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

[Joe Hogan]

In my opinion since there are so many variations in equipment and application, it is best to refer to the specific Manuals supplied with your RV.
If that topic is not addressed call engine mfg support.

Granted this is not the fun discussion path but will give you a defensible documented position should things go wrong.

 

[Racklefratz]

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

Agree. Our 2012 Tiffin has the Cummins ISL 450hp in it, and extended idling definitely asks for issues.

While at the Tiffin factory service center, we had modifications to the tag axle suspension air pressure work done, which required long periods of engine idling to keep the system aired up.

When we left the facility, we were greeted with immediate DPF warnings that only cleared after we have been at road speeds for a while. The extended idling resulted in clear messages from the system that it didn't like it.

 

[Racklefratz]

If facts are needed manufacturers websites, or authoritative sites are the only place I expect anything other than social interaction!!!

Of course. Forums are fine if the question has to do with others' opinions or preferences. For factual info on operation and/or maintenance, I rely on the OEM or their designated representatives.

 

[AbdRahim]

Well, if someone would provide the funding, I would do the research to answer the question scientifically. I suppose we could use DP’s in the middle of the price and engine size range. Let’s see, I would need 30-40 of them and funds for the fuel. Any takers? You all can pool your funds.

 

[Joe Hogan]

Thought I’d ask Grok 2 the question, spoiler alert
, nothing new here

“Extended idling can indeed be harmful to diesel engines for several reasons:

1. **Fuel Dilution**: Idling for long periods can lead to fuel dilution of the engine oil. Unburned fuel can seep into the oil, reducing its lubricating properties and potentially causing wear and tear on engine parts.

2. **Carbon Buildup**: Diesel engines can accumulate carbon deposits in the combustion chamber, on injectors, and in the exhaust system when idling for too long. This can lead to reduced performance and efficiency over time.

3. **Increased Maintenance**: The wear and carbon buildup can necessitate more frequent oil changes and maintenance to keep the engine in good condition.

4. **Regeneration Issues**: Modern diesel engines with Diesel Particulate Filters (DPFs) require periodic regeneration to burn off soot. Idling doesn't provide the conditions for effective regeneration, which can lead to DPF-related problems.

5. **Waste of Fuel**: Idling consumes fuel without providing any significant benefit, leading to unnecessary operational costs.

6. **Environmental Impact**: Extended idling contributes to unnecessary emissions, negatively impacting the environment.

However, there are some scenarios where short idling might be necessary, like warming up the engine in very cold weather to ensure all fluids are at optimal operating temperatures or for systems like the DPF to initiate regeneration. But generally, for most modern diesel engines:

- **Warm-up**: A brief warm-up period is often sufficient; extended idling isn't needed.
- **Heating**: If the vehicle is used for heating or cooling (like in construction sites), alternatives like block heaters or auxiliary power units are more efficient and less harmful to the engine.

Best practice is to limit idling time as much as possible, ideally not exceeding more than 5 minutes unless absolutely necessary. If you're in a situation where idling can't be avoided, consider using technology or methods to reduce its impact on the engine.”

 

[team bradfield]

So you shut it off in heavy traffic/construction or accident?

 

[Texas Clodhopper]

So you shut it off in heavy traffic/construction or accident?

If I think I will be stopped for 10-15 minutes, yes, I turn the motor off and usually get up and walk around and get a snack or drink.

Since the Cummins manual states "Don't idle excessively", and they don't define excessively I err on the side of caution.

 

[Neal]

Since the DEF head crisis I only shut off when I need to so I leave it running while fueling now. I typically shut down when checking into a CG for noise and courtesy to the campers. I don't like surprises after engine start.

 

[MemoriesByTheMile]

So you shut it off in heavy traffic/construction or accident?

If we aren’t moving for a few minutes, yes, we will always shutdown the drive engine. Will then use the gen and house A/C as needed.

 

[MemoriesByTheMile]

Since the DEF head crisis I only shut off when I need to so I leave it running while fueling now. I typically shut down when checking into a CG for noise and courtesy to the campers. I don't like surprises after engine start.

I never thought about this before, would DEF failures only show up immediately after a startup? Or is there a higher incidence rate triggered by startup?

 

[Crazydog]

What exactly is "excessive" or "extended" idling time. Is it 5 minutes or 15 minutes or 30 minutes?
Even owners manual don't give a figure.

 

[AbdRahim]

Ok, so why do trucks idle all night, and some diesel train engines are almost never turned off in cold weather?

 

[Jim]

From ChatGPT, for what its's worth.

Idling your Cummins 605 HP diesel engine for 30 minutes won't cause immediate damage, but it's not ideal for long-term engine health. Here’s why:

Potential Issues with Long Idle Times​

1. Carbon Build-Up – Diesels need heat to burn fuel efficiently. Extended idling can lead to carbon and soot buildup in the EGR system, DPF (if equipped), and turbo.
2. Fuel Dilution – Unburned fuel can mix with the oil over time, reducing lubrication and shortening oil life.
3. Glazing of Cylinder Walls – Low cylinder pressure and temperature during idle can cause improper ring sealing, leading to potential wear over time.
4. Wasted Fuel & Cost – A big Cummins like yours probably burns 0.8–1.5 gallons per hour at idle, which adds up.
5. Cooling System Inefficiency – The engine isn’t working hard enough to keep coolant and oil at optimal temperatures, which could affect performance down the road.

When is Idling Okay?​

• Cold Start Warm-Up – A few minutes of idling (5-10 max) before driving is fine, especially in cold weather.
• Cool-Down After Hard Driving – If you've been pulling a heavy load, a 5-minute idle helps the turbo cool down and prevents coking of the oil inside it.
• If Required for Specific Accessories – If you're using an air compressor or generator that depends on engine RPM, idling might be necessary.

Best Practices Instead of Long Idling​

• If you're parked for a while, use a high-idle feature (if available) to raise RPM to ~1000+ to improve combustion.
• If possible, drive instead of idle, as diesels prefer load over extended low-RPM operation.
• Consider an auxiliary power unit (APU) or generator instead of idling for long periods.

Bottom Line​

A one-time 30-minute idle won’t kill your engine, but making a habit of it isn’t great for longevity. If you must idle, bump the RPMs up slightly to reduce carbon buildup and improve efficiency.

 

[Texas Clodhopper]

Ok, so why do trucks idle all night, and some diesel train engines are almost never turned off in cold weather?

Many of those trucks are pre any kind of emissions controls.