Car can stand for days without dropping at all. But as soon as I go for a drive it starts to drop and the air pump works regularly to pump it back up.

I disconnected fuses 81 and 52 (ride height control and the compressor) and took it for a 5 minute drive around the block and over a few speed bumps - and the result was that one side dropped 25mm and the other side dropped 65mm.

So guess air must be escaping from air bags when it is under more pressure when driving? Any other ideas?

I bought two new bags (Dunlop) and will be installing them myself next week. Anything else I should be looking at ?

Also - does anyone know how to set the height at which the air pump is started to be a lot lower (say 20mm?) then the target pump up height - so that the air pump does not get engaged until the drop is that 20mm ?

 

Another thing that happened a few days ago after pulling into my garage I heard a hissing noise and watched as rear suspension on both sides was getting lower and lower.
This went on until it looked to be fully deflated. Then the pump started and both sides were pumped up to their full height. And that's where it stayed.

Is this some sort of a suspension reset that the ride height control ECU does? Anyone seen this before?

 

An update: I never found what the actual issue was.

First I replaced both airbags and it made no difference: Every time before I got into the car I measured the ride height on both sides and it was always within a few mm of the target of 604mm (I have 17 inch wheels). I could leave the car standing days and it would not drop. But then as soon as I'd start the car the compressor would start - and when the compressor started I could observe the rear suspension first drop - yes DROP - by about 10mm and then get pumped back up to the 604mm or thereabouts level again. And the compressor would work often during the drive - like every 5 minutes or so.
I was able to raise and lower individual sides using INPA, and INPA showed that the interpreted values of the ride height sensors on both sides were withing a couple mm away from the target. I also used ISTA to confirm the ride height settings and everything checked out. But still the compressor would start every time I started the car.

I changed the entire compressor unit - including the solenoid blocks and the discharge valve, I then used INPA to pump up both sides - but this time I pumped them up to 10 mm higher then target - to about 615mm. Then I started ISTA to set the ride height (by entering wheel size and current height) - and even though it was 10mm higher then target, ISTA replied that "it is within tolerance" and did not release any air...

The new compressor does not turn on as often as the old one did. I hear it come on maybe once a drive and that's it. And the car seems to stay at 10mm above the target range...

I took apart the old compressor and it was in very good condition. The valve block and release solenoid worked, all seals were perfect and there was no corrosion anywhere (although the desiccant was damp). I am almost beginning to suspect the EHC (Electronic Ride Controller) ECU has gone haywire. or went into some weird buggy behavior mode - as there did not seem to be anything physically wrong with any components. Maybe putting the car into 10mm above the target height made the EHC ECU work in a different mode? Who knows... would need access to the program that sits inside the EHC ECU to figure that out and if anyone knows how to get that I'd be interested in finding out.

 

Did you recalibrate with ISTA. If not then it needs to be done since you changed parts on the system. The TIS should be in ISTA on the procedures.

 

I had the same thing happen to me. I would use INPA to bring the car back up to correct ride height, after about a week or so the rear would be lower. I did the re-calibrate with ISTA+ "Very Simple" and since has never had an issue.