What Is Diesel High Idle and When Should You Use It?

If you drive a diesel RV, you may be aware of diesel high idle (sometimes referred to as “fast idle”), and your rig may even have a high idle mode or switch.

But, what exactly is diesel high idle, and when should you use the high idle feature?

Is idling bad for the engine? Do you need to let a diesel engine idle on cold mornings?

Does idling a diesel engine bring it up to operating temperature? The answer may surprise you!

In this post, we aim to clarify when, where, and why you should let a diesel engine idle, for how long, and at what idle speed.

What Is Diesel High Idle?

As the name implies, high idle or “fast idle” is a mode that allows you to idle your RV or truck engine at higher RPMs than normal idle speed.

Sometimes a rig will have a switch that must be flipped on in order to engage high idle mode. Many RVs have a high idle feature that can be engaged via the cruise control system.

Either way, the high idle feature exists for use in particular conditions under which the rig needs to idle.

What Does Diesel High Idle Do?

High idle does several things. Let’s take a brief look at the purpose and benefits of using high idle on a diesel engine.

Prevents Wet Stacking

High idle prevents a condition known as wet stacking. This means that the temperature of the cylinder isn’t high enough to burn all the fuel.

When wet stacking occurs, the unburned fuel can get into the oil pan. This dilutes the oil which can lead to significant engine wear.

Keeps Oil Pressure Up

Diesel high idle keeps the oil pressure up to ensure that it’s properly circulating and lubricating the engine.

Reduces Soot

High idle reduces soot in the exhaust which results from incomplete combustion at low engine revs.

This is important because when diesel engines are operating at low RPMs, more soot is generated. That can fill up the diesel particulate filter (DPF) which has been installed on diesel engines since 2008. This can require a regen or running at full operating temperature for some time/distance.

Improves Air Compression Speed

Fast idle improves the speed at which the diesel engine air compressor can re-supply the air system to inflate airbags. This is part of the process of getting the RV ready to hit the road after being parked.

Helps Circulate Oil & Coolant

Before shutting the engine down after driving at highway speeds and/or towing a heavy load, running in high idle mode keeps oil and coolant circulating. This helps reduce the temperature of engine components before shutting down, preventing issues like oil coking or thermal shock.

So when stopping immediately after driving under high engine loads or speeds (such as when pulling into a rest area after driving on the highway), don’t immediately shut your diesel engine down. That’s when engine components are at their hottest, and allowing the engine to run at fast-idle for at least a minute or two before shutting down keeps coolant and oil circulating.

Avoiding the sudden shut-down of a diesel engine that was just worked hard is particularly important for the turbocharger, which rotates at tremendous RPMs when driving at speed. Allowing components to slow down and cool down before shutting down is important for the long-term health of your diesel engine.

Helpful to Trucks with PTO

For work trucks with a PTO (power take-off) used to operate other equipment or a generator, running at high idle ensures that the engine has the power it needs for tasks other than driving.

Where Is My High Idle Switch?

If your diesel rig doesn’t have a switch that says “High Idle” on it, then your high idle feature may be engaged via the cruise control switch. In fact, this is the case with most RVs, including our own.

In order to engage high idle mode, you’ll start the engine, and watch to be sure that the oil pressure warning light goes out, and the oil pressure gauge shows oil pressure (which should always be the very first things you observe at start-up). Then turn the cruise control on, and then push the “Set” button. The engine will then rev up, typically to about 1,000 RPM.

Some vehicles may differ, but however the fast idle is engaged, the point is to bring the idle speed up to about 1,000 RPMs.

Why Doesn’t My Diesel Pickup Have a High Idle Mode?

On some newer makes/models of diesel engines in pickup trucks, the manual control of high idle mode may not be available.

In these cases, the engine computer handles the high idle feature, increasing the idle RPMs when the vehicle is in Park and temps are low, etc.

However, it’s usually possible to add a manual control on these vehicles, enabling you to force high/fast idle when needed.

To do this, a high idle kit like one of these would need to be installed. You’d need to choose the right model based on your truck’s make/model.

Should I Idle My Diesel Engine on Cold Mornings?

Well, first things first: Always check your owner’s/operating manual to confirm proper procedures to ensure a long life for your diesel engine.

Having said that, diesel engines don’t need to idle for very long before driving, even on cold mornings. Idling long enough to get your rig aired up is usually plenty. Then just be sure to take it easy with your right foot as you get your trip started (avoid full-throttle acceleration for example). This allows your engine to warm up gradually as you begin your driving day.

Diesel engines do need to warm up. But, most manufacturers recommend idling for only about 3 minutes prior to running the engine under load. Again, check your operator’s manual for the exact specs for your particular engine.

So when you see people idling their diesel RVs for 10… 20… 30 minutes(!) as part of getting their rig ready to depart the campground, you, and we, know that they are simply wasting fuel, and causing noise and air pollution for their neighbors.

If it’s really cold, using a block heater overnight (when available) is a far better option than trying to warm the engine by idling it.

And while we’re on the topic, can we puh-LEASE, for once and for all, bring an end to the practice of leaving engines (diesel or gasoline) idling while checking in at the RV park office? It’s just flat-out obnoxious and unnecessary. See below for details about engine shut-down/cool-down.

Does Idling a Diesel Engine Bring It Up to Operating Temperature?

No. Contrary to popular belief, no amount of idling an engine (even on high idle) will bring it to the point where it reaches full operating temperature.

Idling a diesel engine at high idle will help it start to warm up from dead cold (and most importantly, get the oil up out of the pan and circulating throughout the engine). But bringing a diesel engine up to full operating temperature requires running it at higher RPMs and/or higher load.

Can Diesel High Idling Harm the Engine?

If used too much, yes, it can, over time. (So can extended periods of low idle.) This is why manufacturers generally consider conditions that require high idle times as “severe duty.” It’s also why they recommend a more aggressive maintenance schedule for rigs that run under these conditions for extended periods, like trucks idling all night in cold weather.

During extended periods of idle, a diesel engine incurs more wear than it would under normal operation. Idling, even on high idle, can cause twice the amount of wear on internal components than simply operating the rig under a normal load.

Not only does this raise maintenance costs, but it can also shorten the life of your diesel engine.

Does Idling the Engine Use a Lot of Diesel Fuel?

On average, a diesel engine uses about a gallon of fuel per hour at idle.

As we write this post the average price of a gallon of diesel fuel in the United States costs $4.282.

This means it would cost you $4.282 per hour to idle your diesel rig. That’s a little more than 7 cents per minute, plus the wear & tear on your engine. Long periods of idling should not be needed for an RVer, and should be avoided.

Does a Diesel Engine Need to Idle Before Shutting Down?

Yes, especially when it’s just been run hard (highway speeds, climbing a mountain, towing a heavy load, etc). Generally speaking, a diesel engine should cool down at fast idle for a few minutes after stopping before shutting down.

This ensures that the engine components (especially the turbocharger) continue to be cooled by oil and coolant while slowing down and beginning to cool off.

While most people focus on the need to warm a diesel engine up before driving, it’s actually allowing it to cool down after driving (especially driving hard) that’s more important.

With this in mind… if you drive slowly through a Wal-mart lot or campground (at near idle) and maneuver slowly into your site, you’ve already allowed your engine to cool while driving gently for at least several minutes. In this case, little or no additional cool-down time should be needed once parked.

When Do We Use High Idle On Our Motorhome?

There are two primary times we engage fast idle — at start-up and at shut down.

First, as a means to air up our rig more quickly. Diesel RVs that have been sitting, especially overnight, need to have the air bags re-inflated before driving. That’s of course the case after dumping the air and leveling with the jacks.

High idle mode runs the on-board air compressor at higher speed, airing up the RV faster, circulating the oil, and getting the engine to start warming up.

So our start-up process is:

1. Turn on the ignition and wait for the “wait” light to go out before starting the engine.
2. Start the engine and immediately look for the oil light to go out, and the oil pressure gauge to come alive. If either of those things doesn’t happen, shut the engine down!
3. Allow the engine to run on low idle for about 30 seconds, giving the oil and coolant time to begin circulating.
4. Activate high idle and hit “store” on the jacks.
5. Once the jacks are stored, and we hear the air compressor “kick out” (that’s the loud whoooooosh sound that tells you the air pressure is all the way up), we are ready for travel. No further idling or warm-up is needed. Just take it easy on the throttle until engine and transmission temperatures are at or near their normal operating range.

At shutdown, we may or may not need fast idle. As mentioned above, coming into a rest area from the highway requires a short cool-down period. The engine was just running at high speed, so needs to be allowed to cool down before shutting it down.

In a campground, or on local roads, parking lots, etc. driving at slow speeds (at or near idle) for several minutes prior to stopping reduces the need for much or any cool-down period. That cool-down is most important when the engine was just run fast and or hard immediately prior to stopping.

We see many fellow RVers idle their engines excessively (like stopping at a rest area for lunch and leaving their engine running the entire time). With the cost of fuel these days, a little education about when idling is, and isn’t, needed should go a long way to saving fuel and reducing pollution and noise.

Do You Use Your Diesel Engine’s High Idle Feature?

We hope this has been helpful in clarifying what the diesel high idle feature is for, how it’s engaged, and when to use it.

Do you use your diesel RV’s high idle feature?

Drop us a comment and let us know about your experience!

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