Well after my mech recharged the ac system with r12 (he has still a small reserve of gas) today I put a thermometer at the central vents but The result is not as good as I expected , yes the air comes out more fresh than outside but not really cold. With an outside temperature of 25-27 degrees Celsius I read not less than 10 degrees at the vents, with medium vent speed and maximum cold (manual mode). How can I increase the performance of the system?
Assuming you have the correct charge, and with no air or other stuff mixed with the refrigerant (good vacuum pulled, IOW) the most common problem in my experience is dirt/clogging in the condenser fins. The debris is particularly thick at the rear–between the condenser and the radiator. Clearing this path (including the radiator fins) can improve airflow and therefore condenser efficiency. You must separate the condenser and radiator a bit to do this. One way to check out the condenser is to spray cool water from a hose onto the condenser during operation whilst noting possible improvement in the vent discharge temperature. IMHO.
Is it possible to visually check the situation of the condenser? From the front of the car the radiator seems to be quite clean
The above post by Roberto is really good advice. Do not proceed down other rabbit holes until you try the cold water spray.
Best way to gain access to the front of the condenser is to remove the upper rad support bracket. I think there are 5(?) torx head bolts holding it in place.
Andrea …
“HOW CAN I IMPROVE THE PERFORMANCE OF MY A/C SYSTEM” ?
Here is an in-depth explanation of the system in the XJ40s …
If your system isn’t cooling very well it’s usually more than just one thing, especially if it’s three decades old. So these are some of the biggies to look at …
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Condenser is clean and not blocked with debris.
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Electric radiator fans are working properly (this is important at idle and low speeds).
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System is properly charged with refrigerant (you can only determine this with a set of manifold gauges) and the pressures are within limits.
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Both a/c fans under the dash are working.
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Both vacuum doors in the fan assemblies are working properly.
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The evaporator is not clogged with dirt and debris (unlike modern cars it has no filter so you’ve had over 30 years of dirty outside/cabin air going through it) This is a tough one because there’s no easy way to clean it, but it can be done.
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Both demister doors are working properly.
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The two flaps (blend doors) in the evaporator box are working properly.
Any one of these problems can bring your system to it’s knees 
Today while i was checking the ac radiator for dirt and grime, i found that behind the fan housing there are some plastic flaps… in the photo one of the flaps is missing and there is a rectangular opening. Should i replace the missing cover with something ? what is the purpose of that closed openings?
The purpose of a radiator shroud is to prevent the fan from sucking air from within the engine compartment–it is forced to suck air through the radiator. But when at speed, the fan isn’t required, as air is “rammed” through the radiator naturally. A problem is that ram air hitting the outer part of the radiator must divert itself so as to exit where the fan is. The flaps, blown open by ram air, add to cooling by increasing the area through which the ram air can pass (adding to the round fan aperture). When not at speed (idling or slow driving), the flaps remain closed…in fact they are “sucked” closed by the suction of the fan trying to get air without pulling it through the radiator.
The flap should be replaced. IMHO.
Mhm so we have a problem… all the flaps have become not more flexible and they remain always closed except the one that is missing…. Which kind of material I can use to replace the missing one and also the others?
Any flexible rubber will do. I cut up an old door mat.
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Andrea…
Yes you should definitely replace ALL the old flaps. The only way they can operate as designed is if they remain flexible.
While any piece of rubber would be better than nothing (Gary … “an old door mat” !! shame on you, this is a jaguar 
) why not measure the old flap thickness and track down something similar, and it has to be black, no pink or turquoise flaps allowed.
As Robert very excellently pointed out the flaps are a rather ingenious way of having the best of both worlds. Allowing low pressure to be built up and suck the air through the radiator at idle and slow speeds and then provide much less airflow restriction at higher speeds. Smart !
I believe right around 30 mph is the magic number where ram airflow becomes more effective at cooling than the fans.
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Andrea - if you don’t have any old door mats - or hey, what about that pink bath mat? 
- handy check with a home improvement / builders supply store (like a Lowes or Home Depot in the US). They carry sheets and rolls of neoprene rubber in various thicknesses. There are also online sites like “rubber warehouse”.
Just be careful googling rubber products, especially clothing LOL
LOL 
i have found rubber mats… i will try to screw them in place
Gary …
Well the dimples are unique !
But I believe if they’re dimpled on both sides (can’t tell from your pix) they may defeat the purpose, well just a little bit. So please forgive me for a bit of Nit Picking, but stay with me here …
When the “flat” rubber flaps are pulled tight up against the fan shroud with the low pressure created by the fan they effectively seal that area. That allows a bit of a vacuum to develop which helps to sucks the air through the radiator… However if your dimples allow air to pass between them and the shroud that vacuum can’t develop. Try putting a pin hole in a straw and start sipping on that cold drink. Vacuums are very unforgiving. May not make much difference and WAY better than no flaps at all but the law of “Unintended Consequences” strikes once again 
I take your point entirely, and although I am silly, I am only slightly silly. The mat was completely flat on the underside.
I’m unsure of the argument that the flaps open with increasing forward motion. The flaps are directly behind the auxiliary electric fan, which when operating would increase the pressure in front of the radiator. I assume that the flaps open when the electric fan is blowing directly on them.
Gary …
Glad to hear your dimples were only “cosmetic”.
That’s an excellent point about the aux cooling fan blowing the flaps open. But remember that it only comes on when the engine coolant temperature gets up to around 93C (but not because the a/c is turned on which is a common misconception).
Mine fans hardly ever come on. When I park up after using the A/C they run for a minute or two after shutdown.
Would a cut up old tire inner tube work?