Rudeney

This came up in Eric's "Buyers Remorse" thread and I think it's an interesting topic worth further discussion in its own thread. The issue is whether or not it is worthwhile to open the bleeder valves at the struts to flush the ABC lines that feed them, or is doing a "rodeo" is good enough. The way I look at it, once the fluid leaves the valve blocks, it's on a dead-end path. Fully expanding and compressing the struts, as a rodeo does, will move fluid in and out of the lines, but it will never truly circulate like it will upstream of the valve blocks. If the struts are fully compressed and the bleeder valve is opened, then it would be possible to change out almost all the fluid downstream of the valve block.

I tend to think if a job is worth doing, it's worth doing right so it would be worthwhile to bleed fluid from these dead-end lines, but in addition to the rodeo.

Nosmo SL55

I agree. Bleeding the struts can only help to improve the the situation when doing a flush.

kbob999

I agree with this logic, and plan to get the double shot of green juice bleed from the corners shortly. Can't hurt, can only help.

Plan is to jack a side of the car, put on jack stands, put the jack under the wheel, break the bleeder loose, then jack the tire up at that corner.
After dropping the side, start the car to raise back to proper position. Let me know if I'm missing something.

A quick comparison of reservoir fluid color vs. the bleeder will tell if it is dirtier in the dead end. Will report back later.

Rudeney

One thing I just though of is that even at the lowest level with the car's weight on the strut, there is still a good bit of fluid in it. This is evidenced by the fact that if you have a loss of fluid pressure, the car will sag and squat much lower than normal height. I assume that opening the bleeder will have the same effect, so the strut will compress more. It might be something to prepare for when doing this bleed.

Considering how the strut fully compressed without fluid pressure, it seems that they could have programmed the system to prevent a full "sag" by locking the valves when the height dropped to a certain level, before the possibility of the tires hitting the fenders. There might could even have been a "limp mode" programmed into the system. If the pump had a bypass valve that could have been activated so that it would keep lubricated, but stop pumping fluid through the system, that and locking the valve blocks, could have worked.

bobterry99

I think everyone would agree. I also believe there are several methods being used to flush fluid, some are better than others, and none are "right" or "wrong". Presumably, a procedure outlined by the factory is not wrong.

The efficiency of exchanging new fluid for old in the struts and their lines is commensurate to the ratio of the volume of strut fluid swept with each up/down cycle to the volume of the fluid in the lines. If someone is using the console switch to raise and lower the chassis, that efficiency is dramatically reduced with respect to a Pitch routine; in fact there may be little or no exchange of fluid at all with the right-side struts. Note that a Star Diagnosis "Pitch" routine sweeps c. 144ml of fluid from the cylinder; in contrast, the console switch sweeps just 1/4 of that volume.

If a shop is performing a flush and charging by the hour, presumably they should follow the factory procedure and be done. But a DIYer who desires to remove the maximum amount of old fluid probably ought to follow the factory procedure and then supplement that by opening the bleed fittings at all four wheels to extract additional fluid.

Factors which will influence that comparison include: (1) method used to flush fluid through the return line for collection, (2) distance from bleed port to its associated valve block. The fluid collected at the right-front wheel will likely be darker than the left-front wheel.

knowbenz

The fluid that is in the strut that leaves during rodeo is going back to reservoir through the return line then filling back up from the supply line. It is getting a good amount of it out but I would imagine a slight amount stuck in there getting diluted with new fluid. I have used a 4 post lift with the car running above to open bleeder, then close, let it fill up, open bleeder, close, let fill up, repeat until it is fresh.

bobterry99

As I visualize this the suspension is not moving, the rate of fluid flow is low, and consequently, perhaps little or no fluid is exchanged within the accumulators and struts.

Rudeney

There is no return line downstream of the valve, so any fluid passing the valve into the line leading to the strut is going to be stagnant. Compressing the strut will evacuate some of the fluid from it, and doing a rodeo will mix in some new fluid with the old, but it's not a true flush. Opening the bleeder will evacuate some of the old oil downstream of the valve, but the strut will still have some. So, with the engine off, if you open the bleeder, can you fully compress the strut (i.e. further than a rodeo does) so that you evacuate a greater quantity of oil? If so, then I'd say for anyone wanting to do the maximum flush, follow the WIS doc doing a rodeo, then with the pump running, drain at each bleeder until the fluid runs clean, then with the pump still, open each bleeder and compress the strut fully. Thoughts?

Regardless of all that, I do agree with Bob re: the dealer's hourly rate vs. a DIYers "free" time to do this. Changing any fluid is better than changing none. This is just for those OCD DIYers whoi have to get every last possible drop out.

knowbenz

You have the entire weight of the car being supported by four struts, when you open the bleeder with the weight of the car on it you get a fair amount, close it and the car raises back up, repeat this and you can get as much fluid as you want. You have a considerable amount of weight pushing fluid out, it comes out with some pretty good pressure. I haven't measured the amount each time.

knowbenz

Maybe I'm misreading this but there is a return circuit. If you had say 1qt of fluid between 2 struts and then running Pitch you will compress both rear or both front at the same time. Now say that gets 0.75 qts of fluid out between the two, they fill back up with even more fluid to the raised position, maybe 1.25qts now, maybe now when it goes down you get 1qt out. you run 12qts through the system it is pretty diluted by then.

ItalianJoe1

The problem is, from the valve to the strut is all one line, like filling and emptying a balloon, you never really remove the fluid that's in the strut even at full collapse. The return is at the valve block, so the path from there to the strut is a single line. Yes, you are gonna push new fluid in most of the way, but it won't really swirl around and replace the old fluid that's in there, has no reason to.

knowbenz

You do remove fluid from the strut. Not every drop I agree, But for somebody to say the fluid between the strut and valve block goes stagnant or never gets exchanged is ludicrous. Following the procedure outlined in WIS will remove the vast majority of fluid in the strut. Next ABC strut I replace I will cut apart and we can find out the exact capacity and design of the reservoir in the strut. If it couldn't change all the fluid then how could you possibly bleed the air out doing the rodeo?

ItalianJoe1

I'm with you, I think the bleeder flushing is overkill.

There can be a little air left in the strut, at the high working pressures it won't matter anyway, and it's not a brake system that relies on 100% air free fluid for proper response.

knowbenz

This thread is going to have me wasting time at the shop tomorrow trying to measure the capacity of the strut LOL Should be simple math to figure the capacity of the line between the valve block and strut. Figure I will raise the struts to max, empty it out through the bleeder then subtract the volume from the line and should be able to get a good idea. If the line is 8mm diameter at 5' of line is roughly 0.076L of fluid (2.6oz). Hard to find cut away images but found this one and it doesn't seem like the cylinder actually holds that much. Does seem like the design allows for maximum removal of fluid though if I'm making out the parts correctly. We'll see....

Aussiesuede

Yeah, it's definitely overkill. The ABC system is actually very robust and is not so sensitive that a new to old fluid ratio of 99 - 1 percent is going to make any difference on either the performance,nor service life of the system. That said, I'm just very happy that R230 owners finally seem to be in general agreement that performing regular flushes has a direct correlation to the health of their ABC systems. Mercedes did a **** poor job of educating buyers on the need to service the system at regular intervals,and the ABC became unnecessarily maligned as a result. So if some wish to go the extra step and bleed out that last 1%, I say more power to em! Just commit to every 40,000 mile flushes and the system will take care of you for every bit as long as they take care of it. So BRAVO!!!

Rudeney

Yes, that is what I meant by "dead end" and "stagnant" because there is only one line leading from the valve to the strut. There is no return line from the strut itself. I'll take the balloon analogy a bit further. Take a balloon and attach its opening to a straw. Fill both the balloon and straw with water. Now, attach a second, deflated balloon to the other end of the straw. Assuming there is no air in the system, no matter what you do, you will always have water in the straw. So, at this point, you want to change out the water, but you can only detach one balloon to do this. No matter what you do, even squeezing the water out of the other balloon, you will still have water in the straw.

This is what happens in the ABC system when you do a flush. The fluid that remains in the line between the valve and the strut would never get changed out. By opening the bleeder, you can clear that old fluid from the line. Thinking about this more, though, I have another theory. Assuming that the maximum volume of fluid that the strut can hold is larger than the volume that the line can hold, if you do a rodeo, it will mix old and new fluid and after so many cycles, you will have effectively diluted the old fluid so bleeding is probably just overkill. But, if the line holds more than the strut, you will never get enough new fluid in there to dilute it. What concerns me about this is that the strut can be the source of contaminants entering the system. It's seal is the one place where the fluid can be exposed to the environment. Of course it should be tight enough to seal out all but the smallest particles, but those small particles will pass and dirty the fluid.

Again, this is all just some "armchair mechanics", so feel free to shoot down my theories and assumptions!

JaySee56

Need to compare the collapsed strut volume plus line to the strut plus line volume at full height. If the ratio is 5 to 1, the first cycle will have 20% old fluid remaining, the second 4%, the third .8% and so on. At some point you can say "clean enough".
Funny that WIS ar32.50-p-0021n says the struts should be bled with the engine off and weight off wheels. I can see why the engine should be off, as a functioning ABC system should not let the struts collapse, but it seems to me that opening the bleed screws with weight off wheels would draw air in not bleed it out.

ItalianJoe1

There is always pressure in the strut, as the valve closes to lock the car at normal height. So if you crack the bleeder with the car on a lift you will still get fluid out. You can push the wheel up and get more, but there will be pressure on it already.

knowbenz

Are you talking about contaminates from contaminated fluid coming back from the strut or the actual strut causing contamination? Not knowing the internal design of the cylinder on the strut I would think the only source of contaminates it could provide would be from deteriorating seals in which case you would have external leaks visible.

Rudeney

Like any sort of shock, the shaft passes outside of the seal as it is extended. Touch the shaft and you ill find it's a bit oily as a small, microscopic layer of fluid passes by the seal. Then as it is compressed, the shaft passed back through the seal, and small microscopic pieces of environmental debris will pass by the seal. It's not much, but over time it will contaminate the fluid. The only other source of contaminates for the fluid will be in wear of internally lubricated parts, like the pump, valves, etc.

knowbenz

Rudeney

Well, OK, then that means no contaminants can enter the system, thus the only debris will be coming from wear of internal parts. That is good news.

knowbenz

Was having trouble with WIS today at the shop but I did exchange emails with Arnott who rebuilds the struts.

Their response as to the capacity and how much fluid is left at lowest point
Response as to contamination possible from shock portion
Here is a decent cut away