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My 958 Turbo has air suspension, and the rear has recently been slowly dropping when parked up - nothing major but definitely noticeable.
Yesterday, I raised the car to maximum height and locked the suspension. I measured the gaps on all 4 wheels, and measured them again 24 hours later.
The fronts were fine, but both of the rears had both dropped by exactly the same amount (40mm). The car was not leaking at all until recently, so I'm suspicious of the fact that both rear struts have developed identical leaks at the same time. (That said, both of my rear ABS sensors recently failed 2 weeks apart)
Other than the struts, are there any common points of failure that are shared between the rears that might cause these symptoms? I will try to spray soapy water around the struts to check for any obvious leaks later on today.
Did a workshop replace the sensors and did they put the vehicle in jack mode when they raised it? It is possible both rear bags are damaged but unlikely. Try doing the same test but leave the car in normal ride height when you lock and measure. Even a subtle difference can show you one corner at fault. If no difference then this is pointing to a leak internal to the valve block. The rear axle adjusts separately to the front and although this is achieved by individual pressure lines, the valve block has a common internal valve which feeds the rear lines.
Did a workshop replace the sensors and did they put the vehicle in jack mode when they raised it? It is possible both rear bags are damaged but unlikely. Try doing the same test but leave the car in normal ride height when you lock and measure. Even a subtle difference can show you one corner at fault. If no difference then this is pointing to a leak internal to the valve block. The rear axle adjusts separately to the front and although this is achieved by individual pressure lines, the valve block has a common internal valve which feeds the rear lines.
I replaced the ABS sensors myself, and it was put in jack mode correctly. Coincidentally, this problem did seem to start around the same time but I'm fairly confident that they're not related.
I've adjusted the car into normal height mode and locked the suspension as you suggested - current heights are 485mm (rear-driver) and 495mm (rear-passenger). I'll check these again in a few hours.
I was going to spray the struts to check for leaks but noticed they're concealed behind the wheel-arch liner so I'll have to come back to that later.
That's useful to know about the valve block, thanks. Is there any way to help diagnose this via measured values in the level control module? I have an iCarsoft tool but not really sure how to interpret the values.
"System air quantity(calculated)" seems to hover around 105L.
"System pressure(sensor)" bounces between 2 bar and 12 bar, depending on whether or not the compressor is running.
"String strut pressure, front left" is 9.06
"String strut pressure, front right" is 8.12
"String strut pressure, rear left" is 6.13
"String strut pressure, rear right" is 6.68
I have had to replace both rear shocks. The rear right is quite easy, and the rear left has a bit more to be removed in order to get the job done.
It was pretty apparent that my shocks were done when they failed, an audible hiss, bouncy ride, sagging corner when parked, etc. If I recall, the vehicle would not allow the highest suspension setting to be reached when they failed. They also failed within 5k of each other. Lots of folks will disagree, but I went with used oem units from ebay with lowish mileage, and replaced them one at a time. (perhaps that contributed to the second failure) I was never able to get any soapy water to show a leak, and suspect that the bags failed on both of them.
Last edited by gutenfreibier; 08-23-2024 at 10:37 AM.
I have had to replace both rear shocks. The rear right is quite easy, and the rear left has a bit more to be removed in order to get the job done.
It was pretty apparent that my shocks were done when they failed, an audible hiss, bouncy ride, sagging corner when parked, etc. If I recall, the vehicle would not allow the highest suspension setting to be reached when they failed. They also failed within 5k of each other. Lots of folks will disagree, but I went with used oem units from ebay with lowish mileage, and replaced them one at a time. (perhaps that contributed to the second failure) I was never able to get any soapy water to show a leak, and suspect that the bags failed on both of them.
Thanks for the info - if I need to replace the struts then I'll probably take a similar route. I'll use either used OEM parts, or try the Chinese strut assemblies. Apparently the Chinese ones are "refurbished" OEM shocks with new airbags, which means the adjustable damping will still function correctly unlike some of the aftermarket ones.
After 6 hours of locking the suspension at normal ride height, I have just taken the new measurements. The rear-passenger side is down by 20mm, and the rear-driver side hasn't dropped at all. This seems strange because both of the rears had dropped by an equal amount after 24 hours in my previous testing. I'll check again tonight or tomorrow morning.
Just re-measured again after locking the suspension in normal mode yesterday.
Rear-driver has lowered by 25mm, rear-passenger has lowered by 50mm. I'm guessing this suggests that both struts are leaking (at slightly different rates).
If the valve block had an internal leak related to the rears, I suspect both sides would drop at an equal rate. They do usually end up equally low after a few days, but I have been monitoring it every few hours and the rear-passenger is definitely dropping faster than the rear-driver.
The car has never shown any air suspension faults and can always rise to any level correctly, but I should probably get it fixed before it starts getting worse.
Are there any guides/tips for replacing the rear air struts (entire assemblies, not just the air-bags) on the 958? I can't find any guides online other than a YouTube video by Bilstein which gives a general overview.
My understanding of the job is below:
- Remove floor from luggage compartment, undo 4 bolts holding the top of the strut
- Remove wheel arch liner
- Undo single bottom mount bolt from the strut
- Remove air-line
- Remove strut
It seems fairly straightforward - is there anything I'm missing?
Not sure if I need to deflate the air from the strut beforehand via software, I have iCarsoft but unsure if it supports this (or if it is necessary at all). I have been told that Alldata has a useful guide but this isn't available in my country unfortunately.
Sorry for not responding sooner. It sounds like your rear passenger corner is the problem. This is the same one that went on mine when I bought the car followed a year later by front same side and then front other side. I am from Australia so it is drivers side but still RH side. I would replace that strut. Forget about the other side unless you have the money to do both it would be better but that one is not necessarily leaking it just appears it is. I also purchased two low Ks genuine struts and used one for the front and have one as a spare for the rear. The front LHS I bought a bag assembly from Miessler and did this myself, replacing the whole strut is obviously easier. If you plan to do it yourself you need to do the research. The valve lines need to be seated correctly at the correct depth and you really need a nitrogen setup (I made my own) to recharge the system. You need to fill the new/replacement strut with around 25psi before fitting as an empty bag can fold and damage. You then need to fill the accumulator with minimum 150 psi which translates to around a reported 130ltrs.
Icarsoft is what I have and no you can’t and don’t need to control anything via software. The residual air valve on the strut is designed to keep some air in the strut once the line is removed the valve closes. Once you reconnect you need to follow procedure for lowering car until wheels are just touching the ground and reactivate air suspension and it should re pressurise the strut.
Last edited by Tim Matar; 09-06-2024 at 09:57 AM.
Reason: Hyperlink added
«This valve for refilling the air suspension system is used in the event of a failure of the air suspension system control. Do you neither have the time nor the inclination nor do you want to avoid high towing costs and other associated inconveniences in order to reach your destination? You can use this valve to refill the system using a simple 12 volt compressor, a workshop compressor or the tire filling station at the gas station.»
I wonder where would be 4 locations within easy outside reach where one could install these?
«This valve for refilling the air suspension system is used in the event of a failure of the air suspension system control. Do you neither have the time nor the inclination nor do you want to avoid high towing costs and other associated inconveniences in order to reach your destination? You can use this valve to refill the system using a simple 12 volt compressor, a workshop compressor or the tire filling station at the gas station.»
I wonder where would be 4 locations within easy outside reach where one could install these?
With the hours spent trying to get that installed in the field, and the fact that something is leaking and you're due for a strut rebuild or replacement any way, it'd be a heck of a lot quicker to just drive home on a collapsed suspension versus try to install that crazy thing! Especially on a rear strut? Good luck installing that thing. I wonder if they've ever sold one of those things!
I have had Cayenne Turbo’s for the last 15 years and am disappointed to find that my 958 rear suspension is sagging overnight.
As noted by gutenfreibier my suspension will no longer allow the highest suspension setting to be reached when they failed.
Tim Matar advised that the valve block has a common internal valve which feeds the rear lines and sounds possible.
Would be helpful to know which part of the system has the “common internal valve”
Here is a great lnk to a procerdure used by Porsche with their PIWIS computer system to investigate problems with the Cayenne Suspension Levelling System.
08-23-2024, 08:28 AM
Rear air suspension leak
