Eolys additive off for FAP particulate filters

EOLYS/ADDITIVE OFF disables the particulate filter additive system in the diesel ECU software. The ECU stops dosing additive into the fuel, stops monitoring the additive level and stops cutting power when the additive tank runs dry. The option is ordered together with DPF Off when an additive-type filter is removed. The finished file with corrected checksums arrives on average in 30–60 minutes.

Why a particulate filter needs an additive

Soot in the filter ignites at 600–650 °C, and a diesel only produces exhaust that hot at full throttle, as VW puts it. If the filter sits far from the engine, under the floor, the gas reaches it even cooler. The answer was an additive in the fuel. It is an organometallic compound: it burns with the fuel in the cylinders, settles in the filter right between the soot particles and lowers the temperature at which the soot burns. In the VW system that is about 500 °C.

PSA was the first to put such a system on a production car: the Peugeot 607 in 2000, with a cerium-based additive. PSA calls the filter FAP, and the additive is known under the Eolys brand. VW had its own system with an iron-based additive, for example on the Passat 2.0 TDI and the V10 TDI. It turns up on other makes too, and the giveaway is simple: a separate additive tank with a pump.

How the additive system works

VW describes its system in detail in its training material, which makes it a good example of the logic:

• The additive is stored in its own tank, on the Passat in the spare wheel well. The tank has a minimum level sender.
• After every refuelling the ECU works out from the fuel level sender how much fuel went in, and the pump adds additive to the tank through the return line. That gives 10 ppm of iron in the fuel, roughly 1 litre of additive per 2800 litres of fuel.
• The additive burns with the fuel and collects in the filter together with the soot. Regeneration runs roughly every 500–700 km and lasts 5–10 minutes: the ECU shuts the EGR, adds a late injection and closes the throttle partly.
• The ECU also keeps a count of the ash in the filter, shown as a separate value in the scan tool.

VW figures from training programme SSP 330 (2004). With higher fuel consumption the filter lasts about 90,000 km. PSA and other makers use their own additives and intervals.

Where the system fails

• The additive runs out. When the tank reaches its minimum, a warning light comes on, and VW then stops regenerations and reduces engine power.
• Ash. Burnt oil and the additive itself leave incombustible ash. It eats into the filter volume, and sooner or later the filter has to be replaced. After a replacement the ash counter is reset with a scan tool, and when the ECU is replaced the old value is copied into the new unit.
• The additive ages. VW rates it as stable for about four years, after which the tank is purged and refilled.
• The pump and sensors. A failed dosing pump or level sender leaves a fault in the ECU memory, and without additive the filter no longer cleans itself properly.

So an additive-type filter comes with two consumables at once: the fluid itself and a filter that will clog with ash in the end anyway.

What EOLYS/ADDITIVE OFF changes

Switching off only the additive while the filter is still in service makes no sense. VW says plainly that the system needs enough additive relative to the soot in the filter. Without it the soot needs 600–650 °C again, and a filter designed for an additive will clog. That is why EOLYS/ADDITIVE OFF is done together with DPF OFF, once the filter has been removed or emptied. EGR Off and Stage 1 can go into the same file.

What to do with the tank and pump

• The tank and pump can stay where they are: when the engine ECU controls dosing, no more additive goes into the fuel once the file is flashed. If another module controls it on your car, mention it in the order.
• Faults should be cleared in all modules after flashing. If an additive or filter message stays on the dash, send us the fault readout from all modules and we will look at the specific car.
• Keep the original file: if an additive-type filter goes back on the car, the system can be switched back on.

How to order the file

1. Read the original file via OBD or on the bench with the tool you use. We need an unmodified stock file.
2. Upload it to the portal and select EOLYS/ADDITIVE OFF together with DPF OFF.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file, clear the faults and check that the additive and filter warnings do not come back.

If your ECU is not in the list below, send the file with a request and we will check whether there is a solution.

Disabling the additive system together with the particulate filter increases particle emissions and changes the vehicle's emissions class. In the EU, the UK, the US and a number of other countries this is prohibited on public roads and may cause an inspection failure. The service is intended for tuners and is used where the law allows it: motorsport, off-road, export, vehicles not used on public roads. Responsibility for using the file lies with the vehicle owner.