I've replaced the two front struts but not the rear since buying a little over a year ago. After sitting for several days, the struts aren't holding air. I ran my SDD to identify any related DTC's, but none show up.
Any thoughts on the source and how to trouble shoot? Leaking rear struts, leaking air compressor, leak somewhere else in the system?
Thanks.

 

"After sitting for several days"

Some people claim their suspension stays up forever.
My car can settle after "several days", it varies, and I consider it normal. Even the handbook says it is normal.

Depends how much is "normal". The "damage" is that the compressor might wear out quicker. However, no air is pumped until the car is unlocked. (on my car, unlocking will pump up the suspension, without the compressor which I think only runs if the engine is running whatever, which means the reservoir is holding air for a long time more than the struts.)

I could spend a fortune, or not worry about it if it is otherwise working.

Edit: I read that if one strut leaks, the others might let out air to keep the car level. Which makes it harder to tell which one is leaking.
You can tell a Genuine X350/358 driver because they keep sticking their fingers under the wheel arch. I'm at 1 Finger/6 Days (normally 3, bottom 0)
Another observation: the suspension can only pump up once without starting/driving.

 

I fought the same problem for quite a while, and spent a lot of money trying to keep the air suspension. Finally gave up and bought the ARNOTT conversion, best money spent. Smooth riding ever since.

 

I put in steel coil overs on my last VDP and thought the ride was fine - didn't have to worry about those sub-zero mornings anymore. But I decided to keep the air suspension going on this model. I can live with it the way it is, but I just wish it were easier to pinpoint the problem. Since I do my own mechanical work, replacing the struts is pretty affordable now that there are more after market products available and even lower priced options that have limited lifetime warranties.
I just can't tell whether it's the compressor that needs a rebuild or whether one of the leftover rear struts is leaking. It seems that if even one strut loses air when sitting that over time they'll all go down evenly. When I start her up, though, she rises fairly quickly when the compressor kicks on.

 

If the compressor was failing you would have a C2303 DTC


"When I start her up, though, she rises fairly quickly when the compressor kicks on."


That shows that the compressor is ok IMO.


Have you tried spraying soapy water on the tops of the struts


and the joints under the spare while you are in there?

 

I do need to try the soapy water test. Thanks.

 

HI
Unlike some people here, I'm still a big fan of the air suspension, and the shocks are easy to diagnose and fix. If I lose the controller or pump I might change my tune, but for now....

An easy and cheap way to diagnose the shocks is to go to a hardware store and buy a small brass fitting, I think 1/4" or 3/8" (I put up a post on this once). Should cost about a buck. It is an 'end cap' and simply is used to plug off a line.
I buy one, unscrew the air hose into the shock, and use the brass cap to plug off that shock. Start car, turn car off, wait overnight. Rotate each night among the 4 shocks, and by process of elimination you'll probably figure out which one is leaking (if any).

I use rebuilt originals for $350 (RMI) when one goes. Have been lucky so far!

 

Clever!
When you do that, does it mean that the car is grounded on the corner that is being tested or is there a no-return valve in the shock that avoids it?
And what kind of sealing is there in or around that brass cap: no wear to do that from time to time? (I know that the air line fittings don't like to be touched...)

 

I went ahead and decided to install rebuilt rear struts with limited lifetime warranties that cost $458 for the pair.

 

If you undo the battery and let it sit for a couple days, won't the leaking shock reveal itself?