Air suspension reservoir not filling above 5 Bar

[Boltondrew]

Hi All,

I know there is a gazillion threads on air suspension issues, but I can’t find something that fits what I’m seeing.

2004 4.4i with 4 corner bags. Compressor was filling slowly over the last few months. Checked with INPA and it was taking a while to fill to 15.2 bar, but got there in the end. Eventually it was taking too long so threw the ‘taking too long ‘ error code. On INPA I could see it was filling, just really slowly, so ordered a replacement piston ring.

Bags were all holding firm so continued to drive for a few days while I waited for delivery of new piston ring. Eventually the ride height got low enough that I stopped using the X5 until I fitted the new ring. Fitted but made no real difference to filling rate. Found a used compressor that was reported as working well. Fitted that - no difference. Fitted the new ring to the replacement compressor - no change. Couldn’t get above 5 bar. But then the 5 bar is steady and won’t decrease unless I pull the output pipe from the compressor.

So I’m pretty sure it’s not a compressor problem, unless both are bad, which seems unlikely. If I pull off the output pipe I can feel it’s pumping air. When I pull off the output it lets the air out of the accumulator, which I thought might be a bit odd, as shouldn’t there be a one way valve?

So in summary, I can’t get pressure to rise above 5 bar despite trying 2 compressors and replacement piston rings. Bags were all holding well. The front left bag was the first to get low after a couple of weeks. Could a leaking bag prevent pressure building in the reservoir? Surely the accumulator can fill even if bags are leaking? Bear in mind all was fine without a working compressor for a while.

Could it be a valve block issue, not letting enough air from compressor into accumulator? Or maybe a relay issue. Something is stopping the pressure from building - can’t work out what it might be…

Any thoughts greatly received! Thanks!

[Clavurion]

So you reached the accumulator 15,2 bar when you forced the compressor active with diagnostics but it won't go up by itself?

[Boltondrew]

Hi - thanks for replying. At the moment if I use INPA to fill the reservoir the pump activates and it very slowly crawls up to around 5 bar. Obviously the compressor temperature gets hot as it takes a while. Pressure then stays steady, but 5 bar not enough to raise either axle. In the past it did reach 15.2 bar, but slowly. I assumed it was piston wear, but having tried 2 compressors with new rings I don’t think it is. Could it be an accumulator valve issue?

[EODguy]

Check your pressure release valve it could be failing at @ 5bar.

I.e. It'll stay closed until 5 bar and then it trips till below 5 bar, causing lack of pressure and over heating of the pump which probably kicked in the thermal protection shutdown.



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[Boltondrew]

Could be that - is the pressure release valve on the accumulator valve block? What makes me a bit doubtful is that the fill to 5 bar is so slow, like there is too much back pressure from the accumulator. The other thing is that the pressure from the accumulator is released when I take the output hose off the compressor. Does anyone know if that’s normal, or should there be a one way valve on the air feed?
Thanks

[Clavurion]

Check this BMW training material.

http://www.e38.org/selflevel.pdf

[Boltondrew]

Thanks - that’s interesting info. I don’t think it’s any of the valves on the compressor as the replacement pump had them all attached too. Maybe it is just that the replacement compressor is also dodgy? I’ve attached the INPA error page ( in German) if anyone can spot anything on it. It’s the EHC_2N2 error. Does the ‘1’ next to the Speicherventil & Kompressorschalter mean anything? (I think that’s storage valve and compressor switch!)