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Summary
  • You should warm up your diesel engine before driving if it’s cold. Engine fluids get thicker when temperatures drop, which means they’ll flow through your engine at a slower rate.
  • It’s always best to follow your manufacturer’s recommended warm-up time, so that you don’t end up preheating your engine for too long. However, it typically only takes 1 to 2 minutes for modern diesel engines to warm up when the temperature is over 50 degrees Fahrenheit.
  • Some of the benefits of warming up your diesel engine include smoother combustion, less engine stress, and decreased emission levels.

Diesel engines are known to be much more fuel-efficient than gas engines. They compress air at a much higher ratio and produce more energy per gallon of diesel fuel. Unfortunately, diesel engines don’t always perform well when the temperature drops. Cold weather can slow down the combustion process, which is why many vehicle owners recommend warming up your diesel engine before you start driving.

Note that most people don’t allow for a warm up unless the weather is really cold; they just let the glow plug “wait” light go out, fire it up, and drive away, because the engine will warm up anyway once it’s started. A diesel doesn’t typically warm up as quickly as a gasoline engine and it may take longer to get the coolant flowing through the heater core warm enough for cabin heat.

What Does Preheating Your Engine Mean?

Preheating your engine is exactly what it sounds like. Before driving away, you warm up your engine for a few minutes. This typically involves activating built-in preheating systems in your vehicle, such as block heaters and coolant heaters.

Should You Warm Up a Diesel Engine Before Driving?

If it’s cold, then yes, you should warm up your diesel engine before driving. Engine oil gets thicker when temperatures drop, which increases the viscosity of the oil (makes it thicker, like cold honey) until it warms up. Coolant and windshield washer fluid can also freeze after a particularly cold night.

How Long Should You Let a Diesel Engine Warm Up?

Your diesel engine’s warm-up time will depend on several factors, such as your vehicle’s year, make, and model. One of the biggest factors to consider is how cold it is. It typically only takes 1 to 2 minutes for modern diesel engines to warm up when the temperature is over 50 degrees Fahrenheit. If the temperature drops to below zero degrees Fahrenheit, you’ll probably need to wait a bit longer.

It’s always best to follow your manufacturer’s recommended warm-up time, so you don’t end up preheating your engine for too long.

What Are the Downsides of Idle-Warming Diesel Engines?

Warming up your diesel engine after it’s first started is a good idea when the weather turns cold. However, you’ll have to be careful of its downsides too. Here are some negative effects that might happen when you your diesel engine by letting it idle: 

Increased Risk of Excessive Idling

Warming up your engine for too long can lead to excessive idling, which causes your engine to burn more fuel than necessary. To avoid this, it’s best to follow the warm-up time recommended by your manufacturer.

Dirty Intake Manifold

Excessive idling can also cause soot to build up faster on your engine’s intake manifold, which can interfere with your engine’s performance and efficiency.

Clogged Diesel Particulate Filter

Diesel engines come with a diesel particulate filter (DPF), a filter that captures exhaust soot and reduces emission levels. This filter typically has to be emptied to clear soot buildup every now and then, but excessive idling can cause it to clog up faster.

What Are the Benefits of Preheating Diesel Engines?

When done correctly, warming up your diesel engine with a preheater before starting the engine comes with some benefits. Here are a few:

Smoother Combustion

Warming up your diesel engine before starting it allows the air and fuel mixture in the combustion chamber to heat up first. This ensures that the combustion process starts properly.

Less Engine Stress

Those few minutes you spend waiting for your engine to warm up with a preheater make a big difference when it comes to protecting your engine. By warming up your diesel car before firing it up, you’re actually putting less stress on the engine.

Decreased Emission Levels

A cold engine block with a low coolant temperature tends to produce uncombusted hydrocarbons, carbon monoxide, and other major air pollutants. In contrast, a warmed-up engine block has a smoother combustion process that produces less emissions.

A preheater (whatever kind you use) will help the engine start as if the weather was a lot warmer so that it burns cleaner from the first moment of run time.

What Happens if You Start a Diesel Engine Without Preheating?

Here are a few examples of what happens when you try to start your engine when it’s cold:

Increased Wear

While the oil pump is designed to pick up and move the oil quickly to pressurize the oil gallery, it tends to do this a bit more slowly when the oil is very thick.

Burning More Fuel

During colder months, fuel efficiency tends to drop during the first few moments of engine operation. One of the reasons for this is that engine fluids get thicker when it’s cold, and thicker fluid takes longer to reach the optimum temperature for fuel efficiency. 

Of course, allowing the engine to warm up first means the engine is running and using fuel, so running the engine to warm it up doesn’t really do much for fuel economy overall, but a warm engine means not burning as much fuel.

How To Start a Diesel Engine in the Cold

Aside from preheating your engine, there are plenty of other ways to help your vehicle warm up. Here are a few tips to help you avoid cold starting your diesel engine:

Make Sure Your Glow Plugs Are in Good Condition if Equipped

Glow plugs and manifold heaters heat up incoming air to improve fuel combustion in a diesel engine. Unless a diesel has a manifold heater (like Cummins engines), it’ll typically have one glow plug for each cylinder. Make sure your vehicle’s glow plugs are in good condition, so your fuel combustion can run smoothly.

, How Long Should You Let a Diesel Engine Warm Up?

Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.

Pro Tip: Newer diesels monitor the glow plugs and will throw a DTC when one or more glow plugs have failed.

Activate Your Vehicle’s Block Heaters

Block heaters are great if you need additional help in warming up your diesel engine. They’re electrical devices that warm the engine and motor oil when the vehicle isn’t in use. Most diesel trucks come with built-in block heaters that are easy to use. For most block heaters, you just have to plug the heater cord into an extension cord and then plug the extension cord into a socket with the appropriate voltage. But don’t drive off with the cord still connected, or you might ruin something.

Use a Diesel Fired Coolant Heater

Similar to a block heater, a coolant heater can also warm up your diesel engine. It circulates the engine coolant over a heat exchanger, which helps heat the engine, fluids, batteries, and hydraulic tank.

About The Authors
Written By Automotive and Tech Writers

The CarParts.com Research Team is composed of experienced automotive and tech writers working with (ASE)-certified automobile technicians and automotive journalists to bring up-to-date, helpful information to car owners in the US. Guided by CarParts.com's thorough editorial process, our team strives to produce guides and resources DIYers and casual car owners can trust.

Reviewed By Technical Reviewer at CarParts.com

Richard McCuistian has worked for nearly 50 years in the automotive field as a professional technician, an instructor, and a freelance automotive writer for Motor Age, ACtion magazine, Power Stroke Registry, and others. Richard is ASE certified for more than 30 years in 10 categories, including L1 Advanced Engine Performance and Light Vehicle Diesel.

Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.

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