The diesel particulate filters (DPF) fitted to all modern diesels are something of a Cinderella feature, in that everybody knows they are there, but nobody takes much notice of them.

As a rule, they work well, except for the occasional regeneration period in certain models, where the vehicles’ computer initiates a sequence of actions that cleans them off.

Yet, as vehicles to which they have been fitted get older, they are increasingly becoming a source of concern, as the number of blockages and malfunctions start to mount up.

What does the DPF do?

As the name suggests, the DPF is there to remove particles of soot from the exhaust – particles which are inherent to the diesel engine, due to it being throttled by the amount of fuel being injected.

These particles can contain unhealthy carbon structures that are best not inhaled, so legislation decrees that they should be scrubbed from the exhaust rather than pushed into the atmosphere.

The filter material used in the majority of DPF units fitted to all types of vehicle, cars and tractors included, is a silica carbide ceramic. This can remove up to 99% of the particles, if it is working properly.

When all is working as it should, the soot that is filtered from the exhaust is burnt off and expelled as carbon dioxide during periods of high exhaust temperatures, typically, when the engine is under load and working hard.

However, there are circumstances under which this process fails and the filter can block, causing a notable drop in engine performance.

Work a diesel hard

The usual cause is underuse of the engine, a combination of low revs and light loads being ideal conditions for the soot to accumulate without the filter being able to burn it off.

This is a typical scenario for tractors engaged in feeding duties over the winter.

The problem is well-known and there are various ways in which it can be overcome, ranging from the manufacturers own built in solutions to aftermarket fuel additives which can be used as a preventative as well as curative measure.

Diagnostic screen
Massey Ferguson diagnostics will display the current state and settings of the tractor’s exhaust system

Tractor makers will tend to fit a heating element between the engine and the DPF unit which triggered by a switch that senses the backpressure in the manifold. This will raise the temperature of the gas to a level where the soot is burnt off.

An additional measure is the altering of the injection timing to increase the heat of the exhaust, which will also encourage the carbon to be burnt off.

DPF servicing

Should these measures fail to keep the filter clean then there are two further options, deep cleaning or replacement.

Deep cleaning the filter can be done in situ by introducing a cleaning fluid into the front of the unit while the engine is running and this is the method employed by most garages when cars are presented with the warning light on.

Unfortunately, this may only be a partial cure. While it will improve matters, the carbon on the filter matrix may change into thermally stable forms while the inorganic ash content will tend to accumulate irrespective of how the vehicle is used.

DPF unit on telehandler
Kubota have recently extended the DPF service recommendation from 3000 hours to 6000 hours on its compact telehandler

Prices charged for the cleaning will vary, but they start at around €150 and go up to €300+ for larger items from commercial engines.

This does not include the removal and refitting to the engine, yet it remains a fraction of the price of a new DPF from a main dealer.

Another step that can be taken is the use of fuel additives. These may be purely for the purpose of decoking a DPF, or a general purpose engine cleaning additive may contain the solvents and detergents suitable for cleansing the whole fuel delivery system, and the DPF.

Permanent removal of the DPF unit is also possible but this requires a careful remapping of the engine and, on road vehicles at least, will result in an automatic NCT failure as well as insurance issues due it to being considered a modified vehicle.