KDSS gurus, I believe I'm a rare failure case (1 Viewer)

[montegofd3s]

The past two weeks I have been working my way around Colorado and Utah hitting the usuals around Ouray the first few days (engineer, Poughkeepsie, Mineral Creek, etc). I then headed to Montrose to get on Rim Rocker trail going to Moab. After about an hour or so of driving the front suspension felt really stiff. It would flex over big stuff, but on smoother parts it would rattle really bad and wasn't letting the front wheels move independently (think a small bump on on side would lift the other side and slam down).
When I stopped for the day I got under to see if I could rebalance the system. Upon opening the screws I knew something was wrong. The screw furthest inboard had no problem opening 2 turns, and I opened that one first, but the one closest to the frame started getting damp with fluid immediately when it was cracked, not even a 1/4 turn. I closed it right when I saw this happen. This did seem to help, but it was short lived as it stiffened back up again.
When it sits overnight, it seems to help a little bit (maybe it balances or fluid cools) but after another hour or so of driving in any condition (on or off-road) it will get very stiff again. I did hell's revenge in Moab, and it will flex, but you can tell it's mostly from the back, as the front wants to stay flat and will eventually give in to the movement. All I can think is that fluid is getting stuck on one side of the shutter valves. Like the side pushing down on the front sway bar?

If @bloc or any other gurus have any thoughts please chime in. Ask any questions you have, I think I will be looking for a new valve system and a tool for service unfortunately. It sucks because I'm now in Aspen, and the highway ride home to new Orleans is going to be very bumpy because of this.

 

[Sandroad]

Where is the bad rattle coming from? Is that a clue to something loose/broken/detached/bent in the front suspension? Have you looked at all the connections there? Others will know if it’s ok for a shutter valve to get a bit wet when opening less than 3 turns.

 

[montegofd3s]

Where is the bad rattle coming from? Is that a clue to something loose/broken/detached/bent in the front suspension? Have you looked at all the connections there? Others will know if it’s ok for a shutter valve to get a bit wet when opening less than 3 turns.

Good question, the rattle I mentioned above was like driving over washboard roads, but the roads were not washboard. There also seem to be a rattle of sorts coming from the kdss rams when I start off in the morning, but the ride isn't brutally stiff at that point. For instance, today I drove from Aspen to Colorado springs. The whole drive was cool and rainy, and the first 2 hours were fine minus the rattle from the rams when hitting bumps/dips (mainly bumps on the passenger side), but all the sudden it's like someone flips a switch and the truck becomes a rock. I'm not kidding, it's within 3-5 minutes its like someone cranks the shock stiffness to 11 and puts 70psi in the tires. I noticed when I just jumped out that the whole truck has a lean to the passenger side, so the front and rear rams seem to be expanding (I believe).

I wonder if air has made it into the system and at some point it heats and expands or traps fluid on one side of the system? I don't see any leaks anywhere else, but maybe a ram could be leaking under the boot and I've never noticed. I spend a lot of time under the truck tinkering so I feel like I would have noticed if something was leaking before.

 

[bloc]

Was the lower section of the valve wet at all before you opened that screw?

Is your system very rusty?

A quick way to explain the valve plumbing is that there are three hydraulic circuits. One is basically the top of the sway bar cylinders, one is the bottom, and the third can be thought of as an overflow circuit. Pressures above expected in either the top or bottom circuit can be bled into the overflow, where the accumulators are... then as the system loses whatever condition caused the top or bottom to overpressure, the excess from the overflow can slowly bleed back into the appropriate circuit.

The balance screws have two effective seals per screw. There is an o-ring in a groove in the the screw that seals against a smooth section in the bore, and this seals the overflow circuit from atmosphere. Then the tip of the screw is a taper that hits a seat that seals the overflow circuit from either the top or bottom circuit. When you open the screws it simply lets the respective circuit communicate with the central overflow circuit. Open both, and they can all balance.

Turning a screw at all and having leakage suggests the lower o-ring and/or smooth section of the bore is damaged. This would then cause me to suspect one side of the system has leaked out to atmosphere, potentially causing a huge imbalance and the strange sway bar behavior you are seeing.

One caveat of the above description where extra pressure within the overflow circuit slowly bleeds back to the respective top/bottom one, is I think if there is a very large imbalance it can lock the pressure in each part and things won't slowly equalize. This is what happens when people used to do the "4x4 trick" to level a kdss-equipped cruiser, and because you are trapping excess pressure in part of the circuit I believe it is very bad for the system.

Not that that's what is going on here..

Basically, it is a complex system with quite a few potential failure points.. even if it is well designed and rarely sees actual failures. Personally I'd be worried about the fluid leaking with any movement of that screw, you are most likely looking at a new valve for that. For the clunking, I'd be checking all of my KDSS cylinder end bushings, trying to check the boots for fluid making it past the seal.. checking the rest of the sway bar mounts for loose bolts or damage.. just giving it a good inspection.

Checking system pressures does require the official tool. Since you are already on the road it may be worth calling convenient dealerships to see whether any happen to have the tool around.. enough 4runners were sold with KDSS that you might luck on to something.

For pictures that better explain what I was trying to get across above, check the following thread.

Anatomy of a KDSS valve

I think you guys and gals are gonna like this.. HUGE thanks to @jimoldguy who shipped me his rust-damaged KDSS valve after it was replaced for him with a new one. That can't have been cheap. Fortunately for me he kept the old one, and when I PM'd asking if he'd send it for a project I'm working...

forum.ih8mud.com

Hit me with any more questions if you have them.. for now I think you are stuck just trying to drive it. If the ride is unmanageable or feels like it'll harm the rest of the truck you *could* open the screws.. but beware, you are effectively disabling your sway bars. This could make emergency maneuvers very unpredictable and potentially dangerous. Proceed with extreme caution if you do that. That said, it is an (risky) option to avoid grenading other parts of the truck if something is seriously wrong with KDSS. Yes you likely will lose oil from the system but I suspect at this point you are looking at a major repair anyway. Oil should not come out under the conditions you describe.

 

[montegofd3s]

Also great questions @bloc, the screws opened freely and been regularly greased. I did have an issue with a bolt weeping past 2 turns in the past, but I have a lift in my garage, so after realizing that years ago I would just open the valve one turn and it would be fine (no weeping).

When I get home I will get the truck in the air and make sure there is no leaks, plus update this thread so others know for future troubleshooting.

 

[linuxgod]

Nothing should weep out, even after 3 turns. I'd be almost certain if it's doing that then the o-ring in the KDSS valve is shot. I have no idea the impact and whether it would make the suspension super hard or super soft, but oil coming out likely means air getting in. The system is pressurized to something like 600 psi.

TBH I'm not sure that just one turn will do much but I would open the valve a turn and try to drive it like that. Essentially just "disconnecting" the swap bar, and just be aware that emergency swerving would be sketchy. Alternately you could leave the KDSS system alone but disconnect all 4 (I think) sway bar points - that will be sure the KDSS system is not active, though again handling will be much less precise than you're used to. People do drive lifted rigs w/o sway bars, and off-road you'll love it, but you will get a lot more lean so take it easy on-road...

 

[bloc]

The problem with disconnecting the links is pressure within the system will jack the KDSS arms down.. in the rear I think it would clear, but in the front it will hit the LCA during driving.

Opening the balance screws effectively disconnects the bars.. or at the very least limits their connection.. fluid will need to pass through the smaller chambers of the valve from one side to the other under cornering and this restriction will put some load on the bars.. but I don’t know how much.

 

[linuxgod]

The problem with disconnecting the links is pressure within the system will jack the KDSS arms down.. in the rear I think it would clear, but in the front it will hit the LCA during driving.

Opening the balance screws effectively disconnects the bars.. or at the very least limits their connection.. fluid will need to pass through the smaller chambers of the valve from one side to the other under cornering and this restriction will put some load on the bars.. but I don’t know how much.

Opening the valves would be the easier method, I'm just not sure if the OP can only turn one of the screws one turn before fluid leaks out if one turn will be sufficient.

 

[bloc]

Opening the valves would be the easier method, I'm just not sure if the OP can only turn one of the screws one turn before fluid leaks out if one turn will be sufficient.

Yeah, at this point I'm operating under the assumption that the system will have to be serviced anyway. It is plausible that there is something wrong with the o-ring but the valve body is ok.. even if I think that is unlikely. Either way they won't be able to get the screw out and inspect the bore without having to dump and then fill/bleed the whole system. For the record if actually trying to dump pressure, I wouldn't do it with the screws. I'd use the bleed port at the top of the valve, and/or the ones at the cylinders.

I don't believe one turn would be enough to get substantial flow through the bottom of the valve.. that's only about a millimeter of clearance at the valve seat. But maybe it could reduce the dramatic pressure issues that seem to be contributing to the banging sensation.

I've been thinking about whether it would damage the system to drive with no pressure, and I don't think it would. There should still be enough fluid in there to lubricate the seals at the cylinders. Aggressively moving any entrained air through the system may make bleeding it more difficult, but then the whole point of the bleed procedure is to get a system previously full of air before install into operating condition.

 

[TeCKis300]

Interesting issue. With anything mass produced, I'd bet it's an issue more people have than they know, as most are not driving on these surfaces, for extended periods, or are not observant enough to notice? But surely not the common failure mode that I would expect from pressure bleed down or loss of fluid, that would generally result in non-functioning roll resistance. Versus what you're describing.

There's more in common with KDSS and AHC than meets the eye. Cross hydraulic lines, accumulators, hydraulic rams. Composed in different manner.

My first guess based on your symptoms is that it's not related to the screw valve.

The way KDSS generally works is to resist in phase motion between axles. And allow articulation for out of phase motions. Then there's everything else, including small bump compliance, which the system relies on hydraulic accumulators to manage. That last sentence in my mind is key to your symptoms.

Hydraulic accumulators have a separator to contain nitrogen pressure. These have been known to leak down or fail over time. They also have limited volume, and will tend to loose accumulator volume over time. I bet the earlier comment of the hydraulic circuits heating up might have something to do with this. It's displacing accumulator air volume. And at some point, locks out as there's no more volume to absorb small motions.

The video below should help explain the system function, especially @2:09.

The question is why is this happening? Are the accumulators worn/failed? Has the system been charged before, and is overcharged? Something else I'm not thinking of?

 

[bloc]

The question is why is this happening? Are the accumulators worn/failed? Has the system been charged before, and is overcharged? Something else I'm not thinking of?

I've been thinking about that a bit. I wonder if it is a problem with one of the internal valves, possibly the top one that bleeds excess pressure into the center circuit. They do have small moving parts, and toyota anticipated some debris because they installed filters in each one. If that pop-off isn't working for some reason, pressure on one side could get very high.

The extremely rough ride makes me believe this isn't a low-pressure issue, as then the bars would just disconnect. But too-high pressure one one side of the cylinders, like the top, could push down on one side of both bars at the same time adding a bunch of spring rate from the sway bars trying to work against the suspension.

Or yeah, maybe the pressure was too high and that was impacting both the oring and operation of the accumulators..

Also, an interesting detail is that they didn't just put the accumulators on the top/bottom circuits, they made the fluid flow through some kind of pop-off valve before it is allowed into them. This suggests they wanted tighter control of pressures than the accumulators would provide.. even if ultimately they are necessary given the cylinder shafts are displacing fluid as they cycle, and that volume had to go somewhere.

 

[montegofd3s]

Thanks for all the replies everyone. I made it home today, and have been thinking about all of these different scenarios on the way home, so here the update.

After the truck sits over night it seems to be riding fairly comfortable again. (more on that later)
When I left Colorado Springs I sort of said screw it knowing that the system would have to get pressure checked and bled anyway and opened both valves 2ish turns. This did allow for a little bit more sway around turns, but in all honesty didn't seem all that dramatic. Yes, it did drip out some fluid along the way, but the valve area was wet, it didn't soak the bottom of the truck or anything. Even so, the suspension got very stiff again around 4 hours into the day. It is teeth rattling over expansion joints or coming off a bridge (think trackcar on the interstate stiff).
I stopped in Amarillo for the night, and the next morning we were again compliant and soft. 4 hours in and we are in wooden wheel wagon mode. At the next gas stop I closed the screws just to see what happened and nothing changed. So here are some of my hypothesis's. It is probably a combination of these or maybe none of them.....

1) My system has been leaking pressure, and I have gotten use to the softness as I drive the truck daily to work, but it is only a 35 minute ride each way.
2) Air has entered the system, is heating up over long drives and causing some unbalanced pressure issues (though if its leaking fluid, this shouldn't be the main problem right?)
3) The BP51s are working overtime since the sway bars are not working appropriately, and are over heating at roughly the same time causing the stiffness after 4 hours on the road. Maybe they aren't properly charged or something? This seems unlikely, but hey....

It is really just a mystery to me that its stiffening in the front and rear at the same time. That why I figured it was the KDSS. I installed the BP51s last year and have been wondering why I can never find a comfortable setting, but maybe these problems are connected, and I have just been blaming the shocks. I could totally understand if the truck was just floaty, but I do not get the stiffness happening unless heated expanding air in the system is locking it out. Like I said, it will ride very stiff, but if there is a big obstacle I can get it to flex only to ride very stiff again after the obstacle is passed.

 

[montegofd3s]

My first guess based on your symptoms is that it's not related to the screw valve.

I agree, though that is something that needs to be addressed now it would seem.

The way KDSS generally works is to resist in phase motion between axles. And allow articulation for out of phase motions. Then there's everything else, including small bump compliance, which the system relies on hydraulic accumulators to manage. That last sentence in my mind is key to your symptoms.

This is a very interesting point.

Hydraulic accumulators have a separator to contain nitrogen pressure. These have been known to leak down or fail over time. They also have limited volume, and will tend to loose accumulator volume over time. I bet the earlier comment of the hydraulic circuits heating up might have something to do with this. It's displacing accumulator air volume. And at some point, locks out as there's no more volume to absorb small motions.

I think you could be on to something here.

 

[bloc]

If the system had any pressure in it air wouldn’t be able to get in. Now that doesn’t mean it wasn’t bled improperly in the past and had air trapped in it the whole time, but with the pressures on a charged system the air would be orders of magnitude less volume than original. And, the whole job of the accumulators is to keep system pressures in check over positive displacement of the pistons, and any temperature changes.

I’m inclined to think shocks, but you are right in that front and rear at the same time wouldn’t make sense.

Also, that screw weeping with only two turns means something is messed up, in a way most of these systems aren’t, even when they have seized screws and more corrosion. I’d have to wonder what exactly is wrong, and what other effects that might have.

One thought.. when the truck is riding rough, with the valves closed, truck off so you can listen, when you crack them again do you hear a whoosh within the valve like significant pressure imbalance is being solved?

Since you’ve accepted having the system serviced, you can put a hose on the top bleed screw and dump the system pressure and catch the fluid. See what it looks like. You may have some junk in there from the bleed screw itself..

Then maybe consider pulling the shutter valve screws and try to determine why yours was leaking as soon as you turned it.

But before you do all of that.. with your screws cracked, and a noticeable difference in roll indicating the bars were disengaged, I don’t see how a KDSS system problem would cause the behavior you describe. I do think shocks need to be given consideration as well.

 

[TeCKis300]

It may be worthwhile to play devils advocate with the possiblity this it not completely a fault, but to a degree, possibly a normal limitation of the system. Perhaps exascerbated by use case and modifications, it's hitting a limitation earlier than expected?

The KDSS as applied to the 200-series is an older all analog implementation. There has alway been corner cases where it may not be ideal and others have noted that it's small bump compliance handling can be constrained.

The later 150-series KDSS as applied to the 4Runners and GX460s, add simple electronic control where at lower speeds, KDSS valves are opened and the system can demonstrate more compliance. This replaces the pop-valve @bloc speaks of. There's mods that can intercept this control and further open up the KDSS compliance envelop into higher speeds.

And there's newer e-kdss versions too.

 

[montegofd3s]

It may be worthwhile to play devils advocate with the possiblity this it not completely a fault, but to a degree, possibly a normal limitation of the system. Perhaps exascerbated by use case and modifications, it's hitting a limitation earlier than expected?

The KDSS as applied to the 200-series is an older all analog implementation. There has alway been corner cases where it may not be ideal and others have noted that it's small bump compliance handling can be constrained.

The later 150-series KDSS as applied to the 4Runners and GX460s, add simple electronic control where at lower speeds, KDSS valves are opened and the system can demonstrate more compliance. This replaces the pop-valve @bloc speaks of. There's mods that can intercept this control and further open up the KDSS compliance envelop into higher speeds.

And there's newer e-kdss versions too.

I can understand that to a point, but in this instance it is on any surface to include fairly smooth highway roads. No denying that abnormal use could have been an underlying factor though.

 

[montegofd3s]

One thought.. when the truck is riding rough, with the valves closed, truck off so you can listen, when you crack them again do you hear a whoosh within the valve like significant pressure imbalance is being solved?

Good thought. I checked and no whoosh, but interestingly in the morning when cold (when I can hear some clunky/rattles from what I believe is coming from the front ram) if I hit the brakes or accelerate hard pitching the vehicle nose up/down I get a burble noise coming from around the valve body. It sort of sounds like if you were to roll a stone ball on a granite countertop, but just happening under my seat. I checked to make sure nothing was rolling around, but it is indeed the kdss system doing....something.

Since you’ve accepted having the system serviced, you can put a hose on the top bleed screw and dump the system pressure and catch the fluid. See what it looks like. You may have some junk in there from the bleed screw itself..

Then maybe consider pulling the shutter valve screws and try to determine why yours was leaking as soon as you turned it.

Good call. I will bleed whatever is left down and pull the shutter valve and report back.

But before you do all of that.. with your screws cracked, and a noticeable difference in roll indicating the bars were disengaged, I don’t see how a KDSS system problem would cause the behavior you describe. I do think shocks need to be given consideration as well.

Yeah, little bit of a mystery. That is why I am thinking the system is pretty depressurized. Here is a thought....Do you think if there is a leak on one side of the ram that it could act like a compressor of sorts? Meaning, during the stroke up and down it is allowing a tiny bit of air to come in, compressing it so a valve opens in the valve body slowly building pressure, then when it sits overnight the system balances allowing air to escape slowly from where it came in? I know that is a bit out there, but just trying to think through things here.

 

[bloc]

It may be worthwhile to play devils advocate with the possiblity this it not completely a fault, but to a degree, possibly a normal limitation of the system. Perhaps exascerbated by use case and modifications, it's hitting a limitation earlier than expected?

The KDSS as applied to the 200-series is an older all analog implementation. There has alway been corner cases where it may not be ideal and others have noted that it's small bump compliance handling can be constrained.

The later 150-series KDSS as applied to the 4Runners and GX460s, add simple electronic control where at lower speeds, KDSS valves are opened and the system can demonstrate more compliance. This replaces the pop-valve @bloc speaks of. There's mods that can intercept this control and further open up the KDSS compliance envelop into higher speeds.

And there's newer e-kdss versions too.

OPs note of ride changing drastically after hours of driving makes me think this isn’t a normal limitation of the system. Plus others have put our dumb KDSS systems through their paces with no problems, and nothing like what is described here. even if like you said previously the head-rocking can be unpleasant.

 

[TimCFJ40]

There's no way this is a normal limitation of the system. Many of us have put many long days (14-16 hours in a day myself) without a hint of degradation to the system. My best guess is that is an accumulator issue. The accumulators give the fluid somewhere to go when the system moves. When the fluid (an incompressible liquid) enters the accumulator, it takes volume up that was occupied by the nitrogen (a compressible gas). If the accumulator lost gas pressure, it could get filled up with fluid over the course of a day without the gas to force it back out. Then overnight, the fluid slowly moves back out, hence why you have good ride for a part of the day until the fluid gets packed up in the accumulator again.

At this point you're destined for a trip to the dealer to have the system recharged at minimum. I'm not sure if the accumulator is a standalone part or part of something much more expensive.

 

[TeCKis300]

Badly worded on my part.

What I'm trying to wrap my mind around is that the common failure modes of the system is a reduction in roll stiffness. Which is what would be expected in a pressurized and hydraulically interconnected system. What I was trying to say is that the softness he's experiencing as normal could be the KDSS operating in compromised fashion due to loss of pressure. And with heat and use, the system comes up to sufficient pressure to engage the pop-valves and accumulators sufficiently where it then operates more normally, but is perceived as harsh? I'm not really keen on that theory, but threw it out there. There's definitely been comments from other platforms with the newer KDSS where it's possible to electronically disengage KDSS at speed, to compare back-to-back, where they start noting some constraints of the KDSS for small bump compliance.

I'm still thinking an accumulator issue. If not a failure, could be overfill. The steps to bleed and charge the system has a tech pressurizing the system to over 1k PSI, bleed at 725psi, and settle at 435 psi. If the system pressure is too high, wouldn't allow the accumulators sufficient reserve volume and possible bottoming out.