Question Is extended idle really bad for a Cummins diesel? Fact or myth?

[Rich W.]

Also found this little tid-bit and my take aways are that “extended” and “excessive” are never defined, and that the 3-5 minute turbo-cool down period isn’t necessary if you’ve been driving at low load for a while, although this time period isn’t quantified either. In the absence of precise system temp monitoring I’ll continue to do it anyway.

 

[Rich W.]

I said I wasn’t going to do this but there is some good info here. These are from various diesel forums and the third bullet is from a dodge pick-up forum, but there is some good information to ponder (and some minor contradictions) and it might answer some questions.

• On a modern diesel, extended idling with EGT's less than about 425 degrees is bad for the engine. The stoich rich programming will cause wet stacking in the cylinders with accelerated ring\cylinder wear.

• Base engine idling for extended periods of time does lots of damage to an engine. What happens is at base idle the fuel is not completely atomized, therefore it leave fuel residue on the cylinder walls where oil should be and in turn mixes with and dilutes the engine oil. This is called "wet stacking" and it will prematurely wipe out an engine. There are other concerns also with extended base engine idling, hot/cold spots in the block from inadequate coolant flow, etc. These have been concern for decades, however with the advent of all the emission tiers the problem has become much more prevalent due to major increases of soot loading from egr and low cylinder temps. If your going to sit and idle for anything more than a 2-3 minutes you should engage the high idle. It will help atomize most of the fuel, keep temps more uniform and overall reduce the affects of extended idling.

• The purpose for high idle is to avoid a condition called wet stacking. An engine at idle does not create high enough cylinder temps to atomize the entire amount of fuel being injected into each cylinder causing the unburned fuel to mix with, and wash down the oil from the cylinder wall, thus dilluting the engine oil with diesel fuel and causing excessive engine wear. All class 8 and 6 engines have had the ability to raise the idle through the use of the cruise control since their conversion to electronic engine controls, and most manufacurers recomend a minimum of 1,000 rpm for extended or prolonged engine idle periods. On my 06 automatic truck, when the temps are low enough it will idle up to 1000 rpm after 3 minutes at base idle, or I can engage the high idle by pressing the cruise button on then pressing the resume/set side, it will instantly jump to 1,100 rpm, if I continue to hold the resume/set button the engine rpm will continue to rise to 1,500 rpm and stay there until I disengage the cruise with the on/off button, or step on the brake.

• The computers control everything in the engines now. The injectors are no longer mechanical or pump timed, they are piezo-electric and the computer tells it how much and how long to emit fuel. Stacking is no longer much of a problem with the new systems and they (the manufacturers) recommend no warmup or no idle down to shutoff. Diesel engines at low idle tend to cool below desirable operating temps so they program in a high idle at 1200 rpm and a pto idle at 1500 rpm. The high idle will keep the temperature high enough to circulate oil as needed.

• Turn on the cruise control, hit 'set' for high idle. 'resume' for pto idle.

• Diesel engines wear the most when starting them up. The seconds after startup when there is no oil pressure can starve bearings and cause premature wear. That is precisely why leaving the engine running when stopped for short periods is recommended and using high idle.

 

[sheridany]

That's right @ARD. Put @Neal in his place.

 

[Kevin D Pem]

This should be a good discussion, using reputable sources to understand what is correct.
We only idle after heavy loads for 3 minutes per the instructions in our Freightliner operator manual, do not idle when fueling or at camp office, etc unless it is still in that 3-5 minute cooldown after heavy load.

But I don’t know if that instruction in the Freightliner manual is due to concern of engine damage due to lower lubrication, concern over cooler cylinder temps leading to incomplete combustion which yields higher emissions, concern of “fuel wash” shortening the life of the engine oil, many state laws banning idling more than 5 min, or ???

I have found it tough to find data backed info on this. Very easy to find fear-backed anecdotal “evidence”.

More than one turbo has been lost in a rest area on the 260 east of Payson AZ that resides at the crest of the rim because the owner didn't take heed of that cool down warning!!!

 

[Joe doggs]

More than one turbo has been lost in a rest area on the 260 east of Payson AZ that resides at the crest of the rim because the owner didn't take heed of that cool down warning!!!

Lots of good points,I’ve been told by numerous Cummins/spartan sources that extended idling should be avoided….one other thing Neal,at least with my coaches,the regen is halted below a certain speed,which I have always noticed to be about 15 mph…so,if there is buildup while idling a regen will not start……..anytime I idle for more than a red light,I definitely bump up the fast idle,as supposedly that minimizes the fuel dilution of the motor oil…..one final thought…when I had the 650 h.p. Cummins,with all the bad valve problems,not idling was supposedly crucial,as well as running it as hard as possible during a regen for maximum heat,may be less of an issue with the newer 600’s,now 605 h.p. I.s.x…..happy travels jb

 

[Fish]

The owners manuals of both my 2018 Cummins dually pickup with 6.7 cummins and my 2007 Monaco Camelot with the 8.8L cummins say to avoid extended idle. If I need to idle my Camelot I bump it up to 1100-1200 rpm with the cruise control.
I do leave the Camelot running while fueling. I figure that coasting into my campsite qualifies for a good enough turbo cool down.

My new neighbor here at the resort has a 2500 ram with the cummins 6.7 and he idles the hell out of it. He lets it run for 5-10 minutes before he drives away. I don't know why and I'm not going to get into his business. I haven't heard so much diesel idle since I was working at the refinery, everyone at the rv park had diesels, it was like 10 degrees outside and everyone was warming them up some to leave for work.

I also didn't get into his business about his floppy awning being out flapping in the stiff winds we've had here lately, yesterday I noticed his awning wadded up and at the road for the trash guys and only a metal frame remaining.

 

[Underboss]

At Gaffney, I asked about this. They said no extended idling. If you must idle, bring the rpms up. I cannot remember but I believe it has to do with DEF

 

[Kevin D Pem]

• Nov 14, 2023

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"I said I wasn’t going to do this but there is some good info here. These are from various diesel forums and the third bullet is from a dodge pick-up forum, but there is some good information to ponder (and some minor contradictions) and it might answer some questions." Rich

So I wonder the sanity of searching forums for solid facts! I know I don't!!! If facts are needed manufacturers websites, or authoritative sites are the only place I expect anything other than social interaction!!!

 

[Joe Hogan]

Even asking Company reps will get generic answers unless you are very specific as to make/model/application.
Incomplete combustion would be a Big issue allowing cylinder wash down and oil dilution.

According to the Spartan chassis rep at the seminar in Tampa, time limits on lubricates are very important. The additives are subject to oxidation and water contamination from air humidity which degrades the additive package.
Cold starts and excess idling are hard on additives. He explained that the Cummins engines are designed for a million miles. The expectation is high milage I.e. 100,000 a year.
So most RVs will follow the time limit on maintenance due to the low usage rates. Sitting in storage or at a campsite subjects the lubricants to oxidation and contaminants from moisture or corrosion.

Consequently we change fluids that require a change every annual visit for maintenance regardless of mileage. Or at the stated mileage if we exceed the recommended limit during the year.

 

[ARD]

Has anyone found a definitive answer/definition to is considered excessive idling? How long is considered excessive? 5 mins, 10 mins, 30 mins?