How to Effectively Warm up Diesel Engines

One of the questions that most motorists ask themselves concerns the methods that it is good to observe to turn on and warm up an engine in the best way, especially in case of a cold start of a diesel engine.

Before giving an answer (or at least some indications) it is appropriate to briefly describe what happens inside of an engine itself and only then jump to conclusions and hints.

The Effect of Heat on the Engine

To begin with, immediately after the engine is started the metal walls begin to absorb heat.

The heat exchange is vigorous as the temperature difference between the gases and the surfaces they touch is high. Heat travels through the metal and diffuses into the whole structure: from the internal components to the exhaust system.

It is obvious that the parts in direct contact with the gases heat up more quickly. If to warm the engine’s head, a larger amount of heat is required, for the drain valves heating is faster. As a result, the latter dilate faster. For this reason, the motor warm-up phase is critical.

Once fully operational, things stabilize, following the gradual passage from ambient temperature to full operating temperature.

What Happens to the Lubrication of the Engine?

It is very important to consider what happens, after a cold start, at the level of lubrication. At low temperatures, the oil is more viscous, and this means that it flows less efficiently within the engine components. The greater resistance to sliding determines an increase in the pressure in the circuit, which is equipped with a limiting valve to prevent the pressure itself from reaching too high values. 

The fact that the oil flows less easily means that when cold it takes longer, after starting, to reach the various parts to be lubricated and to establish the correct pressure in all points of the circuit.

It is for this reason that some engines are equipped with hydraulic devices for adjusting the valve clearance; in these cases, it is sometimes possible to hear a slight (and characteristic) noise, after starting at low temperature, which however disappears after a few seconds.

In summary, when the engine is still cold the oil within the components circulates slowly and in smaller quantities.

Read it: 3 Worst Ford Diesel Engines

The Effect of New Diesel Technologies

Nonetheless today, thanks to new oil formulas with low cold viscosity, things have certainly improved to a noticeable extent. It should be remembered that at low temperatures engine wear tends to be higher, and in this respect, a rapid heating is advantageous.

In the past, when cold starts were difficult, the starter (a tool which enriched the air-fuel mixture to an impressive extent) was often used, and the engine tended to run erratically until it reached a certain temperature!

Today, thanks to the injection systems, the fuel is finely pulverized and tends to vaporize with relative ease already shortly after starting, for this reason, condensation phenomena on metal walls are reduced to a minimum even when cold. In addition, thanks to electronics, the dosage of the mixture are optimal in any operating condition.

Thus, today the engine heats up faster than it once did and the thermal control of the engine as a whole is also greatly improved. Regardless of the environmental and use conditions, there are no longer any significant deviations from the operating temperature, once it has been reached.

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How Long Should You Warm up a Diesel Engine?

Now, after this (necessary) technical introduction, it can be concluded that warming up for minutes and minutes does not facilitate reaching the ideal operating temperature. On the contrary, warming up phases are counterproductive and even prohibited in some countries, such as Northern European ones.

Heating should be rapid precisely to avoid all the inhomogeneities that are created when we wait by standing still.

How to Effectively Warm up a Diesel Engine?

Therefore, the best thing to do is to start shortly after (about twenty seconds or so) the cold start and then proceed going very slowly with the opening of the gas and changing the gears in order to keep the engine at a low regime. Gradually, as the engine warms up, you can then press the pedal in order to reach higher and higher RPMs.

Again, remember that differently from the past, when diesel engine cold starting was difficult, now with the transition from the carburetor to the injectors, the ignition has improved. The “annoyances” caused by the change of seasons and temperatures with consequent re-carburation of the engine are a thing of the past!

What if You Can’t Turn Your Vehicle On When it is Cold?

Having understood that the warming up of diesel engines is no longer a big deal, you can still experience problems if you are not able to turn the engine on because of the cold temperatures…

I will outline here which are the biggest causes – and related solutions – of cold diesel car ignition problems:


Preheating is especially important for diesel engines. Because of how a diesel engine works, starting at low temperatures without heating is simply impossible. If your car doesn’t turn on due to this issue, the preheating plugs probably need to be replaced. Those devices are a resistor that heats the air in the system to allow starting in the vehicle. Especially in cold weather, diesel-powered vehicles won’t start without this system.

Battery status

In order to sufficiently compress the air at ignition, it is necessary to have a sufficiently powerful battery available. As the energy capacity of the battery depends on the ambient temperature, a low battery is one of the most common problems when the (diesel) car doesn’t seem to want to ignite at all.

Please consider that the following normally apply regarding battery capacity vs ambient temperature:

  • at +20° С is 100%;
  • at -20° С it will be halved (50%);
  • at -30° С it will be reduced by up to 20%.

Fortunately, it is also one of the easiest causes to investigate and resolve. As for successfully performing an emergency start in case of a faulty battery, it is possible to get by even without the mechanic: you only need jump leads and the help of another motorist, willing to “share” his charged battery.

Note: as a hint, before deciding to replace the battery, remember to try to tighten the terminals: if they are loose they may not correctly transmit the electricity needed for starting.

You can also read more about preventing batteries from dying out at this link!

Properties of fuel

The evaporation temperature and viscosity depend on the composition of the fuel, which is why winter fuels must be used during the cold season. Diesel fuel transforms into a gel at temperatures ranging from -6 to -18° C. In this case, the engine does not start because the diesel is frozen.

If you are in an area where temperatures are so cold, you may consider refueling at a service station that uses additives that can lower the freezing point of diesel fuel, or buy a special additive.

How to Correctly Turn On a Diesel Car?

Now that you know everything about how to warm up your diesel car and understand what could go wrong in the ignition phase, I want to give you a piece of advice on how to correctly turn on a diesel vehicle, irrespective of the hot or cold season.

  • First of all, turn the key to the start position without starting the engine. The “wait” light will light up on the dashboard (check the user manual for the corresponding light). Do not start the engine before the light has gone out.
  • Wait for the preheating plugs to warm up before starting the vehicle. The preheating plugs can take up to 15 seconds to warm up – even longer in cold weather. The turning off of the “wait” light is used to signal when the glow plugs are hot enough.
  • Start the engine, without insisting for more than 15 seconds. If it does not start, turn it off by turning the key to the stop position.
  • Attempt to start the vehicle again by adequately heating the preheating plugs. Turn the key back to the start position and wait for the “wait” light to go out. If the vehicle has been left in a very cold place, try heating the preheating plugs more than once by turning the key to the start position, waiting for the light to go out, then turning the key to the off position and repeating the process.
  • Turn the key to start the engine again and insist on not more than 20 seconds. Trying to start the engine before the glow plugs have warmed up properly does not cause any damage to the engine, but it is quite difficult.

Also read: Letting a Car Sit for a Month? 6 Things that Can Happen

Do You Need to Cool Down Your Diesel Engine Before You Shut off the Engine?

As a conclusion, a final thought on how and if you should cool down your diesel engine. Do you need to do it? The answer is it depends.

Letting your vehicle sit idle may be helpful if you have been using it hard (think about heavy loads and mountain roads) and you want to let it cool down or want to keep your batteries charged or the air conditioning going for the passengers in the vehicle.

On the other hand, if you have already driven through a town for some time (i.e. 15 minutes) it is not necessary to do this as the slow use of the car would have already cooled down the vehicle (even after heavy use). Finally, and regarding the fact of leaving the car idle (for whatever reason, including cooling down), remember that in some countries you could get into trouble with local police if you leave your vehicle unattended and turned on!