Hi Guys,
I posted sometime back how my windshield jets suddenly “dried up” on me, not emitting wiper fluid at all anymore despite the running of the pump and plenty of fluid in the reservoir. 
So, I finally took the plunge last night (all night, no less) and dug further into things.
I decided to investigate the plenum area under the WW grill. After taking off the wiper arms (and yes, one of the brittle old nut covers crumbled apart in the process - have to get a new one, now 
) I discovered that the 4 screws (with captive washers) holding the grill in place was NOT Pozi-Drive but Torxhead (for once on a Jag, imagine that 
). I think #20s. They had apparently not been removed since the car came out of the factory, as one of them required copious amounts of PB Blaster to finally and slightly loosen up. After removing the grill (be sure to first squeeze both jets fore and aft and push down through their respective mounting holes in the grill), I checked the plenum drains, and was very pleasantly surprised to find them not only clear, but quite over-sized, vs. those used on the sedans/saloons (which is probably why they were clear) 
. I still remember the nightmares caused by the small drain hoses in that area on my XJ40s, X-300, etc., particularly the dreaded “duckbill hose” on the XJ40s. Jag finally smartened up with the XJS and changed to 2" diameter “radiator main”-sized hoses on either end of the plenum chamber. These drain down and through the front wheel arches. To double check free flow, I poured about a gallon of distilled water into the chamber and nearly all of it quickly drained off. 
The bad news is then that I’m still puzzled about the water on my driver’s side floorboard after a hard rain, as me and a convertible owner both posted about earlier. Oh well.
Anyway, the set up for the WW jets is that each has a separate water feed hose that converge at about the mid-point of the chamber via a “3-way connector” (i.e. “Y”) to hook up with the common main feed hose. Shortly after the connector is a one-way valve in the main feed hose. The main hose goes from there to disappear out of sight at the passenger end of the chamber and then, IIRC, travels down the passenger aft side of the engine compartment to run forward from there to the reservoir and pump(s).
The first thing I did to try to find out where the blockage was was to disconnect the one-way valve from the main feed hose, intending to thread my (large E) guitar string down the hose from there as far as possible to see if any obstacle was encountered. I’ve posted before on here how I use a standard (cheap) set of guitar strings to clear out blockages in the tiny water drain or feed hoses on my Jags. IMHO, it’s safer than trying to blow them clear with compressed air, as no risk of blowing apart the hose at some weak point. 
Before I could do that, though, the feed hose was so hard and brittle (I guess with age + the high heat that builds up inside the chamber in the summer and in the winter as well, being located above the heater core?) that it snapped apart off one side of the one-way valve rather than pull free of the valve’s “barb” fitting on that side. O.K., no biggie, but then when I tried to pull off the hose on the other side’s barb it too snapped apart at that point. 
Soooo, sticking the one-way valve off to the side for the time being, un-phased I went ahead and threaded the guitar string down the main feed hose (think “performing an angioplasty on your Jag”). By yanking it back and forth many times, I was able to get a little over 2’ of it through the hose until I could get it to go no further. My hunch at this point is that further progress of the string was not hampered by some foreign obstacle in the hose but possibly due to other factors, such as lack of flexibility of the string, dryness of the inside of the hose, “twists and turns” of the hose, etc… I decided to withdraw the string at that point and instead check out the one-way valve more closely. But first, I tuned on my key and tried the wash pump, to see if water flows out the end of the main feed hose. Sure enough, it did, although I was surprised it was not high pressure - more like that of the stream of water that comes out of a store drinking fountain. Assuming that is in fact the correct pressure, then I guess the one-way valve, when working properly, along with the narrow exit holes of the WW jets, creates the high pressure streams (?) The presence of the water stream further makes me suspect the one-way valve is at fault, rather than an obstruction further down in the main feed hose.
Inside, I first tried blowing air through the valve from either side, like blowing a straw. I noticed that NO air was getting through it, either way. So, is that “normal”, or does that show something wrong with the valve? 
I have never had a new one of those valves, so I have nothing to compare it to.
On the hunch that the valve is in fact blocked up, I tried soaking and shaking it around in a cup of rubbing alcohol, then a cup of 409 Spray Cleaner, and, finally, a cup of hot sudsy soap water. Still, no air will blow through it from either port. 
At that point, I had no choice but to put everything back together until I can find out more about and, if needed, replace the one-way valve. I’m hoping that was the culprit all along. 
I’ll also have to buy a couple of feet of fresh water feed hose (hopefully one of the local auto stores carries it in that size) and some of those double-barbed connectors, so that I can splice the fresh pieces into the old system with them.
btw, I did make a mistake following the Jaguar Factory materials when it came time to replace the windshield wiper arms. Note that they advise how the driver’s side arm should locate about 2" above the bottom edge of the windshield when parked, and the passenger’s side arm at about 1 3/8", both measured near the upper end of the blades where the sub-arm attaches the rubber of the blade to the rest of the blade. That MAY be correct for the pre-face lifts, but it is NOT correct for our face-lifts. When I went to test the wipers afterward there was a slight movement down followed by a CLUNK, with the arms refusing to budge after that point. 
I thought for sure I had damaged something, but when I reset the arms to where they were before I removed them (I had marked that position on my windshield with a couple of pieces of blue painters’ tape) they operated normally once again. 
I wish I could take it apart and repair it, if that is possible, but it looks like a sealed/molded uni-piece construction. So what keeps the water from flowing back in the other direction - is there a movable diaphragm, ball, etc. inside it? I can shake it, but don’t hear or otherwise detect anything else inside of it. Reason I ask about repairing it is I have searched exhaustively online, and found only ONE of them, and that’s a used one, in the U.K… Apparently Jaguar stopped making them long ago and there’s no aftermarket (interest?) in making them. I have run across a few aftermarket non-return WW valves for other makes, but I note that the “disc” part on those is mounted vertically in the line instead of horizontally. More importantly though, it could be the case that the hose fittings are not the correct diameter for the Jag feed hose. 
), which they claim will fit, e.g. “facelift XJS”, although note that the Jag part # is not even close to the one for the facelift XJS valve shown on the Jag Classic website: