Buzzing sound behind rear bay on MCI MC9 and air leak?

[neoneddy]

I've only owned this bus for a month or so.  Hunting down air leaks and as I do I either notice new things that were always there or they are exposing other weak areas in the system.  Here it goes.

When I fast idle from cold (0-30psi depending) , it will come up  quickly to 85-just under 90 PSI then stall  and actually drop back to 70 psi for a few minutes and rebuild.  It's like a valve is triggered and it pumps air into a new part of the system lowering overall pressure.   If I  turn off fast idle and manually rev it up higher I can get it up to 90+ and the low air warning goes off.  Once I get past 90 PSI the second time a straight fast idle will keep the PSI rising until it releases at 120 and the normal cycle starts.

Obviously we have a few issues.  Could it be the compressor governor?   I'm sure I have some leaking check valves and or other leaks letting the air escape.  I'm hoping to build some wood block ramps to get under it and spray soapy water every where.

Once I turn it off I hear a buzzing / running motor noise behind the rear most luggage bay in front of / inline with my drive axel.  Could this noise be air leaking back into the compressor and running it in reverse?  Explaining both the noise and the leaking?

Now for the last item, probably all related. Right rear axel stays aired up indefinitely, left side will bottom out within 6 hours or so.

Other than that buzzing noise only air leak I hear is from the tag axel, it's coming from the metal area where it meats the bag assembly and the frame.  I hear this is common, however  both sides of the tag axel eventually go limp but the right side overall stays up. I suspect the systems are separate because there is a tag axel lift switch up front.

Thanks everyone, you've been super helpful.

[PP]

When your air is originally coming up and then stalls and drops a few PSI, it might be the valve to your auxiliary air tank opening. The brake system should air up before anything else, or at least it seems to on my Prevost. My tags go limp within a day of setting, my right rear leaks down faster than my left side. I've lost count of the times I've crawled around with a bottle of soapy water looking for the air leaks. Fortunately, the front bags will stay full for months, so when I'm going to be parked for a while, I place 4 blocks under the rear assembly and just air up when I'm ready to move and pull the blocks out.
Good luck, Will

[neoneddy]

Will, I should add my front bags are empty too I  think after about the similar time as the left side going down. 

It was used as a transit bus until 2012 or so, and I suspect they didn't care about settling or or anything so long as it worked fine while driving.

[brmax]

I believe your onto something working up a great set of block ramps!
A super heavy duty pair or 4 ramps actually is seriously a basic need.
Many designs around here and do recommend some cut nice plywood pieces.
Anyway them air systems normally are legal to keep the air brake section seperate
with the 60-65psi valves. This saftey thing is pretty good for other future installs fwiw.

Good day there
Floyd

[buswarrior]

relax, your description all sounds pretty normal.

Pneumatics can make all kinds of sounds. Pipe organ, whistle, air horn, hissing, trumpet, flute, MCI ...

All you are hearing is a leak someplace, internal or external, they all make music.

Yes, the pressure protection valve won't open until the air gets way up there, then it allows air to the auxiliary, air suspension, belt tensioners, radiator shutters, etc.

Also, all bets are off on what a previous owner may have done to corrupt the design of the manufacturer.

And stuff wears out, springs in valves get weaker, seals and o-rings get leaky, gummed up, dried out, or otherwise not functioning as intended.

Keep up the good work

happy coaching!
buswarrior

[sledhead]

if you can try to air it up with shop air compressor to see how many leaks you can fix
and make sure it is blocked be for going under it

dave

[bigred]

Just a guess ,but does your coach have a air dryer?? If so that is probably the buzzing sound !!

[Jim Eh.]

Once I turn it off I hear a buzzing / running motor noise behind the rear most luggage bay in front of / inline with my drive axel.  Could this noise be air leaking back into the compressor and running it in reverse?  Explaining both the noise and the leaking?

Now for the last item, probably all related. Right rear axel stays aired up indefinitely, left side will bottom out within 6 hours or so.

You have to wait until the air system is completely fully in all reservoirs before diagnosing any system issues. If you can get it to that point you have half the battle already won.

Like said previously, the "buzzing" is usually air transferring/building from one area of the system to another down stream.

Yes you have three height control valves (as long as no PO has changed the system around) two in the rear and one in the front. This way the bus can tripod and level out much more easily. Much like a three legged chair will always sit firm but a four legged one often rocks.

The never ending quest to repair all air leaks and become "leak free" is somewhat a daunting task. More like an obsession. Kinda like push a piece of cooked spaghetti uphill. With enough patience, it can be done.  Enjoy winning the battles but probably don't expect to win the war. Be satisfied with a solid truce.

[neoneddy]

Jim, thank you for your time and input.  (Everyone else too)

I've  driven it recently and it would purge, I'm guessing by that point she's full up everywhere.

I think other mentioned a shop air compressor, I have a small 6 gal pancake only. It will get it to that first stage , then my phenumatic wipers start hissing, oddly enough that stops  once pressure builds past 100lbs. I can only get over that hump with the engine running.  If I have the shop air and  engine running she's up to pressure luckily split. Which leads me back to my compressor governor as a contributing factor to not building pressure past a certain point.

Last thing I read and want to ask everyone's opinion on. I've read that sometimes driving it for a good 3-4 hours over a few hundred miles can bring oil and enough cycles. Through everything to re soften seals and generally make it  work better.  Since I brought this home it's just been a few short trips here and there.

[neoneddy]

Accidentally double submitted, edited the double post out

[buswarrior]

Your governor is fine.

Yes, leaky teaky do seem to tighten up a bit with exercise, but it just goes back the way it was once they sit again.

There are no free lunches, and no short cuts.

Persistent preventive maintenance. Just keep chasing the leaks, each air valve or component you rejuvenate or replace, gets it that much closer to Nirvana.

Get a photocopy of your air system, so you may write in the replacements. Believe us when we tell you, you won't remember what you did when, as the years go by... After awhile, there should be nothing left that you haven't changed out for new.

Use suitable anti-seize on all metal mating surfaces, remember who gets to replace it again the next time... use the anti-seize or curse yourself a decade from now, the same way you curse the previous owner's mechanic today...

happy coaching!
buswarrior

[gumpy]

It's possible the governor is sticking. Mine was sticking once. It would act similarly, airing up to a certain point under 90 psi and then dropping back to about 70. I had to smack the governor with a screwdriver handle to get it to free up. Eventually I pulled the governor apart and cleaned and greased it. Been fine for 12 years or more now. It could also just be the valves opening to allow air into the suspension, however, typically there's no drop-back when that happens. Just a stall for a few minutes at around 65 psi or so, and then it continues building.

The buzzing is air moving through the system from one point to another. It's compensating for a leak, probably in your air bags or air beams. If you hook up the compressor after you bring it up to full pressure, you'll hear the buzzing pretty much continuously. Don't worry about the buzzing. Find the leak.

Usually you can get the wiper valves to stop hissing if you turn them to park when airing up on the shop compressor. My bay locks button acts the same. Leaks when low air, stops if you hold it in or out till more pressure gets in the system. I have a small compressor on the bay locks, so I often run it to pressurize the lock system, and then it doesn't leak while the bus air system comes up.