Cleaning removes deposits, not faults
A diesel particulate filter blocks with two kinds of deposit. Soot is carbon from combustion, and it burns off when the filter regenerates. Ash is the unburnable residue, mostly from engine oil additives, and it stays in the filter until the filter is taken off and cleaned. An off-vehicle machine washes both out of a filter that is physically sound.
Everything else that can go wrong with a filter is outside what a clean can do. Physical damage needs a replacement filter. Oil or coolant in the filter points to an engine fault that needs repairing. A faulty sensor or a vehicle that never completes a regeneration will block a cleaned filter again. MET’s own instructions say vehicle faults must be fixed before cleaning, and expect filters to arrive free of heavy contamination.
Saying no to a clean when one of these applies is part of doing the job well. A filter that blocks again a few weeks after a clean costs the customer money and the workshop its reputation, even when the clean itself was done properly.
Physical damage
Look at both faces of the filter before it goes in the machine, with a good light. Cracks across the honeycomb, melted or glazed patches, channels that have broken through, or substrate that has shifted or gone missing all mean the filter needs replacing. Melting usually follows an uncontrolled regeneration in a heavily loaded filter, so the cause of that overload needs finding too.
Damage can hide behind a good test result. A crack lets air and water take a short cut, so a damaged filter can show low resistance after cleaning while trapping very little in service. That is why a back-pressure reading is a record, not proof that a filter is serviceable, and why inspection comes first.
Cleaning itself should not damage a sound filter, but a heavily blocked one deserves care. Machines with a soft start bring the water flow up gently for that reason, and the operating instructions suggest a longer soft start for badly blocked filters.
Contamination that points to an engine fault
Oil reaching the filter usually comes from a worn turbocharger, valve stem seals, piston rings or a crankcase breather problem. Coolant usually means a leaking EGR cooler or head gasket. Either can soak into the filter or glaze its surface, and a filter in that state may not clean well. Even when it does, it will be contaminated again as soon as the engine runs.
Signs to look for include wet, sticky or shiny deposits on the inlet face, oil in the exhaust pipework ahead of the filter, and a sweet smell or white residue that suggests coolant. Check oil and coolant levels and history with the customer. A heavily contaminated filter also loads the machine’s wash water and filters faster, which is another reason to diagnose before cleaning.
Why a cleaned filter blocks again
A filter blocks quickly when the engine makes more soot than usual, or when regeneration can’t finish. If the cause isn’t found and fixed, a cleaned filter is simply the next one to block. Diagnose the vehicle as well as cleaning the filter, and record what you found.
Engine faults
Worn or leaking injectors, a sticking EGR valve, boost or intake leaks and poor combustion all raise soot output. High oil consumption adds ash faster than normal, so the filter fills up early even if the engine runs well otherwise.
Sensors and pipework
The engine control unit decides when to regenerate from the differential pressure sensor and the exhaust temperature sensors. A failed sensor, or a split or blocked pipe to the pressure sensor, can stop regeneration or report a blockage that isn’t there. Read fault codes and live data before condemning or cleaning a filter. Many vehicles also need learned filter values reset with a diagnostic tool after a filter is cleaned or replaced; follow the vehicle maker’s procedure.
How the vehicle is driven
Short journeys and stop-start work rarely get the exhaust hot enough for long enough to regenerate, and interrupted regenerations leave soot behind. No clean changes that. Explain it to the customer, because the driving pattern decides how soon the filter needs attention again.
On or off the vehicle: different limits
On-vehicle treatment addresses soot and oil, not ash. It is quicker and avoids removing the filter, which makes it a sensible first step for a soot-loaded filter on a vehicle with no underlying fault. Once ash has built up, the filter has to come off to be cleaned. Neither route repairs damage or cures an engine fault.
Off-vehicle cleaning has its own limit: drying. A wash leaves water in the filter, and some filters can’t be fully dried off the vehicle. Machines that purge with compressed air alone leave some filters needing their final drying on the vehicle. Even with heated drying, a heavily saturated filter, mainly a combined DPF and catalyst unit, may only dry in a filter oven or on the vehicle over a driving cycle. On the S, a heater that trips during drying is a sign of that saturation.
Plan for this when you quote the job. If a filter goes back to another garage, tell them whether it needs finishing on the vehicle, and how.
- METClean E drying
- Cold air purge
- METClean S drying
- Hot dry and cold purge
- METClean V drying
- Cold air purge
Checks before and after a clean
A short routine catches most of the problems above before they become a comeback. Keep a record of each step with the job, so you can show the customer what you found and what you did.
Ask about the vehicle
How it is driven, warning lights, interrupted regenerations and any recent repairs.
Read the vehicle
Fault codes and live data, including the differential pressure and temperature sensors.
Inspect the filter
Both faces for cracks, melting or missing material, and the inlet for oil or coolant.
Test before cleaning
Where the machine supports it, take a before reading to support the decision to clean.
Clean, dry and test again
Dry fully or note what drying remains, then take the after reading and record both.
Explain the limits
Tell the customer about any fault you found and what will happen if it isn’t fixed.
This guide is general. Follow the vehicle manufacturer’s procedures and your machine’s operating instructions, and wear gloves and eye protection when handling filters, which carry soot and ash.
Questions about a filter?
See how a METClean machine handles your own filters, or ask us for a quote.