Idling a diesel

[Tom]

I've read in various places that idling for long periods of time is not recommended for diesel engines, but I have no idea how long is long. If I've dumped the air in my coach the night before, it takes a couple of minutes to bring the air pressure back to where the annoying buzzer shuts off and the air pressure reads something above 90 psi. Of course, we could be stuck at traffic lights for longer than a couple of minutes. Also, when we pull off the freeway having been travelling at the speed limit &/or pulling a grade, we let the engine idle for several minutes to allow the turbo to cool.

What is the common wisdom on what's considered "excessive" idling of diesels?

 

[caltex]

My manual says if you are going to idle for "a while" whatever that means to idle at 1000 rpm using the cruise control.  I assume this is to avoid build up of carbon from unburned fuel at a slow idle. You can easily sit in traffic for a couple of hours idling so what's another 30 minutes if it's convenient to leave it running?  Most trucks at the truck stops never turn off their engines, even leaving then running all night.  What is different about their engines?

 

[Tom]

Good questions Robert and it's all a mystery to me. But I do know that the first thing the CAT tech does when he comes to the boat is plug his laptop into the diagnostic port and verify that my engines have not run at idle for more than some percentage of the time. As you know, the challenge we have is that, in order to keep the wake down on our way home, we have to either run on one engine at idle or run both engines in "slow vessel" mode (lower than idle RPM).

I wonder if the Cummins techs do the same thing with motorhome engines (?)

BTW someone we know well has been running his boat at 9 knots for some time (playing trawler). He complained to CAT that things were getting sluggish and they thought he was getting a buildup of carbon. They suggested throwing the throttles to the wall for a short time while in neutral and it solved the problem.

 

[Ron]

My manual says if you are going to idle for "a while" whatever that means to idle at 1000 rpm using the cruise control.? I assume this is to avoid build up of carbon from unburned fuel at a slow idle. You can easily sit in traffic for a couple of hours idling so what's another 30 minutes if it's convenient to leave it running?? Most trucks at the truck stops never turn off their engines, even leaving then running all night.? What is different about their engines?

Long idle times can be detrimental to your engine.  If you are going to idle for more than 5 minutes or so it is best to fast idle at 1000 RPM.  We have shut the engine down when we get stopped in traffice for a long period of time.

 

[Tom]

Long idle times can be detrimental to your engine.

Detrimental in which way Ron? Is that excessive carbon buildup? If so, why can't the buildup be removed using the method CAT gave to my boating friend (who is also a Framily member)? Is there a long-term problem?

I'm hoping Leo will jump in here with some words of Wisdom.

 

[caltex]

Tom said "They suggested throwing the throttles to the wall for a short time while in neutral and it solved the problem."

I hope Dave had the engines warmed up before he did that.  That is the test CAT recommends to check that everything is functioning OK.  If you can achieve max rated rpm at no load, everything is supposed to be OK.  I certainly don't leave the engines there very long.

He would be better off to run at max throttle in gear, under load for 15- 20 minutes. If he can find a place in the Delta to do that.

 

[Ron]

Carbon build up is the result of long term idling. I was told that on the modern diesel engines repeated periods of long term idling can lead to expensive repairs.  Hope Leo jumps in here on this.

 

[Tom]

You know, maybe they did say full throttle under load. (I might be getting confused).

 

[Alaskansnowbirds]

I've read in various places that idling for long periods of time is not recommended for diesel engines, but I have no idea how long is long. If I've dumped the air in my coach the night before, it takes a couple of minutes to bring the air pressure back to where the annoying buzzer shuts off and the air pressure reads something above 90 psi. Of course, we could be stuck at traffic lights for longer than a couple of minutes. Also, when we pull off the freeway having been travelling at the speed limit &/or pulling a grade, we let the engine idle for several minutes to allow the turbo to cool.

What is the common wisdom on what's considered "excessive" idling of diesels?

At the CAT seminar I attended at FMCA in Albuquerque, they said anything over 5 minutes was unnecessary. By the time air builds up in the morning you're ready to go. Just don't go full throttle on a hard climb until you got above at least 150 degrees. After driving all day by the time you get through local traffic an get to the campground you're cool enough to shut down. Out on the highway if you pull into a rest area, let it idle 3 to 5 minutes to let the turbo cool. On most electronic controlled engines the ECM can be programed to auto shut down after X number of minutes. I never idle more than 5 minutes max. I've heard *RUMORS* that they look at idle time as one of the factors in determining coverage on some warranty items.

I think there is a difference between marine and over the road engines. That may be why you can run a marine engine at full throttle to clear the carbon. I've never heard it recommended for over the road engines.

Many of the engines you hear running in truck stops are the engines of the refrigeration trailers. Some tractor engines are running because that is the only way the driver has to maintain cab temperature while he sleeps. He doesn't have a generator to run the heater or air conditioner. I know, there are still some old school truckers that grew up with the old two stroke diesels that still think you have to warm up an engine to operating temperature before you can go. This is NOT necessary with the new four stroke engines.

 

[Tom]

Good comments Don, thanks.

 

[Jim Dick]

Tom,

I agree that 5 minutes is more than enough idle time. If I am going to idle more than that, such as putting air in the tires from the air compressor, I'll run it on high idle which is around 1000rpm. This can be done with the cruise control on mine and many others.

We used to have a neighbor about 3 sites away that had a diesel pickup. Seems like every morning he would go out real early and start the truck. It would idle for what seemed like  an hour! Then he'd turn it off. Never seemed to go anywhere! Sure glad he sold out and moved.

 

[Karl]

Just wondering if

throwing the throttles to the wall for a short time while in neutral

has something to do with getting the turbo(s) spooled up to full boost?

 

[jerryarlyne]

What is the common wisdom on what's considered "excessive" idling of diesels?

Hi Tom,

Don gave the best answer to the question.

The reason for letting it idle for a few mins. after running hard is that the turbo is cooled by engine oil, as long as the engine is running the oil will cool the turbo to normal temps. This takes about three to five mins. To cool from excesive temps. (over 1,000 degrees) to normal temps. of around 300 degrees. If you have an EGT gage it is easy to see when it it cool enough to shut down. Normally, any longer idle time is just wasting fuel.

Jerry

 

[Tom]

I'll run it on high idle which is around 1000rpm. This can be done with the cruise control on mine and many others.

Well that's another thing I learned today Jim. I'll see if I can do that with our coach.

 

[Tom]

Thanks Jerry. I understood the need to idle long enough to cool the turbo. Unfortunately, I don't have an EGT gauge on the coach, so I tend to err on the side of idling too long when cooling down.

 

[Tom]

Karl,

I believe I had it wrong and Robert had it right - full throttle under load.

 

[Bob Zambenini]

Not that I don't have confidence in the expert opinions here, but I did look around and found the EPA view on this subject.

http://www.epa.gov/smartway/idle-questions.htm#why-idle

Interesting reading, especially from an environmental view.

But my procedure remains the same. When I fire that puppy up and head down the road I don't shut it down unitil I am through for the day. One exception is if I fuel at the RV islands, which is rare. My longest idle is then about 20 or 30 minutes at lunch.

I was shutting down when refueling at truck islands, but couple of years ago I was talking to a golf buddy, who was retired from management in a big trucking firm, and said I see a lot of truckers who don't shut down at the fueling islands. He said if its a diesel we tell them to keep it running until they are through for the day, so I have since followed his advice.

My view is a lot things can go wrong on restarting and there is wear and tear on shutting down and restarting, including it not restarting.

But basically I am a check engine light guy, if the check engine light is not on, I go and I want to enjoy my RVing and not worry about a lot of small stuff.

Bob

 

[Jim Dick]

Well that's another thing I learned today Jim. I'll see if I can do that with our coach.

Tom,

With mine all I have to do is turn the cruise on and hit the set/accel button. It will automatically run up to around 1000rpm. On others I've had to run it up manually and then set it. Can't say they all do it but I'll bet they all are capable of it.

It's like the automatic check of transmission fluid level. The Allison is capable of reading it from the console. On my CC, I would press the up and down arrows together. This would start a test sequence and, at the end if all was well, it would show OL OK flashing alternately. I have read this is much more accurate than the stick. Unfortunately I assumed this was a standard feature and never mentioned it when we ordered the Dream. I found out later it did not have that option and it would cost around $800 to have it installed.

 

[Tom]

Thanks Jim.

I thought the OL check was standard on Allison transmissions.

 

[Jim Dick]

Thanks Jim.

I thought the OL check was standard on Allison transmissions.

So did I! I would guess it's standard with Allison but not with the coach manufacturer. They can make more money by charging for the option.