber seems fine.

Any tips?

 

Sunroof fitted?

1988 s4 Auto ROW black/black West Australia

 

No sunroof

 

There are quite a few possible entry points to check, and it also depends on the year of your car.

If it's an early car, or early with the S spoiler it could be quarter window seals, hatch lock rubber seal, hatch glass seal, or the main hatch seal. The S rear spoilers have fixing screws up from under the hatch trim which have o rings and can leak badly with corrosion. If its the quarter seals you'll see water coming from under the rear quarter panels inside, if its the lock seal it will drip down from there, if its the glass seal it tends to run down to around the lock area as well, and if its the hatch seal it often leaks where the join comes apart in the middle at the rear. If it's later car you can also get water coming in through the tail light fixing nut/rubber washer locations. The sunroof drains at the rear run down either side of the hatch seal and shouldn't give a problem as long as the hatch rubber is OK. If you had a leak from the the sunroof or drains above the headlining then you'll get water running out from somewhere around that.

What year is your car? Exactly where is the water soaking the carpet/where is it most wet?

 

It's a European 928S from '86

Its quite wet in the trunk, specially the carpet on the "cover" with tools and around the lock mechanism

 

remove the tool tray crawl into hatch area close the hatch,
have a helper spray water around the sun roof look for leaks
then move to the hatch seal area
then do the quater windows then the top of the hatch this will get the spoiler area wet
then do the hatch lock.
you can use dow corning 111 on the hatch lock rubber and the quarter window seals
3M strip caulk is used on the spoiler attach points

Amazon has these sealants
IDK if they ship to Belgium

 

Check the rubber ring around the lock and make sure that's not letting water through. If it lets water through it ends up around that area.

Check the wiper penetrations, which will also let water through to that area.

If you take the bottom and side leather trims off under the hatch frame you'll see all the screws that go up through the frame to fasten the spoiler. These have an O-ring under the screw head and can leak badly, especially if someone has had a go at them before. Removing the trims is easy, and gives a good view of the spoiler fasteners, and you'll be able to see if they've been leaking. If you prop the hatch to about horizontal, and have a plastic sheet under, you can use a teapot or something similar to pour small amounts of water around the perimeter of the glass to see if it leaks through the spoiler fasteners. Same technique for the lock and wiper.

Edit: Just saw Stan has covered the same ground while I was typing.