Fast Idle / Losing Air

TheHollands! · March 28, 2015, 08:08:46 AM

Hey guys, I had the bus running this morning MCI-9 6v92T was moving it into position and couldn't release the emergency brake. Had been moving it for a few minutes previously. Saw the air was lowering so put it in Neutral and the fast idle was no longer working and I could see I was losing air. If I hold the throttle I can keep it at 80 lbs but get it no higher, soon as I release I lose pressure. So I'm stuck. I'm a beginner mechanically trying to learn what I can. Any ideas? Thanks, Craig

bobofthenorth · March 28, 2015, 08:15:11 AM

Its pretty hard to troubleshoot something like this from a distance but if it aired up normally and then happened all of a sudden while you were moving it sounds like you are losing a lot of air somewhere. You should be able to hear it escaping but maybe not over the sound of the engine. Air it up as much as you can, shut it down and listen. Then figure out where its coming from.

Jon · March 28, 2015, 08:25:50 AM

Not enough information. If you have the emergency (parking) brake applied and you are losing air you have at least two issues. You have a protection valve problem possibly and you have a leak that is likely not associated with the brake system unless the leak is a relay valve or associated with the brake treadle.

If the parking brake is released and you cannot build air you could have a failed spring brake diaphragm.

Regardless you should be able to hear the location of the leak.

TheHollands! · March 28, 2015, 08:33:53 AM

The parking brake is on and I'm losing the air down to about 65 lbs. Also the fast idle all of a sudden stopped working at the same time.

bevans6 · March 28, 2015, 08:48:06 AM

You have a leak in the accessory air system, AKA the air system downstream of the pressure protection valve. Included are all of the suspension, the emergency/parking brake system, all the engine accessories, air door if you have one. Fast idle is part of the engine accessories and so will not work. the engine stop system is also part of this, and may not work. You may have to turn the engine off on the dash, then go back and manually kill the engine with the engine stop lever (assuming it's not a DDEC engine). then you just need to find and fix the leak, easier said than done, but still required...

Brian

TheHollands! · March 28, 2015, 09:06:57 AM

I haven't been able to locate the leak yet, but what you say Brian is exactly whats happening. I ran the engine a bit ago so I could try and find the leak and wasn't able to shut it off. I got it off now but all that is going on sounds that way. I'm in Indian Head Beach near Melbourne Florida atm, are there any shops down this way to call. Craig

Jon · March 28, 2015, 09:53:05 AM

I have zero knowledge specific to an MCI, but assuming your brake system is building air it should hit somewhere around 75 PSI before the protection valve allows air to the accessory system. In other words you will not hear any leaks or escaping air until both the primary and secondary brake systems are charged. If they will not pressurize you have a problem with the protection valve making your needs somewhat greater.

If the systems build air at least you can have someone outside the coach try to hear where it is escaping. Does your coach sit level and at the driving height? If that is the case it is likely your suspension air bags are OK.

Dave5Cs · March 28, 2015, 10:21:27 AM

If you have a shop compressor with you hook it up to the NP fitting in the engine compartment. Should be one under the drain daily valve on the bulk head. The pressure protection valve on the MCI in set at 65 PSI and the fast idle would kick in higher mine is at 85PSI . So if you can't reach that PSI then it will not work like Brian said anyway until you attain that range. Using the shop air will allow you to put it as far away from the coach as possible so you don't hear anything but air leaks or as much as possible. Make sure you closed all the drains on any air tanks also( don't ask me how I know that). There is a valve in the rear wheel well up on the bulkhead relay valve ( generally with 4 hoses coming out of it and looks like a space ship with a rubber disk on the bottom). If it is leaking it will tell you , you may have a leak in your DD3 diaphragms in the rear. HTH

Dave5Cs

TheHollands! · March 28, 2015, 01:24:05 PM

So I'm not sure what you call this but is seems like it it what Dave described. The rubber disk has air leaking out of it. I attached photo's of it. What do I actually replace and where can I get it. Thanks for all your expertise helping me in my amateur hour. Craig

luvrbus · March 28, 2015, 02:30:49 PM

That is a Bendix R-8 valve shouldn't be hard to find if that is your problem,check the pedal and make sure it is releasing all the way sometimes a small stone can wedge the pedal and the R-8 will not seat

Dave5Cs · March 28, 2015, 02:39:56 PM

Thats it also if not the pedal like Clifford was saying take a pair of vice grips and figure out which is the supply line to the rear brakes from that valve. Put the vice grips on that hose after you wrap a small towel around the line and squeeze it just enough to see if the air leak stops. Do both sides. If it doesn't stop replace the R-8 valve. Napa has them or can get them as well as Luke B. If it does stop the leak then it is one of your emergency diaphrams has a leak iin the rubber and will need to be replaces. truck supply parts place. This is if you have 3 hoses coming out of your rear brake cans which would mean you still have DD3's.

Dave5Cs

TheHollands! · March 29, 2015, 11:51:08 AM

when system is below 75psi and foot brake is applied and held no air leak at RE-8 valve. When system pressure get to 75psi or higher (will not go above) air leaks from the exhaust port of RE-8 valve. There is no leak below 75psi.

bevans6 · March 29, 2015, 12:54:41 PM

Is your parking brake on or off, at the push/pull valve? The R-8 valve acts (amount other things) as the quick release valve to exhaust air from the rear brake chambers. When you apply the brake the quick release valve is closed, so air cannot escape. When the brakes are not applied, all air inside the service brake section of the rear brake chamber is exhausted. If you have the parking brake applied at the push pull valve and air pressure gets over a certain point, air is sent to the parking brake section of the rear brake chamber. If there is a leak in one of the parking brake diaphragms air will leak from the parking brake section to the service brake section and will come out of the R-8 valve. I would first chock the wheels just to be safe, disengage the parking brake at the push pull valve and see what happens when you air up. If no air leaks out, this is point towards a diaphragm leak. You can then clamp off the parking brake air lines (the ones on the extreme rear of the brake chamber) and see if clamping the lines off changes anything (with the parking brake re-applied at the push pull valve). If clamping off one line stops the leak, that is the bad chamber.

Brian

buswarrior · March 29, 2015, 07:05:23 PM

Pay close attention to Brian's diagnostic instructions.

Yes, that MC 9 has DD3.

Proper diagnosis avoids unnecessary parts changing.

Happy coaching!
buswarrior

TheHollands! · March 30, 2015, 10:17:42 AM

Thanks Brian for the information. I had a friend over who has more knowledge than I and your information was real helpful to him. We were able to get it moving temporarily by capping off one of the lines to the chamber and get our air pressure back. Will be in the shop tomorrow. Craig