Hello all, been awhile. Been doing a massive overhaul on my SV8 and now that it is all done, I've got a little issue. Here is the short of it: Needed to replace the rear subframe due to rust, so did that, along with all the other 'while you're in there' stuff like rusted fuel filer neck, tank straps, brake lines, etc.. Due to delays in getting parts and life, this took 3 months! Anyway, buttoned it all back up about two weeks ago and everything was good! Took it for a little drive, no loud noises, quiet, smooth and no bits fell off. I did lose a bolt or something into one of the recesses of the body panels and it like to rattle over harsh bumps, gotta track that down, but other than that, perfect. Now, I parked it up and when I came out the next day guess what, the car was flat on the ground. Oops! Figured I didn't get one of the rear shock airlines tight. Checked them, checked everything, turned the car on, aired up perfect, then waited. Again, dropped, but this time I had disconnected the battery to see what corner was bad, turned out to be the Front Left. Well, good news was my front shocks were completely shot anyway, figured this one finally had it, so ordered new Arnott airs for the front, and just installed them today. Guess what, same issue! Now, since this was happening with both the old and new shock, I ruled out bad shocks. I have done some diagnostic testing with my IDS and think I may have found the odd problem. Here it goes:

I noticed that the front left would drop in like 5-10 min, and to me an air leak like that should be easy to spot with soapy water, no luck there. So, as I was working in the truck area for a while maybe I knocked the valve block/reservoir or something. Got out IDS and monitored live data for the front height sensor (and yes, I could watch it drop about 1-2mm/min), and the pressure in the reservoir. Guess what I noticed, it appeared that the air from the shock was leaking back into the reservoir as it dropped. How did I come to this conclusion? Well, over the span of ~15 min, the front shock would drop to -30mm and the pressure in the reservoir would increase from 3.0 to 3.5 Bar. All else was perfect. One it dropped to 31mm, it would fill back up, pressure levels would jump, then when it was done, it would reset to 0mm and 3.0 Bar. Then as I sat there watching everything, again, mm by mm the height dropped and every few mm the pressure in the reservoir would increase 0.1 Bar. Now, being a scientist, you must repeat, which I did and same results another 3 times.

So here is what seems to be going on, as the front shock drops the pressure in the reservoir increases. The only thing that jumps to mind for me is that the valve in the valve block that controls the air flow from the reservoir to the front left shock is slightly stuck open. There is another possibility is the reservoir is too low. If I'm only getting 3.5Bar in the reservoir isn't that too low? When it went to re-level the front, the compressor would have to come on, the pressure in the reservoir jumped up to like 9-10 Bar, got to the right height, then stopped, pressure valve released and got a steady 3.0 Bar. Since this is probably too low, is is also possible that due to low pressure (and maybe a slightly stuck open valve) the air from the front left shock is pushing it's way back into the reservoir (as the pressure isn't high enough). Also, the car can't raise itself until it's running, suggesting not enough pressure in the reservoir.

To me this all points to something wonky with the valve block. I know you can get these pretty cheap (not OEM) on amazon/ebay so would the easiest thing be to just swap it out? After all the work I have done on the car, what's another $100 valve block! So, anyone have any other ideas, or does my diagnosis seem reasonable?

Just realized that if I'm right with my scenario, I wouldn't be able to find an air leak since it seems to keep pressure at around 3-3.5Bar and any air leaking seems to be leaking back into the reservoir, not to outside. meaning really there isn't a leak after the initial pressure drop!

Thanks!

 

Just was reading over the Air suspension thread on here (great resource!) and noticed this:

From pdf page 8 of the Air Suspension section of the Dealer Training manual: "This means that the system is operating within a pressure range. This is done to prevent the air pressure held in the air springs from being transferred into the reservoir.


Seems this is exactly what I am experiencing, air pressure in the springs being transferred to the reservoir (dropping the front left). Guess my issue is the system isn't getting up to pressure. Also, it says that the reservoir shouldn't get below 9Bar under normal conditions, well, if I'm only at 3Bar, something ain't right! So, I am now fairly sure my diagnosis was right (air leaking back into the reservoir from the shocks due to low pressure) but what is causing the low pressure? Leaky/defective valve bock? Bad compressor/pressure relief valve?

 

Just adding to what I know. I just took a look at the car and as you would expect, the front left is right down, however, the front right is now also down, not all the way, but I would say 50% lower than earlier. Not as much, but still down. The rear is fine. So, it seems whatever is happening is happening to both the front right/left, just faster on the left. I can't really monitor this side as there is no height sensor on the front right. I'll probably tinker with it some more tomorrow and see what's what. As this problem is new and happened pre/post strut replacement and seems to be affecting both front sides, I'm leaning a little more to valve block. Mine is pretty corroded and maybe I knocked it/messed it up working in there. Maybe I bashed it with the spare tire when taking it out/moving it around!

 

Update:

Took the car for a drive today. Front left was all down (as usual), front right low too. Started the car, front rose up perfect once the compressor kicked in. Drove it around for a while, again, no apparent issues. Got back home and all 4 corners were perfect. Parked it up and about a hour later checked on it. Same as before, front left right down and front right definitely lower. Now, when I opened the door just now, I could hear the reservoir open and the car try to raise the front, but it couldn't. Sounded like an empty reservoir. Now, I had been driving around for a while so there should have been ample time to fully fill the reservoir to 15 Bar, but seems it didn't as there is no air in there to lift the front back up. At least the car knows is low and is trying to raise the front, just no air pressure!

So, all this seems to point to the reservoir not filling properly. I don't have time to play around with IDS today, being that it's New Years Eve, so I'll wait on that til I have some time.

Other than that I hope you all have a great New Years!

 

brain off

parts cannot on

 

Well, to continue the conversation with myself, I got another little bit of info. The car has sat for the last few days, and when I went into the garage today, it was a bit lower all the way around. Okay, not surprising. However, then I opened the door, the car lowered itself all around a bit (normal, trying to balance it out) BUT, what happened after some lowering was new. I heard it switch to raising and for a brief second, it added some air to the front left! This has never happened. Now it didn't bring it all the way up, only a little bit. So, my conclusion: The air in the shocks is somehow leaking back into the valve block/reservoir and when left for a long time, it actually added enough air to the reservoir to give a glimmer of attempting to raise the front left. This means the system is actually working in that it senses low height and tries to level. Good. The bad is not enough air in the reservoir. I started the car, the compressor fired up and the car arose! Then, after an hour (front left dropped already) I opened the door and guess what, tried to rise but no air (like normal).

At this point, I've got two ideas and they are the same ones I came up with at the start. Either the compressor isn't filling enough (not likely in my mind) OR the valve block has decided it no longer wants to do it's job (most likely). I think I will get another valve block ($75 all day on ebay for aftermarket ones). At this stage in life everything on this car is basically new, so even if a new one doesn't fix it, I don't mind having a new one in there for a minimal investment.

 

I'm following your self conversation closely, but can't really make sense of the problem. i can't really imagine that with such a significant drop, the air in the system is just being pushed back and forth between the components. I rather suspect that there is a leak somewhere.

But i can't think of a practicable way to measure the pressure exchange between the left front shock and the reservoir.

Anyway, I'm curious to see if replacing the valve block solves the problem.

What still interests me is to know how you read the height change of the level sensors with SDD. Do you need the VMM therefore?

Fritz

 

flatsix: Glad you're listening to my ramblings!

I've got a VCM and full IDS setup to I'm locked and loaded when it comes to monitoring all the sensors! While I agree that a leak is what I thought, I can't find one anywhere. And with the front left dropping so fast, that should be an observable leak. As to the pressure moving around, basically what I am seeing on the sensors is as the shock drops (monitored via IDS) the pressure in the valve block/reservoir increases. Happens every time. So, while I agree that it would seem odd pressure is just moving around, it appears to be doing so according to the sensors. Also, immediately after the compressor runs, there is NO reserve pressure in the reservoir (ie. can't raise the car at all). However, after the car sits for a few days, all the corners are low BUT somehow there is magically some pressure in the reservoir to at least attempt a raise (front left comes up like 10mm). Where did that pressure come from? The only explanation I can conceivably come up with is it bled back from the shocks into the reservoir as the pressure in the shocks is greater than that in the reservoir. Why is the pressure too low in the reservoir, don't know! The compressor is working fine (as far as I can tell) so that just leaves the valve block.

Either way, I'm going to check off possible problems in order, with the valve block being first up!

 

Yes, all this is not entirely plausible. For pressure equalization to take place between the strut and the reservoir the pressure in the tank would have to drop below 3 bar and the corresponding valve for the shock at the front left would have to be at least partially open, as would the fifth valve for the container.
But where is the 15 (-3) bar from the reservoir if there is no leakage? And if the reservoir is not sufficiently filled, why doesn't the ASCM set the C2302/03 plausibility error?

In principle, it is probably a closed system. But to some extent the air must be able to escape in a controlled way, for example when the ride height is set in sleep mode by lowering to the level of the lowest strut. I think this is done via the vent solenoid on the compressor. Also not to exclude that there is a little leakage there.

Anyway you are right, for 70 bucks it makes little sense to think about it. Although I like to verify suspected causes before I replace parts I would proceed in this case the same way.

Fritz

 

^^^
Correct in that the only outlet for venting air from the system is back through the compressor's vent solenoid. No other outlet to atmosphere should exist. If pressure is dropping anywhere in the system and the vent is not open, it is a leak; either a fitting, a valve, or the strut bag itself.

It's also entirely possible that the vent valve is not closing properly, although i would expect a much quicker loss of pressure, I think.

You read about many many people using Bagpipe Andy's compressor repair, but you don't necessarily see those people talk about looking deeper into the compressor, and they should. The dryer desiccant beads should be replaced, and if water had gotten past the dryer and into the system, that vent solenoid (along with valves in the trunk valve body) could be rusty. As I've said on the forum many times, I got sick of chasing the issues on mine and went to Arnott coilovers and couldn't be happier with the car. (When I opened my compressor, the dryer compartment was actually storing water, meaning there was water throughout my system!! There is NO WAY to get that water out.)

 

wfooshee: I haven't taken the compressor itself and/or vent valve off the list! But like I said, for $70, a new valve block is an easy decision. Also, mine is fairly corroded up AND none of this was happening until I was messing around in the trunk with the subframe swap and I know for a fact I bashed the valve block (by accident!) a few times with the spare tire/tools. Maybe a coincidence it all happened right after this, but easy to see with that new valve block.

I can now state for sure, that it is not just the front left affected, it is just the faster corner! The front right also drops, not as far and takes longer, but it drops. Seems that it reaches a point where it balances out and doesn't drop anymore. That is what makes me think it's just pressure moving around somehow (along with the reservoir suddenly getting more pressure while sitting off). Again, if there was a straight leak to the atmosphere, you would think it would just keep leaking and dropping and I would come out to both front sides on the ground, not stop suddenly at some pressure point leaving one side down and the other only 50% down.

Also, I can't explain how magically with no compressor running, I can see in real-time the pressure increasing in the valve block as the shock drops. If it was venting to the ATM, pressure should be dropping, or at the very least not increasing, as the shock drops, with the compressor off.

With the compressor, I know that it has been replaced prior to me getting the car, but I don't know exactly when, and I have put on about 75K miles since I got the car. Also, based on the corrosion I am seeing on the valve block (and other places on the car) I wouldn't be surprised if the compressor might be a bit corroded up too! But, that is second on the list. I should be getting the new valve block this weekend and if it is a bad block, I should know if a few hours to a day if it worked. If it doesn't, well, I've got a shiny new valve block and I will have a look at the compressor!

Again, I have checked every connection/air line and not a single leak (at least that I can find) and while this air system can be problematic, no way I'm going to coils. The car was born with air and will die with air!

ps. Just decided that even if this fixes the issue, I'm going to take that compressor out and give it a look. If by chance water got in and messed up the valve block/struts, with new struts and a new valve block there is no way I want water getting into these shiny new parts!

 

All over the place for $20-$25

For Mercedes W220 W211 Audi A6 C5 A8 D3 Air Compressor Cylinder Head Piston Ring

 

Valve Block! Let's vote.
The only other "moving bit" (from an air/sealing perspective) is a valve in the compressor. Q: What leaks? A: Valves do. Unless something else was "messed with".

In my case, I just monitor and do nothing, for 4 years now. Sometimes it goes down overnight, "nek minute" it stays up for weeks (Sunday driver embarrasingly). May not be your symptoms, but I take mine as a clear sign that valves are not resealing properly, and apparently re-seal "next time". If the compressor brings the suspension up, all one is wasting by doing nothing is a shorter life of the compressor.

For an unknown reason, I have never got as low as an error message.

 

Small update. Haven't got the new valve block yet, BUT the car appears to be healing itself! I got my state inspection today and, as it was sitting, as expected the front dropped, the left all the way and the right about 50%. However, instead of always trying to raise the front when the car wakes up and failing due to what seems like no air in the reservoir, this time, I started the car, and guess what, it rose itself back to normal WITHOUT using the compressor, it was all from the reservoir! I actually did this twice today, but the second time was not all the way up. So what the heck is going on. The front is still dropping as normal but now is filling up the reservoir? In my mind, this almost confirms something wonky with the valve block, like stuck solenoids. As the car is getting used again, they solenoids are trying to move again, and that is loosening up. That would be my guess! Good news is car passed with flying colours so obviously I put it back together right!

I should have the new block in the next day or two so hopefully this will be the solution.

 

I will say these cars do seem to be self-healing to a degree. When I purchased my about 6 months ago it had low milege but was not used often and sat around most of the time. I've since been driving it every day and some of the little nagging issues that I was going to get around to seem to have cured themselves. Guess its true what they say, these cars love to be driven.

 

Well, new valve block installed, problem still remains! Now, my old valve block was pretty nasty once I got a good look at it, so I don't feel bad about replacing it (and now I have a spare!). Again, checked for leaks with the new block and none found. So, that points me to the compressor.

Now, as I have mentioned, as I have started driving the car again, these issues seem to be kinda fixing them selves sometimes. First, once the car drops, even when it wakes up, you can hear the reservoir solenoid opening to try and raise it, but nothing happened. Then, sometimes it will open and air will come out and raise the car. But it only does this randomly. For example, today when I went out, it wouldn't raise with the reservoir only, perfect once the compressor kicks in. However, I just checked the car in the garage and guess what, I woke it up and it raised the front from the reservoir! I'm wondering now if either the compressor vent/exhaust valve is sticking, thereby not getting the pressure up to what it needs to be. That could explain the reservoir issue. It doesn't explain why there is no error code in the system since if the compressor isn't getting up to pressure and the car is dropping I should see a plausibility error or something. Another option might be a tired compressor? I think I'll take it out, rebuild it and give all the valves a good cleaning. Again, the compressor immediately raises the car once it turns on in short order.

Now, all of this could be a mute point and I do have a very very small leak somewhere and am just missing it. I came across another thread here where someone outlined how to test each line/compressor/block/reservoir for a leak with a few push to connect air line fittings and a nice big compressor (all of which I have), so I may have a go at that. Either way, once it is aired up and driving, it stays up, so I'm not worried about excessive damage to components and the compressor only runs for about 30 secs or so on startup to get the car back up to height. And since it seems to sometimes fix itself, maybe it just needs more driving!

 

The cylinder bore also wears, & scars, from a worn piston ring.

 

Wingrider: Yep, have thought about that too. I'll take the compressor out and apart to have a look and see what's what! Just because I was told the compressor had been changed out before I bought it, doesn't mean it actually was.

 

I have been dealing with similar issues Winterjag. Had various mechanics give things a try. In my case at present, the fronts as a pair are at high extension and the rears dropped, but not all the way like the are low riders so that every pothole I run over is painful. I have had them at the bottom and it's like having a hard tail. Every bump is painful. The car would scrape over speedbumps and low spots in the road. At some point in the past 2 months it would all be level then not. I had the right rear sag and all the other sides seem to be level. The last mechanic thought, replace the distribution block that's back near the pressure tank. It like yours was about $100 and a fairly easy replace. Didn't change anything unfortunately. Fittings had been soapy water sprayed with limited success. Some have said one shouldn't use soapy water but some special fluid. Not sure on that but was some thread somewhere.

Next mechanic said shocks should be replaced in pairs. I had a blown left front shock and didn't know that they should be replaced in pairs, so got something on ebay from china that it said was a match. Not sure how much I believe of that, but I do know that my rears are Arnott. My fronts haven't matched the rears since I bought the 2004 XJR 7 years ago. So I don't think mismatch is the issue as they have been fine for long periods of time. There's a sensor on the right front wheel, if I recall correctly that if the alignment guy hits, will give the " cruise control unavailable" and something like front warning alert disabled or something to those words.

Mine is the sport suspension model shock, which I understand is more uncommon, but standard on the XJR. I hear the compressor kick on and have had all air a couple of months ago, up so don't suspect that's the weak point. Currently I don't seem to find anything rising or changing when it kicks on but there's a leak somewhere. The compressor is located under the left front fender, necessitating taking off the wheel and wheel liner. It's an on your back job but not too bad. I've cut little pieces of cardboard to hold the springs in position while I bolt on the other bolts. I think there are 4 on springs. I've seen compressors on Ebay, new for about $450 but have also used bagpipingandy's repair kit for about $60 and done that on another car or maybe this one, don't recall years ago. I have heard the refrain about the desiccant ( absorbing pellets one puts in the jag compressor) but they don't come with the kit he supplies.

I know that there are leveling sensors, a pair for the back and only one for the front. My backs had been replaced or tested and ok. It doesn't make sense with my car's condition to spend the several thousand dollars to replace all the shock with coil overs or even with new( rebuilt) Arnotts.

Your chasing as I have, wondering where in the system it's leaking. Brutal ( a member mechanic in Texas I think) has a thread where he shows how he made 4 air pressure valves assemblies that he pumps up on each shock and let's sit for awhile to see where or which one is leaking as small leaks are really tough to figure. If all hold then it's some of the plastic line or a reservoir tank leak more than likely.

I think many with a nice car go the coil over and get the module that tells the idiot light "vehicle too low in red" and yellow "suspension failure" to turn off and have no more sleepless nights.

I believe that there's a module that I would think tells the computer where or to what level to raise or lower the shock. In my case with the sport suspension, it drops down the car with speed and changes the shift points on the transmission. I'm at a bit of a loss and ready for something without a headache, as in another brand car.

 

Yeah, it's called the Air Suspension Module, and it's located on the wall behind the rear seat. As for the level it's trying to reach, that's a standard level, although it's lower (for aero) above 105 mph. The system tries to keep all for corners at the same height at all times.

The air Suspension has nothing to do with shift points. As far as I know, shift points are determined entirely be the speed of the car, the engine load (i.e. hard throttle or pretty much coasting,) and whether in Sport or normal mode by the button on the console.

If you want to get rid of headaches from air suspension that doesn't work, get a set of coilovers. It's what I did when I found so much water in my system that I had no hope of recovering any good function from it. The dryer chamber of my compressor was actually a water tank! The desiccant was clumped like cat litter! Finding that meant that water has been pumped throughout my system... into the reservoir and into the shocks' air bags. There is no way to get the water out, or to remediate the corrosion it causes in valves and other structures, so I gave up, went coilovers, and transformed the car from an expensive mistake I was going to regret ever being involved with, to the best and most comfortable car I've ever owned.

There will be people who will tell me I should have persisted, that there's absolutely nothing like a Jag with working air suspension, but at what cost? The car is worth maybe 7 grand on a good day and I'd need at least half of that to get everything actually working, pretty much a complete overhaul of everything: compressor, reservoir, struts, and valve body, along with all the fittings in between all the pieces. I spent less than 1300 on Arnott coilovers and have not had a care about suspension issues since. The car rides better than it ever did while I had air struts on it, it drives great, it's reliable, and I'm loving it! I've never been in a Jag with working air suspension so I can't make a direct comparison to function, but I can certainly make a direct comparison to cost and value!!!