I wrote earlier that I found old stagnant water under the rear seat. It's back fresh now, after a torrential rain.

Where re should I be looking looking/testing for water entry?

Are is there any drainage tubes / ports in the vent at the rear of the windows that could be plugged? Can water enter the vents in this area? Rear window seal looks to be watertight, will test.

 

I can't remember anything about the drains on the XJS coupe....too many years have passed since I had one. I do recall the existence of drains in the trunk/deck lid areas, though. Others, with better memories, will chime in.

As a start I would suggest removing the rear seat, climbing inside, and having a helper give the car a very thorough soaking with a garden hose. Watch and wait for the trickle of water to appear....which will at minimum give you a clue as to where the leak is coming from. It might take some time.

Cheers
DD

 

Dog's suggestion very sound. The chances are it is coming through the rear window.

 

I own a 93 XJS coupe. Had similar issues. YOU WILL NOT SEE THE DRAIN TUBES BEHIND THE REAR SEAT. In the trunk. At the metal ridge that goes along the top there are 2 drain holes. One on the left . One on the right. The one on the left has 2 drain tubes. One for rain run off from the roof. The second one for the fluid excess in the fuel portal area. The fuel tube connects at a Y junction to the main left tube. The one on the right drains the rain run off. The end of each drain tube is under the car. THIS IS WHERE THE PROBLEM MIGHT BE. The ends on mine were clogged and not protruding clear of the under body. Each end should be about 1/2 inch visible. ALSO, each tube needs to be ramrodded with some bailing wire. The wire has to be long enough to reach the end of the tube. The tube on the left takes some patience to make sure the wire goes into the lower tube after the Y junction. If for some reason you find the tubing to be cracked or broken. Aquarium tubing will work fine or any hardware store that carries tubing will do. Mert in Charleston.

 

My car also leaked and had water under the rear seat. After repairing and replacing the rear screen seal, rear 1/4 window seal and C pillar vents I have not water in the back of the car. If you do replace these window seals make sure you use Polyurethane window sealant, this is the stuff they use on bonded screens. I had to pull the rear 1/4 windows out and re-seal them as the shop that installed all my windows used mastic and it started leaking after a year. I removed the rear screen and 1/4 windows and re-sealed them with PU sealant. I have not done the front screen yet and it is only one that leaks.

The main place and my main suspect would be a leaking rear window. In these 2 pics you can just make out the rust hole in the bottom left corner of the rear screen lip





Here you can see how the vent deteriorates, both of mine were cracked and leaking.

 

They usually are. I mean it's God spelled backwards after all.