Fast idle races up when turning off

[lostagain]

6V92TA mechanical. My fast idle works fine, but when I switch it off at the dash, the rpm races up to 1800 or so for a couple of seconds, then goes back to normal idle. I am without a service manual now because mine on a CD has crashed, I can't upload it anymore. (I am looking for a paper one, like the real book). I remember from the last time I adjusted it, that one air line pushes on the fuel lever on the governor cover, and that there is a limit in the back of the governor that you adjust to set the fast idle rpm. There is also an air pressure regulator that controls the air pressure for that system. It feels like the limit turns off first, before the air cylinder that's on the fuel lever is released of pressure, making it run fast momentarily. Is there an exhaust orifice I should be looking for on that cylinder, or elsewhere? Other ideas?

Thanks

JC

[Red Rider]

Sorry to hear JC. I'd share any info That would be helpful but I don't have a clue.

[luvrbus]

JC, the fast idle and holding cylinder should be on the same line with a tee,both exhaust through the skinner valve your fast idle cylinder is just dirty,spring broke or with the combo fast idle and Jake buffer switch the roller may have a flat spot or a piece of the rubber boot has wedged between the switch binding the rod momentary .You can take the cylinder apart and replace the o-rings it has a small snap ring on the air supply end remove it and it will come apart fwiw 

I would say it is buildup on the rod or in the cylinder,I will check manuals I may have a extra set If I do I will send them by Tim  

good luck

[lostagain]

Thanks Clifford. I will be in Alberta away from the bus for a week or so. I will take the cylinder off when I come back and take a good look.

JC

[lostagain]

I took the air cylinder that pushes on the fuel lever off and cleaned it and lubed it. The rpm still comes up a bit when I turn the fast idle off, although not as much as before and for just a fraction of a second now.

I think what is happening is the idle lock at the back of the governor comes off first, then a half second later the fuel lever push cylinder retracts. The air comes from the Skinner valve to a T right behind the fuel lever cylinder. So that cylinder is getting 120 psi. From the T, it continues to a pressure regulator set a 85 psi, that supplies the idle limit at the back of the governor. I wonder if I could try lowering or increasing the pressure for the lock on the governor. There is also a buffer switch. I could take it out and look at it too.

JC

[luvrbus]

I never saw one with regulator do you have a air throttle ? You see a inversion valve on some with the supply coming from the parking brake so you cannot engage the fast idle with parking brake off but I never saw a regulator between the skinner valve and fast idle control unless their is a air throttle in the loop.Something is holding the buffer open for a second longer it could be the switch, there is not much in a manual about the fast idle 

[HB of CJ]

When you turn off the high idle before shutting down the engine, try bipping the throttle once as you turn off the high idle.  Then let the engine settle down to low idle before shut down.  HB of CJ (old coot)

[lostagain]

There is no air throttle, just a regular cable.

Old Coot, I could try blipping the pedal, but I don't think I could, since, as long as the fast idle is on, it is limited to the rpm you set it at with air operated stop around the idle screw in the back of the governor. Mine is at about 1000, could be as high as 1200 rpm, or whatever you want to set it at.

I won't be able to look at it tomorrow, I am driving a truck.

I'll keep you posted.

JC

[luvrbus]

Ok I have a 8v92 here with that setup I'll dig into it Thru that is a different ball game remove the buffer switch if it comes back down in hurry then the switch is bad if so let me know and I'll send you a new switch, I didn't know that bus had such a late model of engine 

That is a neat setup it just pushes the idle screw in for high idle found mostly on fire truck engines for pumping because they are so easy to adjust to what ever speed desired     

[lostagain]

Looked into it some more this afternoon, and didn't find anything.

With it running, I moved the air pressure regulator up and down, with no change, so I put it back to 85psi like it was.

Looking at the governor standing at the back bumper, there is the Jakes switch on the left. I removed it, didn't see anything funny, and replaced it. Then on the right of the governor, facing me, is a screw (buffer screw I think, I don't have a manual right now, so not sure if that is what it is called). I screwed it in and out a 1/2 turn at a time with no change to the surge when turning the fast idle off.

I rigged it up so I could turn it off and on from the back to observe the piston on the fuel lever. It seems that the piston retracts off the lever first, the lever goes back to idle position, then the rpm surges briefly . So the surge comes from the governor somehow...

It really is not a big deal, maybe it is normal? Now I can't remember if it always did that, or if other buses I drove did that too or not.

So I still don't have any idea what is causing this little rpm surge when turning off the fast idle.

JC

[luvrbus]

That is the staring aid adjustment facing you JC put that back where you found it 

[lostagain]

Yea I did. I just found a tune up manual with a picture of the governor. I see that the screw on the right is a starting aid screw. I was careful to reset it where I found it. The Jakes switch on the left is the buffer screw. I adjusted it back to where it should be. Thanks.

JC

[Larry B]

Hi JC--- Have been following your issue with interest as my fast idle sound same type . It has been a couple of years since had that apart but if I remember correct the small rod from the buffer switch/ fast idle cylinder that passes into the govenor has an O ring (stop oil leak) and a small metal washer (to limit the amount of OUT travel the rod has). The rod has a step in it at the buffer switch nut connection. Maybe the O ring or washer is hanging up. As long as you do not undo the jam nut on the govenor you should not loose any orig. settings. Undo the  mount nut and take buffer switch and fast idle air cylinder off. Pull rod out and inspect. That O ring is an odd size ,do not loose or oil leak - been there. I am almost sure there was a spring on end of rod(govenor side) to get ride of engine roll at idle. Some thing else you might want to consider checking.
       Larry B

[luvrbus]

He doesn't have that type Larry his works off the idle adjustment JC I am looking for the service bulletin for the setup when I find one I will PM it to you the manuals will have nothing about it fwiw