My 2004 XJR sat for about 18 months. When I got it running again the suspension pumped right up, but it leaks down completely in about 2 days if I don't drive the car. Are there any common failure points? The compressor was replaced in 2012, and I've replaced 3 of the 4 shocks, the last being about 3 years ago. All four corners of the car are down, so I don;t think it's an individual shock. Any ideas where I should start looking?

 

I would be suspicious of the tank in the trunk and the valve body near it. The valve body is probably more likely, at least that was the problem with my car. I somehow had a torn o ring on one of the hose going into the valve body.

If I remember correctly, there are check valves between the valve body and shocks, so air only flows towards the shocks. If theres a leak (and they all leak a little) the car will try to keep itself level until the tank runs out, then everything settles.

Run the car long enough for the suspension to inflate and the pump to turn off, then put some soapy water on the valve body connections and look for bubbles.

 

The fourth shock as well as the valve body in the trunk are the first suspects.

 

Assume that one shock is leaky. Even in sleep mode (parked), the system will lower the other corners to suit. (Jaguar put it more delicately, but who wants to see a wonky Jag?). This doesn't use air. Obviously, it makes it hard to tell which shock "leaked" first.

The reservoir and valve block can possibly be checked separately from the corner shocks. The reservoir will or should hold air even when the car is parked and the shocks are down. Assuming you haven't unlocked the car since parking, just unlocking should cause the air reservoir to pump up the shocks immediately. There's only enough for one raise. If this doesn't happen, there could be leaks in the reservoir/valves OR you've unlocked the car to retrieve your handbag too many times.

My own car can lower over days or weeks of unuse (variable). But even after several weeks, unlocking the car pumps it up immediately. My take is that the shock(s) leak a bit, but the reservoir/valve block doesn't.

However, if the air reservoir is depleted, then running the engine, or by some accounts having to drive, is necessary to run the compressor...to restore the air reservoir...

I am also of the opinion not to jump in too quickly, if your car has sat idle for 18 months. The seals or valves or whatever might benefit from actual use. In my case (I think it sat in a dealers yard for 6 months), it used to go down overnight but doesn't now (without doing anything) -although it still goes down eventually.