Can You Add DEF While the Engine Is Running? Equipment DEF Systems Explained

Most diesel engines currently have a DEF system that is aimed to reduce harmful emissions. DEF stands for Diesel Exhaust Fluid. This is a special fluid that is gradually added to the exhaust of a modern diesel engine to make it less toxic. Without a DEF system, any diesel engine is good for the environment because it produces quite a lot of particles and harmful elements when burning diesel fuel.

DEF is not only presented in cars and trucks. All modern heavy equipment also has DEF tanks and you should add DEF as soon as the dashboard tells you to do so. But equipment operators have so many questions on how and when they should add DEF that we decided to write the article about this.

So, today, we'll tell you if you can add DEF to heavy equipment when the engine is still running and why this is not the best idea. Also, we'll tell you how exactly you should add DEF to avoid problems.

Adding DEF - why is that so important?

The first rule of all equipment operators and truck drivers, if they want to preserve the environment, is to add DEF as soon as the vehicle says the liquid level is too low. Usually, it's signaled by a special light on the dash or just with a message on the screen.

It doesn't mean that the engine will stop working when there is no more DEF in the tank. In most cases, it will keep on working but signaling you all the time that you are killing nature. Not the most pleasant thing, we should say.

DEF is sent to the exhaust system when the engine works and transforms nitrous oxide into water and oxygen. Yes, some part of the harmful gas is still emitted because no exhaust system is capable of completely neutralizing all harmful elements. But still, DEF makes the exhaust of your equipment much more nature friendly.

If you don't add DEF when needed, the system will not be able to work as it should. It will light up the DEF level light on the dash and will probably signal in some other ways depending on the type of software you have in your equipment.

In trucks and even passenger cars with diesel engines, the situation is more or less the same. It means you will need to add DEF or AdBlue to keep driving your vehicle safely for nature.

Can you add DEF to your equipment when the engine is on?

Actually, yes, you can. In most cases, it's not specified in the manual for your equipment or truck that you should turn off the engine before adding DEF or even opening your DEF tank. It means that manufacturers don't care about this.

But we still recommend you look through the manual and make sure the manufacturer gives no specific instructions.

Actually, DEF is not pressurized or hot, so it's quite safe to open up the tank even when the engine is running. Also, adding DEF is pretty straightforward, so you can just pour it into the tank and that's all. So, we see no reason why you should turn off the engine before adding DEF.

But still, here are some considerations:

1. If there is no DEF right now in the tank, better turn off the engine and then add new DEF to the tank. This will obviously help you avoid any shocks to the exhaust system and DEF injection system.
2. If the DEF tank is located close to some other parts that can be hot or vibrating, better turn off the engine. This will save you from potential danger or just from spilling DEF all over the place.
3. If the DEF tank is located in a very inconvenient place for adding the fluid, better turn off the engine. For example, if you need to climb up the tractor's wheel or something, excessive vibrations will not be good and safe. Also, you may accidentally put your hand in something hot. So better give it some time to cool down and then add DEF.
4. If the manufacturer says in the manual that you should turn off the engine before adding DEF, make sure you do so.

In all other cases, you may open the tank and just add DEF without paying attention to the engine. It may be on or off, it doesn't matter much.

It may seem like a weird question. People who have never dealt with a diesel engine will say it's easier to turn it off and just add some fluid. But diesel-powered equipment owners know that turbo timers can keep them on for 10-20 minutes until the turbocharger is cooled down. So, sometimes it's all about saving your time.

How to add DEF to your diesel engine?

Unfortunately, we've seen a lot of heavy breakdowns in equipment just because people didn't know how and when to add DEF.

The process of adding DEF is pretty straightforward:

• once you understand that DEF should be added to the tank, find DEF of good quality;
• when you have the fluid, locate the DEF tank in your equipment or truck;
• open the lid on the tank and see if you need some funnel for adding the fluid;
• use a funnel and slowly add the needed amount of DEF into the tank;
• take the funnel off and make sure you close the lid tightly;
• make sure the DEF light is off on the dashboard;
• if the light is still on, try turning the engine off and on again.

If the light is on after the procedure, it may also mean that some of the sensors or DEF injectors are broken. Further diagnosis is needed.

Unfortunately, a low level of DEF or problems with injection may eventually lead to worse problems with your engine. First of all, in vehicles and equipment with a DEF system, catalytic converters are made to get the exhaust without a huge amount of NOx. Otherwise, some issues may start ruining the converter or DPF and this will eventually lead to an expensive repair.

Is any DEF OK for your equipment?

Basically, DEF is two-thirds of deionized water and one-third of urea. This solution is pretty hard to make in any different way. But some companies that offer too cheap products may use the wrong water or provide the wrong ratio of water and urea. It will mean that the solution is less effective but it still won't ever affect the work of your engine.

So, it doesn't matter which brand you choose. The only important thing is to check whether DEF you buy for your equipment has a proper certificate and is made in accordance with requirements.

Bad DEF may affect DEF injection systems, exhaust pipes, catalytic converters, DPFs, and other equipment. But it doesn't come in contact with the engine itself, so the engine performance may only be affected indirectly.

What to do if I don't have DEF to add but need the equipment to work?

Well, you better have some spare DEF to add to your equipment when needed. But if you don't have it and still need to work, just make sure you add the fluid as soon as you can. Keep working and keep an eye on any other lights or error messages that can appear on the dashboard of your equipment. It's not always going to happen, but still.

If the diesel engine will go into limp mode, it may be just impossible to do all the work you need to do. So, we would send someone to get some DEF, order it online with delivery, stop working for an hour or two and get some DEF, etc.

The more technological your engine is, the worse it can react when DEF is over. In some cases, it can even stall and not start again until the problem is solved. It all depends on how the ECU is programmed to react in these situations.

But you should know that modern diesel engines are very limited in terms of ecology rules. They may sacrifice performance, power, and reliability for the sake of ecological standards and improvements. So, the 2023 diesel engine may well die when DEF is over while the 2005-made engine will still work as if nothing happened.

Final words

We know that buying DEF is sometimes out of your budget and you would like to save some money. But DEF is obviously not the fluid that will help you economize some funds. If your equipment requires DEF to work well, you better have some spare fluid. Also, you should avoid low-quality fluid, otherwise, problems will be there for you.

But in terms of turning the engine off to pour some DEF into the tank, we can't give you any certain advice. Just look in the manual and if you don't find there anything, you can easily add some DEF when the engine is on. Be careful and don't hurt yourself because of a hot engine or other parts.