Activated carbon container in the car: Function, causes of faults and diagnosis

The activated charcoal canister protects the environment against unburnt fuel vapours. This page explains how the system works, what could cause a fault, and how malfunctions affect the running of the engine. You will also find valuable practical tips for troubleshooting in connection with the activated charcoal canister

The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for suitably qualified personnel only.

Basic principles of the activated charcoal canister

Activated carbon container basics — 1 Fuel tank, 2 Bleed line, 3 Activated charcoal canister, 4 Recirculation line, 5 Tank bleed valve, 6 Air-mass sensor, 7 Throttle device, 8 Air intake section

Ever since the introduction of lambda control on vehicles with a catalytic converter, all emissions from a vehicle have been subject to legal regulations.

This also includes evaporating fuels (hydrocarbons) from the fuel tank. Since these so-called volatile hydrocarbons are not allowed to enter the atmosphere, they need to be captured and stored for reuse. In order to comply with legal regulations, retention and recirculation systems for fuel vapors are used here.

One important component in this system is the activated charcoal canister.

Functional principle of the activated charcoal canister

The activated charcoal canister is connected to the tank bleeder via the pressure compensation line and is used to capture and store fuel that evaporates under the influence of heat.

The timed valve is located in the connecting pipe between the intake pipe and activated charcoal canister. As soon as the lambda control is active, the timed valve is actuated and the line between the suction pipe and activated charcoal canister is released. As a result of the vacuum in the suction pipe, ambient air is sucked in through an opening in the activated charcoal canister. The air that is sucked in flows through the activated charcoal and guides the captured fuel to the engine for mixture formation.

Since the system affects the composition of the mixture, it only becomes active once the lambda control is operating.

Checking the activated charcoal canister

Check activated carbon container: Activation of cycle valve — Triggering of timed valve

When troubleshooting, proceed as follows:

Check the activated charcoal canister for damage.
Check the hoses, lines, and connections for damage and to ensure they are correctly seated/mounted.
Check the timed valve for damage.
Check the electrical connections of the timed valve for damage and that they are correctly mounted.
Check the ground and voltage supply. To do so, remove the connector on the timed valve. When the engine is at operating temperature, a voltage of approx. 11 – 14 V must be present (the engine must be warm for lambda control to be active, otherwise it will not be possible to actuate the timed valve)
Check with the oscilloscope: Connect the measurement line of the oscilloscope to the ground wire of the timed valve.
Adjust the measuring range, X-axis = 0.2 seconds, Y-axis = 15 V.