'88 XJS for reference.

The tires that came with my car, they dropped pressure fairly rapidly, they were old, so i didn't keep them long.

Replacced with new Cooper V rated tires, same thing, tire pressure drops much faster than on any other car I own, even with the same basic Cooper tire, or Michelins, or Goodyear, etc.

Valve stems are new, no leaking from the valve cores, but pressure degrades 2-3 times as fast as any of my other cars.

Doug

 

Have you checked the rim flange to see if they're leaking? First, thoroughly check the rim flange for damage(corrosion or oxidation) or dirt before mounting the tire.

 

Generally not, but it's also worth checking for cracks. I had an X Type rim crack after hitting a pothole and it wouldn't hold air.

 

I've had this occur only with the "Lattice" style XJS wheels, FWIW. They leak down a bit faster than others.

Cheers
DD

 

Any Aluminum / Alloy wheel can be porous. Highly unlikely in this day and age, with the newer manufacturing technology. We used to pull the tire, clean the rim and scuff it with sand paper, and then paint the inner area, (not the bead) with Rustoleum. Which cured the problem. This was also a common trick for older Harleys which suffered from porous engine cases causing them to leak as the oil seeped through the aluminum itself.

Jack

 

Well, the wheels were made almost 40 years ago, and older Brit cars have a reputation for leaking oil, so I wouldn't put leaking air out of the possibility.

Good tip on the Rustoleum as a sealant

Doug

 

I was 13 years old in 1971 and my Dad had bought a Basket Case 65" XLCH Sportster. In the process of rebuilding the engine, I saw him paint the inside of engine cases with "Red Rustoleum". Being the curious newbie, I asked why and he told me. I can attest that his Sportster didn't leak a drop when it was done. Problem solved as Most Harley's in that day and age needed a diaper. Lol.

Jack

 

That's the Factory Authorized Rust Thwarting System (FARTS)!

 

I was always told British cars leaked oil because it helped maintain the gravel driveways in front of the manor homes...