Please be gentle; I don't need any criticism of my "mechanic" skills. I have a 2011 GL550. I have the same love-hate relationship most of you have with your GLs. I have an airmatic problem that has me stumped. Over the last 5 years, I have had to replace 2 compressors, 1 valve block, both front air struts (Arnott), and both rear air struts (done by an independent shop that was horrible and went out of business). Over the last week, my GL has been tilted. This morning when I checked the relative heights, the SUV was tilting down towards the driver side. The driver rear strut was almost completely collapsed. The front driver strut was partially collapsed, and the opposite side gave the impression that it was raised. I connected my OBD reader without starting and got the following reading on the levels: Driver Front (DF) -30 mm (2.84 volts), Driver Rear (DR) -44mm (2.24 volts), Passenger Front (PF) -26 mm (2.12 volts), and Passenger Rear (PR) -7 mm (2.61 volts). I then turned on the vehicle and read the level readings: UNCHANGED. Of course when the OBD is reading the live values, you cannot change the suspension settings or raise the vehicle to the off road setting. After exiting the module, I raised the vehicle to the off-road position and re-read the live values. The pump kicked on and leveled the vehicle. Values were: DF 17mm (2.04V), DR 19mm (3.29V), PF 12mm (1.9V), and PR 38 mm (2.04V). I then exited out the module and pressed the button to put it back to normal ride height - NOTHING. The car remained in the elevated position. I then did a bubble test on the front struts - no leak. There is no audible air leak in the rear struts. Tried to spray the valve block, but most of it and the compressor are hidden behind the bumper. Anyway, when I turned of the car and locked the doors, I could hear the valve block doing its thing and the car lowered to about right height. Any ideas of what is going on? Obviously, the driver rear has me concerned since it was the most collapsed. But, why wouldn't the car return to the normal position after being raised, until the doors are locked? Any thoughts would be greatly appreciated.

 

Your car should level itself using central reservoir pressure when either front door is opened, or with ignition on.
The compressor fills the central reservoir when the pressure drop commands it to.
The valve block distributes pressure from the central reservoir to each air spring, based on what the level sensors on each wheel is telling it to do.
Sounds like you've had a mish mosh of people throwing parts at a relatively simple problem, which in the first case was a leak in one or more air springs.
The leaky bags in turn caused the compressor to burn out, which caused a replacement.
At some point you swapped a valve block, why? Unless you've pushed moisture through the system the OEM valve blocks rarely fail; however, the aftermarket ones are utter crap and fail very quickly.

So this reads like a valve block problem to me, but let's discount any leaks first - some of your bags read like they may be as much as 5yrs old, which is service life.

What I would do is to level the car on a level surface, pull the airmatic fuse, measure distance from center of wheel arch to the deck, and walk away for a day.
Pulling the fuse should deenergize (i.e fail closed) the valve block solenoids and level sensors. With the airmatic fuse pulled, the car should not try do do any autoleveling which will help in pinpointing the problem.

Any droop on one wheel is then the result of:
1. a leak in that bag
2. that bag's connector
3. that bag's line
4. that bag's connector port on the valve block.
5. that port's internal solenoid in the valve block.