Overfills and underfills
Many drivers are suspicious of situations when more fuel goes into their car's fuel tank than stated in the technical documentation. There are also opposite cases when car owners are outraged to see the "wrong" amount on the fuel pump display after trying to fill the tank to full.
Overfills. Contrary to popular belief, the fault most often lies not with gas station workers or equipment tricks (although that happens too), but with the actual tank volume. For the vast majority of cars, the rated capacity is not designed to be filled to the very top. In other words, if you fill up until the nozzle automatically shuts off, the tank will hold, say, 45 liters (as stated in the documents). But if you top up slowly, "drop by drop," you can pour in 50-55 liters.
It is worth noting that Japanese cars are considered the most accurate in this regard – their tanks rarely hold more than 10% above factory data. European models, on the contrary, may have a real volume exceeding the rated one by 20-25%. As for Korean, Chinese, and Russian brands, their overfill above the norm is about 15%.
Underfills. You shouldn't immediately get indignant and demand a complaint book if you think you've been shortchanged at the gas station. A warning light or an on-board computer message about zero range does not mean the tank is empty. Almost all automakers provide a so-called "reserve fuel supply." This is the minimum that will last for another 30-50 kilometers of travel (with calm driving), even when the electronics have already reported zero. In essence, this is the same safety margin as the inflated speedometer readings – instruments always show a speed somewhat higher than the real one.
Let us add that some manufacturers even specify the reserve amount. For example, the VAG group (Volkswagen, Audi, SEAT, Skoda, and others) has a reserve of 10-13% of the rated tank volume depending on the model (most often 7 liters). That is, when the electronics show an empty tank, there are actually still a few liters of fuel sloshing around in it. With Japanese cars, the situation is different – their reserve is significantly smaller (3-5 percent of the tank volume).
There is another curious point. Recently, more and more automakers are pushing their customers not to delay a visit to the gas station otherwise. They deliberately configure the computer so that it stops showing the remaining range when it drops to 30-50 km. The second method is that the number of models (mainly Japanese and European) is growing, whose on-board computers simply "do not see" less than 10 liters of fuel in an empty tank.
Why is this done? Automakers pursue a single goal – to protect owners from breakdowns that would have to be repaired under warranty. An empty tank can indeed lead to unpleasant consequences for the fuel pump (and not only). Therefore, we strongly advise not to let the car run completely out of fuel.
Filling to the neck
Filling the car to the brim makes sense before a long trip. For example, when going on vacation, say, to Crimea. It is logical that 60 liters in the tank is better than 45 – the range is greater, meaning you can stop at gas stations less often. Moreover, some Ukrainian roads still do not abound with gas stations – there is a risk of getting stuck without gasoline in a deserted area.
The difference between the rated and actual tank capacity is not accidental. Factory characteristics do not take into account "safety voids." But since these voids exist, they must serve some function. And fuel dispensers for some reason automatically block the supply when the tank is nominally filled.
Modern cars have a whole fuel tank ventilation system, which usually includes three valves:
1. Gravity valve.
Shuts off the ventilation tube when the car tilts more than 45°. Located between the fuel tank and the canister, which absorbs gasoline vapors from the tank.
2. Solenoid valve.
Controlled by the fuel injection system, opens access for gases from the canister to the intake manifold. Located in the engine compartment.
3. Ventilation valve.
During driving, atmospheric air enters the fuel tank through it as fuel is consumed, so no reduced pressure is created in the tank. Located near the filler neck, which is why most drivers know about it.
To understand the purpose of fuel tank ventilation, let's consider the Audi A4 owner's manual as an example. Here is what it says:
• A ventilation tube is connected to the upper part of the fuel tank. First, it goes through a connection point on the filler neck to the gravity valve. It closes the tube as soon as the car tilts sideways more than 45°. This prevents fuel from leaking out through the ventilation tube in accidents.
• Next, the ventilation tube is not simply vented outside but is connected to a charcoal filter. The purpose of this device is to capture fuel vapors harmful to the environment that escape through this tube (they arise, for example, when the contents of the fuel tank heat up). When the engine is running, under certain loads, the gases are sucked back out of the charcoal filter. The moment this happens is determined by the solenoid valve controlled by the fuel injection system.
• Parallel to this system, there is high-speed ventilation, which leads to the filler neck through a compensating volume in the form of a special-purpose container in the fuel tank filler shaft. This ventilation path is important primarily during refueling, when the fuel level rises quickly, and vapors must be displaced but not released into the atmosphere in excessive amounts.
• During driving, atmospheric air enters the fuel tank through the same path as fuel is consumed, so no reduced pressure can occur in the tank.
If you fill the car to the very top (some even pack the fuel by rocking the car), the operation of the tank ventilation system can be disrupted.
Alexander Chalenko, Head of the Service Department at UkrAVTO Corporation:
"Filling gasoline 'to the brim' is not the best idea. In this case, fuel may leak both through the filler cap and through the drain tubes of the fuel system. In the second case, the leak will be barely noticeable, since the tubes are in a hidden place (under the rear fender or side panel). In this case, fuel can get on various parts of the car: rubber suspension components, wiring, transmission parts, and the brake system. And the most dangerous thing – on exhaust system parts (muffler), which, as is known, heat up to high temperatures! The danger is the same with the brakes. If rubber parts are simpler – they will just soften, then with brakes and muffler the situation is more serious. Ignition is possible!"
In some cases, after filling to the neck, the on-board computer may display an error, informing the driver with a "Check" message. Increased fuel consumption also happens. It is possible that the car may not start at all.
Let's summarize
The consequences described above are related to the fact that fuel supply deteriorates. But it should be like this: in the volume in which fuel is consumed, air must enter the tank, otherwise reduced pressure forms in it, and fuel supply worsens or stops. Also, the tank must be ventilated so that fuel has the opportunity to expand when heated. Now remember that it's hot summer outside, and imagine the possible consequences of filling fuel to the brim at +30°C. Our advice: if you do pack fuel, then not to the extreme point when it is visible from the neck. And the second important point – do this only in situations where after such a fill you will be able to drive at least 80-100 km to use up at least part of the "excess." But it's better to fill up until the nozzle clicks off (it's enough to have a canister of fuel in reserve) and learn the rules of economical driving.
