EVAP OFF — disabling the fuel vapour recovery system

EVAP OFF disables the fuel vapour recovery system in the software of a petrol ECU: canister purge valve control and the system leak check. The file removes the EVAP faults that appear when the purge valve or the leak detection pump fails, or when the canister has been removed. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

How the vapour recovery system works

Petrol in the tank evaporates all the time, especially in hot weather. To keep the vapour out of the air, the tank vents through a canister: a plastic box filled with activated carbon granules that the vapour sticks to. Evaporation accounts for about 20% of a car's hydrocarbon emissions, so it has its own limit: in the EU, no more than 2 g per test in a sealed chamber.

The carbon can only hold so much, so it is purged regularly. With the engine warm, the ECU opens the purge valve a little, and intake vacuum draws the vapour from the canister into the cylinders, where it burns. The valve is normally closed; the ECU pulses it open and meters the flow. Fresh air enters the canister through a filter to replace the vapour.

Since 2019 the European evaporative test has been tougher: the diurnal part was extended to 48 hours and an aged canister is now tested. The regulation states plainly that earlier purge strategies did not work well enough in town driving.

How the ECU checks the system for leaks

Cars built to US rules (CARB and EPA) have to detect leaks from 0.5 mm. Some BMW, Porsche, VW, Volvo, Mini, Land Rover, Kia and Hyundai models use the Bosch DM-TL electric pump for this; before it, BMW used a vacuum LDP. The test runs after the engine is switched off: the pump blows air through a 0.5 mm reference orifice and the ECU stores the pump motor current, then a valve switches over and the pump pressurises the tank. From how the current changes, the ECU decides whether the system is sealed.

The test only runs under strict conditions. On a BMW, for example, the car has to be switched off after a drive of at least 20 minutes, which itself followed at least 5 hours parked, with 15–85% fuel in the tank, an ambient temperature of −7 to 35 °C and 11.5–14.5 V on the battery. The test starts every second time these are met, usually once a day. That is why an EVAP fault comes back a day or two after clearing rather than straight away, and a quick test drive does not prove a repair.

Why EVAP faults appear

• The purge valve. On VW, Audi, Nissan and Mazda it is the most common cause of P0441. A valve stuck open lets vapour into the intake uninvited: the engine is hard to start straight after refuelling and runs rough for the first few seconds.
• The canister. The carbon granules clump together and the tank can no longer vent properly: popping from the tank, a fuel smell, and in hot weather the engine will not hold idle.
• Leaks. A loose filler cap, a cracked hose or a cracked canister housing.
• The leak detection pump. In practice wiring and corroded connectors are to blame more often. The DM-TL often sits in a wheel arch, and some have been flooded by an overfilled tank.

Codes per SAE J2012. The leak detection pump and the tank pressure sensor have codes of their own, and manufacturers may also use their own codes.

What EVAP OFF changes in the software

Mixture control, lambda sensors and the rest of the calibration stay as they were, and there is no power gain from this. Stage 1 and CAT/LAMBDA OFF can go into the same file, and individual faults unrelated to EVAP are handled by DTC OFF.

What to do with the tank and canister

• The tank has to breathe. If the canister is removed, you have to vent the tank yourself, for example through a fuel filter as on carburettor cars. Never leave the tank sealed with no vent.
• The purge port on the intake must be blanked off once the valve is removed, otherwise the engine will draw unmetered air through it.
• Smell. Without a canister the vapour goes into the air, and the car will smell of petrol, especially in hot weather and after refuelling.
• If the canister stays, bear in mind that without purge the carbon saturates and the vapour will escape anyway.

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 EVAP OFF. You can add other options at the same time.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file and clear the faults. If the EVAP fault used to return after a day or two, check the car again after a couple of days.

The option is available for petrol ECUs including Bosch ME, MED, MEVD and MG, Continental Simos, SIM and EMS, Delco and Kefico, and the full list by make is below. If your ECU is not there, send the file with a request: we will check the software and tell you whether there is a solution.

Disabling the vapour recovery system increases hydrocarbon 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.