Hissing from airbag — guessing this isn't normal lol

[bevans6]

On a stock mc-9, the front air beams are connected together, so a leak in either beam, any of the air bags, or the connecting piping and the central leveling valve will make the whole front suspension not inflate.

[gumpy]

On a stock mc-9, the front air beams are connected together, so a leak in either beam, any of the air bags, or the connecting piping and the central leveling valve will make the whole front suspension not inflate.

The only connection between the two is the 1/4" line that comes from the T off the leveling valve output.  But, yes, a large hole in one of the bags might affect the other 3, as it appears to be in this case. 

[skytripping]

Find out where exactly that air is coming from. It's enough that you will be able to feel it if you get your hand in there. You should probably jack it up and block it. Remove the wheel if you have the means to do so. Ultimately, you're going to have to do that to fix the problem.

You either have a hole (crack) in the front passenger air bag, or the air beam has a hole in it.

As far as how they work:  The front and rear bags on the front axle are tied together with the air beam. The two sides are separate (i.e. they have two separate air beams, with two bags on each, Left and Right). The drive axle is set up similarly. Main difference is the drive axle uses larger air bags, and has a leveling valve on each side, whereas the steer axle only has a single leveling valve which fill both side air beams.

You have a major hole somewhere. Either in the front passenger air bag, probably in the upper lobe, or in the passenger side air beam.

BTW, here's a page with a bit of detail on replacing the front airbags: http://www.gumpydog.com/Bus/MC9_WIP/Mechanical/Steer_Axle/steer_axle.htm

Thanks gumpy. It sounds like I'll have to get the bus jacked up as you suggest since I can't quite reach the airbags through the wheel well.

[Scott & Heather]

So question guys, if he's leaking air that bad, how on earth is he getting 120 psi to the dash gauge?


Sent from my iPhone using Tapatalk Pro

[skytripping]

My friend and previous owner of the bus (owner of a truck sales company) says he's pretty sure his guys put in a brand new air compressor when they rebuilt the engine. Would that explain the healthy performance despite the leak?

[chessie4905]

120? I dunno. Sounds like something may be frozen, unless he lives in the south. Also, how long has it been this way? Has it set for a looong time and he just obtained it?

[gumpy]

After watching your video again, I'm kind of wondering it it's not coming from the air dryer moisture ejector valve. That's a lot of air noise  coming out of there. Can't really tell direction from the video.  Do you hear the air dryer blow off when it hits 120 psi? If so, it's not the moisture ejector valve.

The line that feeds the air beams from the leveler valve is 1/4" and it's split between the two beams. The one coming from the compressor is much larger than that. Also, the suspension is the last thing to get aired up. It doesn't kick in until somewhere around 65 psi. So, it's definitely possible for the system pressure to reach 120 psi with a large leak in one of the bags. It's going to cycle rather quickly, though, as it is doing. Based on the fact that one lobe of the front bag is not inflating, my professional armchair diagnosis is on that particular bag having a hole in it.

[skytripping]

Thanks Gumpy. I guess my specific question in response to your last post is whether or not this airbag position (picture below) is within normal tolerances. I wasn't able to reach my hand back there, but by using an extension stick I manipulated the upper bag and thereby augmented the sound of the hissing. Perhaps that's not enough data to make a definitive determination, but again, my question is this: is the seating/position of the upper airbag normal and expected in a non-inflated state, or is the airbag placement in and of itself a "faulty" position? Once I can get the wheel removed I'll probably have a much better idea of what the problem is, but it seems to me like the airbag is in a non-standard/working position.

[chessie4905]

When the air pressure drops, does it go to zero or stop or noticeably slow when it gets below 85 or 65? If it noticeably slows, then major problem is in air suspension. Check in that gap with a prybar and flashlight. Good chance of crack or hole there. Do any of the air bags inflate. You can hit them with a stick. They should thump like a tire. You can check the area with a small hose placed to your ear and probe around that bag. I'd consider using an earplug incase noise is very loud. Or mechanic's stethoscope without metal end attached. Very useful item to have down the road for noise and air leak diagnostics. Not too expensive either.

[Oonrahnjay]

    Couple of things ... the air suspension has many components that all have to work together.  You can have a leaky or torn bag, a bag out of place or incorrectly installed, leaks in an air beam, rust holes in brackets, problems with valves, leaking fittings or pipes etc.  It can be difficult to sort out the difference because the effect of leaks from any source have similar effects on the overall air system of the bus.  Once you have a major problem, you have to really dig down into all the parts of the air suspension system and positively locate the problem.
    It is not uncommon also to have several problems at the same time.  You can have a corroded mounting plate that's not sealed to the air beam at one place and has damaged the bag itself in another place.  Or a bag that's slipped out of position might have bent the mounting plate. There are a lot of opportunities for "combination" problems.  Probably, most of those components probably were installed at the same time so if one is deteriorating, there's a good chance that others are also damaged or reaching the end of their design life. 
    In my opinion, you're going to have to work through all the system and check out all the parts themselves, their installation, and their effect on the other parts around them.  My guess (WAIG) is that you have multiple problems now and making the system right is probably going to require changing some parts that appear to be working OK now but are worn or damaged or ending their design lives and are just ready to fail.

    Thanks for the photo.  It helps for people to see how the parts really look when they're trying to understand what's going on on a bus.

[bevans6]

At this point I would get the bus up in the air, front wheels off,  and find the leak.  Poking at it from the outside is pretty much a waste of time until you get stuck in.  The air spring can certainly look like that if it's on a missing bumpstop and fully deflated for a long time.  Might be OK, might not be.  AD2 purge valve kits are still available from Bendix, PN 287053N.  I have one on my desk that I will install when it's warm again.

Brian

[gumpy]

Brian is right. The air bag can certainly be compressed to this extent if there are no bump stop rubbers left.

Understand the upper and lower lobes are not individual bags. There is only one chamber in there, with two lobes. If you augmented the air noise by messing with the bag using a stick, it's most likely a hole in the air bag itself. The photo does not indicate a problem in the seating, though it is possible. I've had one that developed a leak in a seat over a period of time due to a pinch that occurred when it was installed. I've also had one that cracked in the lobe where it gets stressed because of the severe deflation such as you have there.

As Brian says, get it jacked up and get the tire off and then you'll be able to confirm where the problem is. I expect you'll be replacing that air bag, which may well be the best scenario.

[Ed Hackenbruch]

If it is the airbag and you have to replace it, you may as well do them all. Save a couple of the best ones for spares but you will probably never need to use them.

[skytripping]

Thanks for the detailed info guys. So about getting this behemoth in the air....

Here's my current (limited/likely incorrect/incomplete) understanding of the steps I need to take. Please feel free to copy/paste and correct any of these items (or add new ones):

0) Loosen lugnuts on any tire which will need to be removed while wheels are still on the ground (thanks Buswarrior!)
1) Since I can drive it, should I pull the front end onto some blocks or ramps in order to get the majority of the height I need?
1b) Chock rear wheels (thanks Richard5933!)
2) Stick a couple 20 ton jacks under either the axle and/or vertical frame columns as shown at the bottom of this post: http://www.busconversions.com/bbs/index.php?topic=31266.0;all
2b) Remove wheel blocks or ramps
3) Once airborne, stick blocking under the axle and/or vertical frame columns as shown above
3b) Lower jacks until bus rests securely on blocks
4) Find a big @$# breaker bar and wrench (or large 1/2" socket of the correct size) to remove lugs
5) Take a short moment to lament ever wanting to own a bus. Recall the visions of waking up in the heart of Banff. Shake it off. Plow ahead.
6) Remove wheel, set aside
7) From here I'm fuzzy. I'm guessing I can take care of the air system inspection/airbag replacement (if necessary) without removing rotors or anything else wheel-related?
8) Find any and all air leak sources, film/photograph, and return and report...

Any other tools I need? Any other steps I'm missing? Any misconceptions? I'm sure there are things I'm missing here, and to be completely transparent I am quite apprehensive about getting the bus up in the air, properly blocked, and ready for the real work. I'm not really familiar with the anatomy of the bus structure/body yet, so when I hear references to "bulkhead", "axle jacking points", etc, I don't have a clear picture in my mind of what those mean. If you can be gentle in explaining this like you would to a newbie, this newbie would be ever grateful.

Ron

[richard5933]

I cannot comment much on the air bag replacement itself. But, however you get the thing in the air, you need to properly chock the wheels and block the bus so that it cannot fall on you. The jacks cannot be relied on to hold up the bus without proper blocking. Proper wheel chocks need to be large enough to hold the bus from rolling and not just the little things used to chock a compact car.