Limp mode in Cummins 8.3??

[WimMorre]

So bus starts up and everything, but there is nothing happening when i step on the gas, either in park mode or not. My doors and back window are all closed. I let it run first for about 15 min and let it warm up. When i put it in gear or in reverse, it moves very little, like 1/4 inch. But nothing happening when trying to give it gas.
Worked fine up to a month ago, strange... Anybody have any idea?
Thanks in advance!

[Sehnsucht]

My first instinct is that you shouldn't be giving it any gas because the Cummins 8.3 is a diesel engine. Beyond that, does the engine rev up but no motion? If not, probably a throttle signal issue. If it revs but no motion, probably a gear selector issue. Beyond that, may need more detailed info so maybe folks here with experience in those models can ask more specific questions.

[EastCoastCB]

Talked to OP yesterday on the phone. It shifts, it just won't rev.

I think its either a bad pedal or tps. I don't have any Cummins 8.3 experience so hoping PNW steve or someone can help this fellow out.

[WimMorre]

Thanks for the quick reply guys!
Engine automatically revs higher while in park mode. It's just that nothing happens when pressing on the pedal.

[Booyah45828]

what year?

[bus-bro]

Brakes stuck?

[EastCoastCB]

Originally Posted by Booyah45828

what year?

Either a 98 or 99. Its got a mechanical 8.3 and 3060.

[EastCoastCB]

Originally Posted by bus-bro

Brakes stuck?

Nope. They get up to full pressure and disengage. It was moved a month ago.

[WimMorre]

ok, so just moved cable back in the engine dept and it revs! so it has to be somewhere between the pedal and the engine. when i move the pedal, there is that sound of air pressure...
someone else also told me to check and see if there's a cruise control unit somewhere, and if it's turned on it may turn off the air throttle, therefor making the foot throttle inoperative
my kids and the neighbor kids play in the bus quite a lot, so perhaps they pushed a button i'm not aware of?

[EastCoastCB]

could be man.

Kids at my wedding screwed up a bunch of stuff in my last bus.

[Sehnsucht]

Okay, more info, more suggestions. If it has cruise control, I would imagine there's at least two switches, a master on/off and a set/cancel or set/decel/accel or something to that effect. I think cruise control on school buses is very rare though so the other switch I'd look for is some kind of high idle switch. My experience with these on mechanical engines is that it uses a small vacuum module to pull on the throttle cable enough to bring the idle up to about 1k, which is adequate for long term idling and supporting AC and such while not moving. If high idle is engaged, I'd like to think it would have a neutral position defeat so that as soon as you shift into gear it would throttle back down but this doesn't sound like the symptom described so it's just a shot-in-the-dark suggestion.

If the throttle is a cable, it could have snapped somewhere in it's length, or if the pedal to engine response is air-actuated this could be an air leak and you've indicated an air sound when the accelerator pedal is depressed. On the plus side, either of these leads us away from a more complicated electronics issue so that's a positive. Thankful for mechanical engines!

[cadillackid]

Iwas thinking alot of these busses have air throttles on them.. I see where the air is pumping up so it seems to me either a valve in the pedal up front or the rear actuator is causing the issues..



im assuming both of your tank gauges come up to pressure (120 or so )

[Booyah45828]

It likely has an air throttle. A lot of your older RE buses and coaches had them.

Make sure it's getting air pressure to the pedal and from the pedal with it pushed down.

If it's not, figure out why. The pedal might need rebuilt, or you might have to rebuild the cylinder in the back.

They're pretty straightforward devices though.

[WimMorre]

Wow, you guys sure know your stuff! Thanks for the great info!
I was busy all day welding on roof, but tomorrow i will be getting underneath the bus and see if can find a leak somewhere or other. If not, I'm sure i'll be back on here going deeper on some of the explanations...
Thanks again guys!

[WimMorre]

More info. There's air pressure both in front by the pedal and in the back back the engine. Someone told me to start it out of parking, and low and behold, there was reving. Was able to move the bus, and then within a minute no more rev. Turned ignition on and off a few times, back to rev, then again no rev.
Does anyone know where the air throttle censor is? I looked on youtube but no luck so far.
Also, you guys mentioned either a valve in the pedal up front or the rear actuator is causing the issues.. and The pedal might need rebuilt, or you might have to rebuild the cylinder in the back.
So that's where i'm at. Rev comes and goes... more no go than go
Thanks guys!

[12vtech]

It may be different on your bus but on my 98 Thomas RE its connected directly to the P7100 injection pump linkage. My pedal and air throttle valve is manufactured by Williams Control.

[WimMorre]

Williams Control eh? Will have to look them up and give them a call.
Yeah, i have lots to learn when it comes to all this stuff
Thanks for the info!

[12vtech]

No problem. Heres a picture of my air throttle attached to the intake manifold. There will be linkage from the air throttle to the P7100 pump. You might also find another air line that connects to the air throttle. This would be the high idle solenoid.

[WimMorre]

Ok, thanks. So what should i do next? See if can order new air throttle pump and/or censor? or new pedal? Wonder if they still make it/have parts.
I wonder what people out there do if eventually they don't make those parts anymore...

[Sehnsucht]

Originally Posted by WimMorre

I wonder what people out there do if eventually they don't make those parts anymore...

Scrap yard or rebuild.