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Author Topic: FAST IDLE DOES NOT ENGAGE  (Read 4813 times)
coloradorich
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« on: February 06, 2011, 09:07:04 AM »

ON MY 1976 MCI-8 WITH AN 8V-71, ON START THE AIR BUILDS TO AROUND 60 TO 65 LBS AND STOPS. I HAVE TO MANUALLY REV. THE ENGINE FOR AT LEAST TEN MINUTES TO BUILD AIR PRESSURE TO AT LEAST 95 LBS. THEN FAST IDLE WILL WORK. I HAVE BEEN ALL AROUND THE COACH AND CAN HEAR NO AIR LEAKS. WHEN I STOP FOR FUEL I HAVE TO KEEP IT RUNNING AS THE AIR WILL DROP TOO FAST. I HAVE REBUILT THE AIR DRYER AND PRESSURE RELIEF VALVE. ANY SUGGESTIONS.
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bevans6
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« Reply #1 on: February 06, 2011, 09:20:48 AM »

It's considered polite to not type in all capitals, fwiw.  It's a little hard to read, is all.

If your air pressure builds somewhat normally to 65 psi and then stops increasing, you have a leak in the accessory air system or in the parking brake  system, both of which are past the pressure protection valve.  the pressure protection valve won't pass air on to those systems until the air pressure is over 65 psi.

The fast idle system uses the air pressure in the accessory system to operate.  the air pressure has to get up to a certain level to allow it to go on, so if you have a big leak it's normal for the fast idle system to not operate.  So look for the leak, when you fix that the fast idle system will almost certainly work properly again.

I would start by listening and looking for the air leak.  If your suspension settles really fast, look there.  If your suspension stays up, then it's check valves are operating and the suspension may not be the problem,  Can you set and disengage your parking brake?  if so, then that system is probably OK.  Look for leaks in the accessory system - all the other air operated systems.

It should be said that if you can't reach 120 PSI in a reasonable time, if your air system as a whole is losing pressure at over 3 PSI per minute, and if your compressor recovery time (time to go from 85 PSI to 100 PSI with the engine at 1000 rpm) is longer than 45 seconds then you really shouldn't be driving the bus because the bus is failing the minimum DOT standard for operation.  If you do, please be real careful!

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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rv_safetyman
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Jim Shepherd


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« Reply #2 on: February 06, 2011, 06:03:10 PM »

Brian hit it perfectly.

What bothers me, is that you have a major leak and you can't hear it.  A leak that big should be able to be clearly heard.  Try getting a young person (if you have a few years under your belt) and/or a lady.  Someone with good hearing should be able to hear the leak.

You are new here, so we need to say this to you loud and clear:  *****DO NOT GO UNDER THE BUS UNLESS IT IS WELL BLOCKED****** so that it can not drop on you!!!  {{here using caps is considered emphasizing and is acceptable - at least to me Grin}}

My guess is that you will probably have to go under the bus (start at the front) and listen for the leak. You can also check it with a heavy soap solution sprayed on anything with hoses on it.

Accessories are things like wipers, air seat, door hardware, etc. 

Get the coach up to pressure and shut the engine off to check for the sounds of a leak.

Like Brian said, I would ****NOT**** drive the bus until you get this resolved.

Jim
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Jim Shepherd
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papatony
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« Reply #3 on: February 06, 2011, 06:32:17 PM »

If you have access to a air comp. air it up with that less noise
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Nellie Wilson
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« Reply #4 on: February 07, 2011, 01:18:12 AM »

ColoradoRich -

As in 'Molly Brown' stinkin' rich or just your given name?   Smiley

First off, welcome to the board. Second, where in Colorado? How's anybody gonna help if nobody knows where you izzzz?

To the point: I sure agree it's a leak and (from my limited experience) that the leak comes at or after the accessory tank.

PS.  I saw a terrific 'air leak finder' in Arcadia... some sort of infrared device, as I recall. Anyway, it eliminates the need to actually hear the air leak (not always easy) or crawl around with a squirt bottle. Jack Conrad demonstrated the thing... maybe he'll chime in?

Nellie

 
 
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Tony LEE
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« Reply #5 on: February 07, 2011, 02:56:13 AM »

"I HAVE REBUILT THE AIR DRYER AND PRESSURE RELIEF VALVE. ANY SUGGESTIONS."

Probably a silly question but did you do this to try and fix the problem, or did the problem occur after you did it.

Regardless, my experience with the pressure protection valve suggests as others have said, that it may be the problem. Either it has failed or the filter immediately before it is blocked.
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Ed Hackenbruch
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« Reply #6 on: February 07, 2011, 05:20:27 AM »

It could be that you do not hear it leaking because there is not a leak, but a plugged line somewhere.  Ran into this situation this winter trying to find a "leak" on a guy's bus that would not air up.  Took a while to figure it out. Smiley
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
bevans6
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« Reply #7 on: February 07, 2011, 05:41:22 AM »

I thought, when crafting my initial response way up there, of the plugged line scenario, and in particular a plugged or malfunctioning pressure protection valve.  Obviously it's still a possibility, air can be weird sometimes...  I looked at two things - bus apparently airs up to 65 psi OK, or not bad anyway, and then really slows down it's rate of air compression, and secondly it loses air so fast that he has to keep it running when fuelling up.  Losing air equals leak to me, and downstream of the PPV.  It's all I can think of.  Remember that by 1976 on an MC8 the pressure gauge is probably reading the dry tank, so if there was a plugged line after the dry tank it's pressure would still build normally, and if there was a plugged line before the dry tank it wouldn't build at all.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
Ed Hackenbruch
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« Reply #8 on: February 07, 2011, 06:07:08 AM »

The not hearing any air leaks part of it made me think of a plugged line since i had just encountered that. Smiley The other thing is that the governor may be going bad......i think that that could cause the not building pressure and not holding pressure while also not hearing air leaking anywhere scenario.
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1968 MCI 5A with 8V71 and Allison MT644 transmission.  Western USA
robertglines1
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« Reply #9 on: February 07, 2011, 06:44:05 AM »

On a past coach a 78 MCI 8 I had a fantom leak.  Turned out to be the small tank in the drivers side front lower compartment.  was hidden underneath when I started poking it with a screw driver(after loosened up so I could get to bottom) it went right threw.  Another hard to hear place is the bottom of the air bags where the metal of the air bag is bolted to the mount and last I will mention is air beam.  Could be one or combination ..Soapy water in a squirt bottle is your friend here! Ck small tank first as it is a age related failure.
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bevans6
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« Reply #10 on: February 07, 2011, 07:21:45 AM »

that small tank in the drivers lower compartment is the accessory tank, so it's an excellent place to start!

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
buswarrior
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« Reply #11 on: February 07, 2011, 07:23:31 AM »

It is leaking, badly.

Don't drive it until resolved.

find leak, be...

happy coaching!
buswarrior
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Nellie Wilson
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« Reply #12 on: February 07, 2011, 07:32:59 AM »

Ed -

Bingo! Why didn't I - of all people - think of that? Last year, after Arcadia, I experienced the same scenario as Rich. Bus was fine, had just driven several thousand miles without incident (though I had noticed a minor inconsistency when airing up). Stopped for fuel after the rally and the bus wouldn't air up again.

For whatever reason, it dropped lots of pressure while fueling, then wouldn't build past 60psi or so. Finally nursed it around to the station air hose, aired it externally and BK drove it back to Jack's (on a route requiring no stops). Lo and behold, the bus starts making air again... but too much air (about 140 psi).

We discounted the governor as the problem because it had been recently 'replaced' (huge saga about that debacle). But the same problem kept recurring and no obvious leaks! Finally eliminated everything but the governor or the unloader. And voila! it was the governor (turned out to be a piece of old crap).

Learned a lesson: Eliminate the easy stuff first!!

(Thanks again to BK, White Eagle (Tom) and Jack C.)

Nellie
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rv_safetyman
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« Reply #13 on: February 07, 2011, 08:15:44 AM »

Pardon a bit of a rant here.  Rich, you asked a question (1st post) and have gotten great input.  However, you have folks stumbling around on your specific problem. 

When you ask a question, there should be some dialogue on you part to get the best information.  How about you updating us on where you are with the problem so that we can focus on solutions?

Jim

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Jim Shepherd
Evergreen, CO
85 Eagle 10/Series 60/Eaton AutoShift 10 speed transmission
Somewhere between a tin tent and a finished product
Bus Project details: http://beltguy.com/Bus_Project/busproject.htm
Blog:  http://rvsafetyman.blogspot.com/
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« Reply #14 on: February 07, 2011, 01:18:05 PM »

My experience with failed air governors is the same, they always allow too much pressure rather than too little.

Do they ever allow too little?
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PD4107-152
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