Ever since having CX's this sealing area has been a problem.
All of my 3 cars have leaked engine fumes here.
I decided to investigate in detail.

The first problem apparent on all of my cars & only fixed on my latest CX25 was the joining of the bonnet & its "frame"
Citroen only used dobs of sealant between these.

First the whole seal area was cleaned, then sealed properly with White Sikaflex,
Thus eliminating that tinny noise the bonnet used to make when closing it.

Next I installed a 20mm sponge rubber seal to cover the Vent Tower seal mark below.
For a couple of days the vent was fume free, only to start leaking again as the domestic grade open cell sponge rubber went hard & inflexible.

Having recently fitted replacement leather upholstery I was determined to fix this problem immediatly by whatever means.

CHECKING VENT TOWER to BONNET GAP FOR PARALLEL

After removing the vent tower sealing rubber, I made up a piece of thick plywood to fit inside its inlet and measured the vent grille to tower gap. The diagram shows tha the vent tower is leaning forward relative to the bonnet, otherwise it was parallel to the bonnet.
I then measured the seal, which indicates a sponge rubber seal of only 6.6mm. is needed.

So far it didn't seem there was a great reason for seal leakage.

MEASUREMENTS & IMPRESSIONS

I then greased the top of the tower sealing rubber & started to get somewhere. The vent tower was 7.8mm too far forward. The previous depth measurements verified this. By adjusting the top vent tower to body bracket I was able to get correct alignment. An impression of the Vent tower seal on the bonnet was then made, by moving the sealing rubber 6mm. or so above its landing in the vent tower.
The problem starts to become obvious. Citroen have made a mistake here. The vent tower rim should be further forward than it is. In its present setup, like all my previous CX's the seal is too close to the frame/bonnet join. Added to this the new sealant I had put in had a high spot. Removal of the high spot improved the seal contact, though it still wasn't perfect.

FINAL RESULT

I then purchased a small sheet of 6.5mm. black "Closed Cell" sponge rubber and fitted it on the bonnet underside.	The original vent pollen filter & its drain frame was refitted & so far after a week sealing is perfect.

To date I am geting a 100% seal