I have a problem that seems to be getting worse over time. I think a small amount of water is getting into the rear shock because practically every time the temperature drops below freezing they extend fully and stay there until they thaw out. Once they're above freezing they return to the correct hide height. I've done some experiments with the local European luxury car shop. We disconnected the airlines from the tank and let the car drop to the bump stops. Then we reconnected the lines and started the car, it rose to the correct height, but the next time it froze it was right back up in the sky. Their theory is that the compressor is no longer drying the air sufficiently with the result that water is getting into places where it shouldn't.
When the suspension is working well I usually will get an "Air Suspension Fault" error after about 10 to 15 minutes of driving. When the suspension is in the air it's about 1 minute before the error shows up. Obviously I need to do something about this and I'm willing to replace the compressor if that's what's needed.
If it's relevant, one front shock blew out at the end of August and I replaced the pair. As far as I'm aware the rears are original. It's a 2004 XJR with 105k.

Does the compressor theory sound reasonable?

 

If you're still on the original compressor you've done very well ! I had to replace mine at about 35k miles. The second one then lasted until I part-exed the car at 115k for a 2007 Sovereign on 54k.

The only way I can think of ice causing your problem is ice preventing the valves in the valve block from opening and closing properly, and also ice in the line to the compressor and its vent valve preventing venting.

The system relies on the height sensors telling the ASM when the height is correct and also the system being able to react to that by providing air to the air springs, and also to allow air from the springs to be vented to atmosphere if the height goes over the registered height value. This latter would happen when a really heavy load was carried and then unloaded. The ASM will vent air from the air springs to lower the car back to its defined value.

So ice could cause anomalies, hence a water trap in the compressor assembly. I have to say, though, I haven't read other posts that describe the same problem and I have been posting here since 2010.

If the ASM cannot maintain height correctly it should throw a fault code and put up the fault message. This would be done when the ASM has tried to move the suspension to its defined setting but the height detectors tell it otherwise.

Does it do this all the time when the temperature is very low ?

 

I think their theory is that the ice was preventing the valves from opening when needed.

It has been getting more common since the first of the year or thereabouts. It happened once in December, now it's almost guaranteed. Before this started happening the rear would sag in the cold, sometimes the front would too, but the compressor could always bring it back up.

 

"it rose to the correct height, but the next time it froze it was right back up in the sky."

Why did it rise up?possibly because it can't vent?

The vent valve is on the compressor I suggest you remove compressor

and check for water and check operation of ?two valves assoc. with it.

 

I agree that the vent valve on the pump looks to be the most likely candidate as it is this that allows surplus air to be vented to atmosphere. After a run, the system normally opens this valve and vents some air from the reservoir to clear out the water trap.

 

IIIC, only your rear shocks remain extended under cold.
That in principle should mean an issue at the level either of the rear shocks or of the valve system feeding the rears in the trunk.
Though not excluding also a worn compressor or frozen valve therin...

 

Hi Gareth,

I'm trying to think through the theory about ice causing the valve block valves for the rear air springs to stick, but I have to wonder why the same wouldn't be happening to the valves for the front air springs. If the valves for the rear air springs were mounted lower in the valve block manifold than the ones for the front air springs, I suppose that could make sense, but the valves are all at the same height in one row along the top of the manifold. And the valve for the left rear (blue code) is at the right end of the manifold, and the valve for the right rear (red code) is second from left, with the port for the right front (yellow) and air reservoir (magenta) in between the two rear valves. Why would frozen water affect the valves for the rear springs but not the ones for the front springs or the air reservoir?

Furthermore, why do the valves only freeze after allowing the rear air springs to fully pressurize? Why wouldn't frozen valves prevent the air springs from pressurizing properly at least some of the time? And why do both rear air springs behave the same way? Shouldn't one of them sometimes not freeze, since the valves and height sensors are separate?

Another possibility along that line of thought would be that because the air hoses for the front air springs are 6mm diameter but the hoses for the rears are just 4mm, perhaps frozen moisture in the lines more easily blocks the smaller hoses for the rear springs. But again, why would the hoses only freeze after the air springs are fully inflated? Shouldn't they sometimes freeze before the air springs are fully inflated and prevent them from inflating further? And shouldn't we sometimes have only one rear air spring frozen and one that is working properly?

Although Fraser uses the term "water trap," the stock X350 does not include a water trap in the sense that that term is used in the U.S. Instead, the system has an "air dryer," which is a cylinder full of molecular sieve desiccant beads that is part of the compressor assembly. Perhaps in the U.K. the terms are synonymous, but in the U.S. a water trap is a different type of device, with a container that collects condensed water droplets and which must be periodically drained. There is no water trap of this type in the stock X350 system. However, it is certainly possible for the desiccant beads in the compressor air dryer to become saturated and allow excess moisture into the system.

If the problem were the exhaust vent valve in the compressor, then why wouldn't the front air springs exhibit the same behavior as the rears, since the exhaust valve is the only path for air pressure to exit the system, at least in a properly-operating system, and it doesn't sound as though your system has a leak?

See the link below for a summary of the system operation and some links to additional information and photos that may help you and your mechanic as you try to sort this out:

Air Suspension & ECATS Summary: Components & Operation

It's easy enough to disconnect the valve block to see if any water drips out. Just take photographs so you can get the color-coded hoses reconnected in their proper ports. And take care not to overtighten the fittings when you reattach the hoses.

You can also disassemble the air dryer to check the condition of the desiccant beads and also check for corrosion on various parts of the compressor. The post at the link above includes links to photos showing how I rebuilt the compressor on our '04, including replacing the desiccant beads in the air dryer bed.

Please keep us informed.

Cheers,

Don

 

A little bit more information. It's been pretty cold recently here with quite a bit of snow, but it got up to 33F or so today and the rear end came back down. However, when I got out of work tonight the temperature had dropped to 25F according to the car. As I was leaving the parking lot I heard a creaking or groaning sound from the back of the car. This corresponded with the rear end raising. It was sitting level in the parking lot as I approached the car, but it is high in the rear now parked outside my house.

 

Lucas demon is hiding in the rear pressure tank, haha.
More seriously, very strange behaviour.
Rear end rising may be due to the daily periodic check by the ASM and levelling up the rear. However, it should also in principle raise the front so as to have the car level balanced.
Unless there would be ice plugs in the front parts of the air circuit (but 25F is too warm for that) or some leaks, hindering full front raising

 

More strange behavior. I drove the car 500 or 600 feet, dropped someone off and drove back. The rear was still high from last night and, at 28F, I didn't expect that to change. On the way back however, I got the same creaking sound from the rear, possibly just the left rear corner. When I parked the car the ride height was back down to where it should be.

I've been wondering if there is a slow air leak in one or both of the rear shocks. Before the car started rising up when cold the rear would sag. The compressor would almost always manage to raise the car. Occasionally, I would have to go around the block to "shake it loose" but it would raise itself to the correct height.

 

For the suspension to rise when the vehicle is unattended there must be a leak from the air reservoir to the suspension unit air line. That means two valves must be passing (three valves if two suspension units rise); the valve that connects the reservoir to the 'air-bus' inside the valve block, and the valve that connects the air-bus to the suspension unit's air line.

 

It doesn't rise while unattended. It will rise as soon as I start driving.

 

I think you have an issue with the rear height sensors. Try spraying with a lubricant to see if this makes any difference.

 

On the mechanical parts? We did clean the contacts already.

 

Yes the mechanical parts. I doubt the contacts would exhibit such as temperature issue. I wonder if the adjuster is sticking.

 

Apologies, I must have been confused with an earlier thread - there are so many on the air suspension.

 

Well, I replaced the compressor almost a week ago. There was quite a bit of moisture around the air line fitting. With the new compressor the "Air Suspension Fault" message has not come on at all. The weather has been quite warm for February, so I haven't been able to fully test the fix until tonight when it got down bellow freezing again. I'm pleased to report that the suspension stayed where it should at the correct ride height. I am willing to declare this "solved", or at least "fixed".

 

Good that it seems solved.
But it looks to me very awkward that only the compressor replacement could fix this strange behaviour.
Maybe the new dessicant beads contributed to dry the lines if it was a matter of water/ice.
Cross fingers for the future