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Author Topic: Bus wont go off fast idle!  (Read 1711 times)

Offline trey

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Bus wont go off fast idle!
« on: July 25, 2007, 12:22:57 AM »
I have an MCI 9 with detroit 8v92tt and it wont go off fast idle. on the engine there is this pill bottle size thing that pushes out a little pin that keeps the throttle from moving/stuck on fast idle. I have to push the brakes a lot letting all the air out before this little pin will go back in. what is this and whats wrong with my bus that causes it to get stuck on fast idle? Im new to the bus/diesel engine world can anybody help me out?
thanks
trey

Offline Barn Owl

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Re: Bus wont go off fast idle!
« Reply #1 on: July 25, 2007, 12:33:31 AM »
Your electric/air solenoid valve up stream is stuck in the open position. At the moment, my mind is blank on the name of that thing. Someone help! Starts with an “S”.
« Last Edit: July 25, 2007, 02:56:31 AM by Barn Owl »
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Offline Barn Owl

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Re: Bus wont go off fast idle!
« Reply #2 on: July 25, 2007, 12:39:43 AM »
I remember! It's a skinner valve.
L. Christley - W3EYE Amateur Extra
Blue Ridge Mountains, S.W. Virginia
It’s the education gained, and the ability to apply, and share, what we learn.
Have fun, be great, that way you have Great Fun!

Dreamscape

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Re: Bus wont go off fast idle!
« Reply #3 on: July 25, 2007, 02:18:39 AM »
Barn Owl is right, that pill bottle thingy is a small pneumatic cylinder that works off the skinner valve. It is probably all gummed up and wont' let the air dump when you take power off the skinner valve. HTH.

Happy Trails,

Paul

Dreamscape

Offline FloridaCliff

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Re: Bus wont go off fast idle!
« Reply #4 on: July 25, 2007, 02:22:31 AM »
Remember, its broke.

Not much to take apart and clean up.

This is all heavy duty and rebuildable equipment and most are very simple to rebuild.

Give it a try!

Cliff
« Last Edit: July 25, 2007, 02:26:14 AM by FloridaCliff »
1975 GMC  P8M4905A-1160    North Central Florida

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Offline RJ

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Re: Bus wont go off fast idle!
« Reply #5 on: July 25, 2007, 05:01:51 AM »
Trey -

To elaborate a little more:

The unit on the engine that pushes on the throttle lever can be removed, disassembled, cleaned and reassembled easily.  Lube it with something other than WD-40, use 3-in-1 or similar.

Next, follow the air line that attaches to the piston assembly you just cleaned back to the engine bulkhead.  Here you'll find the skinner valve, and chances are it's stuck.  A little more complex to clean, sometimes it's better to replace, they're not that expensive.  (Of course, you can try the "cheap" method first - with the engine running on fast idle, switch off the F/I.  If it doesn't drop, then rap on the skinner valve with a rubber dead blow mallet to see if that unsticks the valve.  Regardless of the results, order a new one!)

FWIW & HTH. . .

 ;)
RJ Long
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Dreamscape

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Re: Bus wont go off fast idle!
« Reply #6 on: July 25, 2007, 09:05:29 AM »
Can someone post some phone numbers for Trey? I suggest replaceing the parts as that is the best way to go. I am traveling and don't have these available.

Paul

Offline Don4107

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Re: Bus wont go off fast idle!
« Reply #7 on: July 25, 2007, 06:16:25 PM »
Trey,

Are you sure that the fast idle switch is not turned on?  Could have been bumped.

Sounds like the cylinder is working since when you pump down the air it retracts and allows the engine to idle.  Indicates the problem is the skinner valve.  It is stuck or power is being applied to activate it.  Trace the air line back from the cylinder to the skinner valve. 

Check to see if power is being applied to the valve.  It should only have power when the fast idle is turned on.  If there is no power, try tapping on it.  If that causes it to close, plan on changing it anyway.  It will stick again.

Good luck

Don 4107
Don 4107 Eastern Washington
1975 MCI 5B
1966 GM PD 4107 for sale
1968 GMC Carpenter

Offline Stan

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Re: Bus wont go off fast idle!
« Reply #8 on: July 25, 2007, 07:06:15 PM »
I don't know what the costs are now on Skinner valves, but when I was working with them a lot, a rebuild kit was about 10% of the cost of a new valve. With the valve in a vise, it is about a ten minute job to replace the plunger and gaskets. I guess the decision to repair or replace is determined by how much you value your labor.

Offline FloridaCliff

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Re: Bus wont go off fast idle!
« Reply #9 on: July 25, 2007, 08:22:54 PM »
Stan,

I bought one new and rebuilt my used one.

You are 100% right on the price difference.

Found this out a little to late the first time.

I now have a good spare, and will rebuild any future ones.

Cliff
1975 GMC  P8M4905A-1160    North Central Florida

"There are basically two types of people. People who accomplish things, and people who claim to have accomplished things. The first group is less crowded."
Mark Twain