Common misconceptions about marine diesel engines have led to many boaters acquiring bad engine idling habits. A marine diesel engine is not meant to idle before going under power, but unfortunately, it happens often. The outcome is wasted efforts on the boat owner’s part at best, and at worst, behavior that puts your diesel marine engine at risk.

All boaters need to know the specific practices that apply exclusively to marine engines and avoid believing misconceptions about letting their engines idle to warm up. Ultimately, it is bad to let your diesel engine idle, but perhaps not for the reasons you’d think.

 

Idling Wastes Fuel

One of the biggest misconceptions regarding diesel engines is that, because they burn such little fuel compared to gasoline models, it’s okay to let them idle. This belief is only partly true. Diesel engines do burn less fuel than gasoline ones, depending on the engine size.

The reason for diesel’s comparative fuel efficiency is that diesel is a richer energy source than gasoline. It takes less diesel to produce the same amount of energy that gasoline creates. Because diesel is a different, richer energy source, its combustion properties are also different. Gasoline requires far less heat to combust, producing more energy at a faster rate. Diesel, on the other hand, requires more heat to combust but burns a smaller amount once combusted.

Diesel fuel doesn’t combust completely until the engine is under load, which is how diesel engine manufacturers design them. By idling a diesel engine, you’re only combusting a small amount of fuel, meaning the boat is still using and wasting the limited fuel it is combusting. The result is a wasteful use of fuel, an unnecessary increase in air and marine pollution and engine health risks due to incorrectly warming up a diesel engine.

 

Unburned Fuel Causes Pollution

Diesel engines, in general, emit high levels of particulates and nitrous oxides. Diesel engines also release carbon dioxide (CO2) emissions. Therefore, if an idling diesel engine can’t fully combust fuel because it’s not under load, the wasted non-combusted fuel unnecessarily adds to marine pollution. Within reason, any amount of excessive idling is a pollutant problem.

The best way to reduce diesel fuel emissions is to turn the engine off rather than letting it idle for more than a short period. Exactly how long can you let a diesel engine idle? A marine diesel engine should not be left to idle any longer than 10 seconds unless it’s being prepared for final shutdown. Sticking to this practice significantly lowers the amount of pollution caused and saves fuel, money and your personal time.

Pollutant emissions are bad for the environment and the engine. The pollutants caused by unburned fuel — and the unburned fuel itself — seep past the piston rings when not combusted. Over time, this can cause the engine to smoke up as oil viscosity is reduced. Allowing an engine to idle too long and too frequently is the likely cause when you start seeing and smelling pollutants emitted by your diesel engine.

 

Idling Does Not Warm up the Engine

Understanding diesel’s combustion point is crucial to being a diesel marine engine owner. Diesel fuel does not warm up entirely, nor does it combust until the engine has reached its operating temperature of roughly 185 degrees Fahrenheit or 85 degrees Celsius. To achieve the proper operating temperature, the diesel engine needs to be under load. Therefore, any idle time spent “warming up” the engine will be futile and wasteful because it will remain cold until it’s under load.

The steps below offer better ways to let a marine diesel engine warm up properly:

1. Bring up the Operating Temperature Quickly

The first step to warming up your marine diesel engine is to bring it up to operating temperature as quickly as possible. There’s an unfortunate habit in the boating community of letting a marine engine idle for up to 30 minutes before leaving the berth. As discussed, this is a wasteful practice that does more harm than good.

Instead, put the engine in neutral and set it at a speed just above a low idle. Give the oil roughly five minutes to warm up and circulate throughout the engine.

2. Engage Light Load Transmission

After the engine has safely warmed up in neutral, the next step is to get underway using a light load. Under a light load, the engine can now safely warm up. During this step, it should never achieve more than 50% of its maximum revolutions per minute (RPM) since it’s still in the warmup phase.

Continue this method until the gauge indicates the engine has nearly reached its operating temperature.

3. Run at Normal Speed

Once the gauge indicates the engine is now at or near its operating temperature, you can put it into normal cruising speed. A safe cruising speed should be set at no more than 75% of the engine’s maximum RPM. At this point, you may need to perform some diesel engine troubleshooting if you hear any unusual sounds. Put the engine in neutral, check the propeller and clear away any debris.

To avoid the engine developing a fixed wear pattern, occasionally vary your RPM between 75% of maximum RPM and any level below. Don’t remain at a certain RPM for too long without changing up your speed.

4. Returning to the Berth

Another best practice for maintaining a long engine life is allowing your diesel engine to idle before final shutdown. Since an idling diesel engine doesn’t cause heat, allowing it to idle after being under power for an extended period helps it gradually cool down before being turned off. Consider letting it idle for up to five minutes before turning off the engine.

During this cool-down time, do not rev the engine at all, especially just before turning it off. A revved engine causes the rotors to keep spinning, and if it has been shut off, then no oil will reach these parts. This practice may be specific to the engine model, so always review your operators’ manual to learn the best engine shut-off practices for your machine.

Despite how inadvisable it is to let a diesel engine idle before being warmed up, modern marine diesel engines are well-engineered — the designers accommodate for bad engine idling practices. With longer wear resistance on engine parts and better overall durability and reliability, many marine engines still have long lifespans, even under poor engine idling habits. Regardless, boat owners should always reduce their idling time to further care for their engines while reducing fuel waste and harmful emissions.

Choose Diesel Pro Power for Detroit Diesel Marine Engine Parts

Preserve your investment, save on fuel costs, protect the environment and enjoy your vessel longer by developing better anti-idling habits. Reducing the amount of idling will keep your engine parts working better for longer.

When you do need routine engine parts replacement for your marine diesel engine, choose Diesel Pro Power. As your source for Detroit Diesel marine engine parts, Diesel Pro Power has the widest online selection of parts for smooth-running marine engines. With 24-hour worldwide shipping, Diesel Pro Power can help you get your marine engine back up and running in no time.

Browse our inventory of Detroit Diesel parts, or get help finding the right diesel engine components for your boat by contacting us today.