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

[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.