The car smells of gasoline – is this normal? Not really

In a gas tank, gasoline evaporates, like any volatile liquid. Especially intense – in hot weather, as well as when driving, when the fuel splashes. When gasoline evaporates, fuel vapors are formed in the upper part of the tank which, when the pressure rises, can blow up the tank and therefore have to escape somewhere. But this is not possible: both ecology and the requirements for the fire safety of a car are against it. Yes, and expensive gasoline is wasted.

Previously, all these problems were viewed in a simpler way: gasoline vapors were sent into the atmosphere. On Muscovites, a leaky cork was used, which they exhaled under the license plate mounting platform. On the Zhiguli, an arc-shaped tube connected the tank to the atmosphere. Of course there was a smell of gasoline.

With the introduction of Euro-2 eco-standards (or the second eco-class in relation to Russia), systems appeared that prevent the release of fuel vapors into the environment. Today, almost all cars use the same tank ventilation system, which differs only in the location of individual elements.

How it works?

The main element of the ventilation system is adsorption. This is a plastic tank filled with small granules of activated carbon. Gasoline vapors from the tank through the pipe enter it, where they condense on the surface and in the volume of coal particles. This leads to the purification of air from fuel vapors.

The adsorber can operate in different modes.

• First – when the car is parked and the engine is not running. When the car and the tank are heated in the sun, the pressure in the tank rises. Fuel vapors with air are forced out of the tank and pass through the adsorber. There, the coal particles are saturated with gasoline, and the air is cleaned and, opening the tank ventilation valve with positive pressure, goes into the atmosphere. The tank is ventilated and fuel vapors do not enter the atmosphere and do not pollute it.
• Second mode – with engine running. The capacity of the adsorber to deposit fuel vapors is not infinite: periodic regeneration is necessary to remove excess fuel so that it can continue to fulfill its function.

The vacuum in the intake manifold of the engine is used. The electronic control unit (ECU) closes the tank vent valve that connects the canister to the atmosphere and opens the canister vent valve.

Purge usually does not start immediately after starting the engine, but when the engine is warm. The vent valve does not open fully: it meters the flow of fuel vapors. This is achieved by supplying a pulse-width-modulated voltage to the valve through a PWM controller, allowing the ECU to change the coil speed. Or maybe give up altogether if it decides that the purge has already happened. After all, the system knows how much fuel has passed through the nozzles and what “appendix” the tank ventilation system has given. The composition of the mixture is reported to her by oxygen sensors.

The operation of the system is under strict control of the ECU. To do this, most modern cars (Euro‑5 and Euro‑6) use an absolute pressure sensor. He immediately sees any leak in the system, ranging from a leaky pipe, a cracked canister body, a worn fuel tank vent valve to an open fuel tank cap.

How is it diagnosed?

Due to the environmental importance of the system, the ECU monitors the health of the valves through the resistance of their windings.

The pressure sensor controls the density and the feedback to the lambda sensors indicates whether fuel vapors are coming out of the adsorber. All of these failures lead to the ignition of the Check engine indicator.

Even with a simple scanner like the ELM327 you can often read the error code.

Adsorber stands and engine

The adsorber must remain operational up to 80,000 km (on vehicles up to and including Euro 4) and up to 160,000 km (Euro 5 and above). Then manufacturers sometimes have to replace the adsorber with a new one. In practice, not everyone is concerned about environmental problems: the average car owner is convinced that the ‘coal bus’ should last as long as the car itself.

Unfortunately, the technology has not yet reached this point. On most machines, the adsorber really lasts for decades, but it happens that the granules stick together into a dense pile, the normal ventilation of the tank is disrupted or even stops altogether. At the same time, the pressure of saturated gasoline vapors in the tank increases, causing plugs in the fuel line – especially in the heat. The engine starts to work – it starts badly, does not hold revs and even stalls. The car loses momentum, vibrates when the accelerator pedal is released, and so on. To complete the sensations, pops are heard in the fuel tank, the smell of gasoline appears.

Is it possible to refuse the adsorber?

Unwanted, but possible. This should be done only in cases where your car of a rare model has an adsorber damaged as a result of an accident, and you cannot get a new one anywhere.

Then you can disconnect the pipes and simply connect the tank to the atmosphere through an ordinary fuel filter of carburetor cars – this will prevent dust from entering the fuel tank.

A fuel filter on carbureted vehicles must also be installed at the inlet of the canister breather valve. This solution is suitable for vehicles up to Euro 4 that do not have a pressure sensor.

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