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What an excellent write up. My wife and I purchased an 2000 XK8 about 4 months ago. Since then we have fixed issues as time permits. The car has several rattles and the vents are one of them. As a temporary fix I have used the gentlemans backer rod fix with pretty good results, but still have some rattels on the drivers side at slow speeds. I'll be pulling out the vents and implementing the permenant fix in a few weeks. Thanks to Keith and others for your fixes and suggestions.
When you get a minute, please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST section and post some info about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice on posting to the forum.
In lieu of paper, I opted for 6 feet of 5/8 inch caulking backer rod (available from building supply stores in packets or bulk). Backer rod is a continuous foam "rope" of various diameters that is stuffed into large cracks ahead of caulking. The rod prevents caulk from going farther into cracks than is necessary. Any color is fine because very little, if any, will show. Be aware that 1/2 inch is too narrow and slips out in places and 3/4 inch is too wide to fall out of sight at the mid-dash. Just stuff the rod atop the dash from one end to the other with something thin and handy. Keep stuffing until the rod is out of normal sight (i.e., behing winshield black edging) but not beyond the front lips of the vents. If you screw up, just pull the whole or part of the rod out and start again. The whole process took about 20 minutes, looks neat and professional, and WORKS super well. Good luck
Thanks for this solution. It literally took 5 minutes and has solved the demister rattle issue in my 97 XK8. I had some difficulty tracking down the backer rod in this size, so I am attaching a photo of the package. Also, I initially thought that I would have to install it between the windshield and the demister vent with a bead of foam filling the gap and showing based on the note above "but not beyond the front lips of the vents." I actually went further, stuffing the rod entirely past the vents so that it doesn't show at all using a plastic trim pulling tool that had a wide flat blade. It wasn't necessary to have the bead of foam between windshield and the demister because the foam rod puts enough pressure on the demister beneath the surface that it pushes the entire assembly away from the windshield. No more rattle, and you don't see a thing! I'm sure the correct repair would have the demisters tilt down more at the windshield, but nobody would ever know that the foam rod solution that I did wasn't the original appearance. I'm pretty picky about doing stuff right, but given the risk of breaking the demister during the full repair, plus the time and complexity, I'm happy with the result that I achieved using Boris' recommendation.
Just used soft gray silicone to stop the vents whacking against the base of the windshield. Great color for my black interior, if you even try to see it. Advantage: 230C temperature rating means everything else on the dash will melt, but the tubing will still keep the dash/vents from rattling against the windshield.
High temperature gray silicone tubing from Mcmaster-Carr Supply Company, 3/8 OD, Partially installed, before being shoved it fully in place between the dash and windshield. Fully installed it spans the entire base of the windshield and is tucked in on the sides.
As an update to my repair using the foam backer rod, when installed, I pushed it fairly far down the crack between the vent edge and the windshield because my dash is tan and the backer rod is gray. In trying to hide it, it eventually slipped entirely below the vent, expanded and pushed the vent edge upwards. Corners began rattling, so I stuffed some additional foam in those areas. I've decided to do a proper repair, but haven't gotten to it yet.
I can't recall what photos were originally on this post, but the full procedure is documented here: JagRepair.com - Jaguar Repair Information Resource . Gus, who runs jagrepair.com, which works off donations, is an exceptional resource and an active participant on jaguarforums.com. He helped me out on an engine issue, which I otherwise would not have tackled on my own.