Bad DPF Filter Symptoms and Causes

A DPF is mounted in a diesel engine’s exhaust system and is designed to reduce the amount of soot and other particulates expelled during the combustion process, thus improving overall air quality.

The DPF does this by capturing the soot and breaking it down into smaller particles before entering the atmosphere.

The DPF comprises of three main parts, a canister, a catalyst, and a ceramic substrate. 

Over time, the filter element can become clogged, resulting in a decrease in performance. 

There are other signs, too, excessive smoke coming from the exhaust, poor performance, and increased fuel consumption. 

When working well, the DPF will clean itself without any input from the driver, but for various reasons we’ll discuss below, this doesn’t always happen.

How A Functioning DPF Cleans Itself

Passive regeneration regenerates the DPF when the engine runs at a certain temperature and speed. 

Regeneration means the DPF releases all the soot that has built up during the release of normal exhaust gases.

The engine will automatically start the regeneration process when the exhaust temperature, EGR flow rate, and exhaust gas recirculation (EGR) rate are all above a certain threshold. 

It is an exact procedure and relies on the EGR flow rate being enough to ensure that the exhaust gases are being recirculated at an appropriate rate.

The EGR rate is monitored to ensure the exhaust gases are not too hot for the filter to handle but hot enough to be expelled. 

When this doesn’t happen automatically, the following can problems can result

DPF Clogging

DPF filter clogging can also be caused by prolonged idle periods and frequent short trips that do not allow the engine to reach its optimal operating temperature to regenerate.

The DPF filter needs to reach a certain temperature for the trapped particles to be burned off. If this does not happen, the soot and debris will continue accumulating, resulting in clogging.

Finally, some vehicles are more prone to DPF filter clogging than others. 

The older the car, the more likely it is to have an outdated DPF that hasn’t gone through the same research and development as newer models.  This is very clear in the Volkswagen diesel saga, where they are older diesel cars DPF when not efficient as they claimed. 

Later entrants to the DPF market, such as Toyota, have not had the same bad publicity as Volkswagen’s earlier DPFs.

Certain driving habits may also increase the risk of clogging. Driving slowly and never getting the vehicle hot enough to expel the soot often happens in urban town driving, where the vehicle never gets up to temperature or the driver always accelerates gently.

Other Causes of A Bad DPF

Bads Fuel Injectors’

Poor fuel injection spray patterns can also lead to a bad DPF. If the fuel injectors are sooted slightly, they won’t spray a consistent pattern inside the engine. This results in poor combustion, which produces even more soot.

This cycle continues until the DPF cannot clear the soot generated any longer and stops regenerating automatically.

Bad Oil

Bad oil can damage a diesel particulate filter (DPF). But what does “bad oil” mean in this context? Bad oil can refer to various things, but generally of an inferior grade.

Motor oil with a high SAPS rating can cause serious damage to a DPF (Diesel Particulate Filter). SAPS stands for Sulfated Ash, Phosphorus, and Sulfur; these three elements are found in motor oil. When motor oil with a high SAPS rating is used, it can cause a build-up of soot and ash within the DPF, leading to a decrease in performance and an increase in the need for more frequent regeneration cycles. This can cause a decrease in fuel economy, increased emissions, and an overall decrease in the life of the DPF. Using motor oil with a low SAPS rating is important to avoid any potential damage to your DPF.

Short Trips

Driving for long periods at low speeds or idling for extended periods can lead to the build-up of soot in the DPF. It is important to drive at higher speeds to increase the temperature the DPF needs to start generating.

Signs of A Bad DPF

1. Warning Lights On – The DPF light is yellow and, on most cars, looks like a box with circles inside attached to a piece of pipe.

2. Poor Fuel Economy – A clogged or damaged DPF can also cause your vehicle to use more fuel than normal. 

3. Loss of Power – A bad DPF can also cause your vehicle to experience a loss of power or performance. 

4. Excessive Smoke – A clogged or damaged DPF can cause increased levels of smoke from your vehicle’s exhaust. 

Can A Bad DPF Be Repaired?

A bad DPF can indeed be repaired. Depending on the cause of the DPF’s poor performance, a repair can take one of two routes.

A clogged DPF can be cleaned through a process known as ‘forced regeneration. 

This involves using an electronic gadget to speak to the car’s ECU and tell it that it needs to regenerate. Sometimes this works, and the regeneration will be completed, but often, it will throw up an error code saying that regeneration was impossible. 

The next option is chemical cleaning. 

Many DPF cleaning services will come to your home and attempt to clean your DPF for you. There is no guarantee that this will work; however it is far cheaper to use their services in the hope, you can repair your DPF and then buy a new one. 

In some cases, the DPF may be damaged beyond repair. If this is the case, the only solution is to replace the filter entirely.

This can be extremely expensive, and older diesel vehicles are often not financially viable, and the car may need to be scrapped instead.

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