Fast Idle Question

scirocco22 · 04-19-2008, 01:38 PM

Can somebody tell me what I'm up against here. I haven't see this addressed in my manuals ...but I'm overwhelmed at all the separate manuals I received so I may have missed it as I've only had the coach for a bit over a year now.

I know that to turn the fast idle on and off that the cruise control switch is used. However, I now notice that when I turn the engine on, after about a minute it goes into fast idle by itself. The crusise control switch hasn't been touched and is in the off position. Clicking the switch back and forth doesn't push the idle speed down. The only way it will cut itself off is by stabbing the brake pedal. BUT then it comes right back to fast idle after a few seconds.

This has just started happening. It's never happened before this.

Where do I start troubleshooting this?

Thanks.

scirocco22 · 04-19-2008, 01:38 PM

Can somebody tell me what I'm up against here. I haven't see this addressed in my manuals ...but I'm overwhelmed at all the separate manuals I received so I may have missed it as I've only had the coach for a bit over a year now.

I know that to turn the fast idle on and off that the cruise control switch is used. However, I now notice that when I turn the engine on, after about a minute it goes into fast idle by itself. The crusise control switch hasn't been touched and is in the off position. Clicking the switch back and forth doesn't push the idle speed down. The only way it will cut itself off is by stabbing the brake pedal. BUT then it comes right back to fast idle after a few seconds.

This has just started happening. It's never happened before this.

Where do I start troubleshooting this?

Thanks.

fun time · 04-19-2008, 02:31 PM

I had to laugh when I read your post as mine does something similar but with a slight variation. Mine will not idle high until after it has been running for about 5 minutes then goes into high idle without the cruise control. Once it runs for five minutes then the cruise control can be used to activate the high idle. Once I step on the break it goes off and stays off for about another 5 minutes then turn itself back on until I repeat the brake pedal stomp. It will continue to do this for several times at least. I wish I could tell you where to start troubleshooting but I am in the same boat so look forward to reading the answer.

SargeW · 04-19-2008, 03:44 PM

I'm not sure either of you have a problem. My coach will randomly fast idle as well. However, there is a sensor built into the ECM that monitors engine temp and fuel delivery. If the engine is not warming up fast enough and burning all of the fuel that the injectors are putting into the cylinders, the excess fuel will actually start to "wash" the cylinder walls. By increasing the idle, you are getting a more complete burn of the fuel and not leaving excess fuel in the cylinder.

I would only worry about the high idle if the engine was already warm. Diesels warm up VERY slowly. That is why they can run so cool on a hot day. Playing with the cruise control won't over ride the ECM. It is designed to do that to prevent an unsafe fueling condition. As long as it drops when the brake is touched, all is most likely working according to spec.

Sarge

scirocco22 · 04-20-2008, 06:19 AM

Thanks, Sarge!

Perhaps I can cross that off my list of things to worry about! I had no idea that the ECU was monitoring that. I drove transit buses for years and we had to manually engage the fast idle during warm up.

Which leads me to a related question. It seems to me that the high idle on my motorhome is exceptionally fast. On buses, when we threw the fast idle switch on, it would only kick up the idle a few RPM. In the motorhome, the fast idle almost seems to make the engine race. Is this normal? What RPM is the fast idle supposed to be running at?

Thanks.

SargeW · 04-20-2008, 03:03 PM

Yes I noted that too Rocco. But since it is built into the software program that Cummins designed, I figured that they must be OK with it.

fun time · 04-20-2008, 03:13 PM

My fast idle is right at 1100rpm on mine if that helps.
Thanks SargeW for the info. Very helpful.

Teach · 04-20-2008, 04:02 PM

1100rpm on mine, too.

diesellabs · 04-22-2008, 06:21 AM

One of the triggers for an automatic fast idle is low battery voltage; the engine will accelerate to increase the charging rate.

With my C7 I can invoke a faster idle using the cruise control just about any time after starting -- I don't have to wait 5 mins -- and I can set the idle speed wherever I want it.

I think, but don't know, that some of the Cummins engines have an automatic fast idle controller that cycles it up and down for cooling...

SargeW · 04-22-2008, 06:32 AM

I believe you are right Steve. With the software programs that run our engines these days, manufactures have a whole range of options that they can build into their designs. Anymore it's almost like driving a CPU with wheels.........

Pusherman · 04-22-2008, 03:39 PM

Mine too will go into a fast idle if the battery voltage is low -- I think below 12.6V.

On mine the display system on the dash will tell me it's in fast idle for low voltage.

scirocco22 · 04-23-2008, 06:53 AM

Thanks, fun time, Teach, and diesellabs for the feedback and good information.

Yeah, Sarge, it just seems a bit too high for me.

Don, what display system on the dash are you referring to? The Freightliner info center? ...or did you install an auxiliary Silver Leaf or TripTek- type system?

I'm just now figuring out different settings and displays on the Freightliner display but did not see that one. I'd sure be interested if you're seeing that readout on the stock Freightliner display. And this is the chassis battery voltage that you guys are talking about, correct?

Thanks.