hissing when removing gas cap: Causes and Fix

Not so long ago, car fuel tank caps were not airtight. They had a small hole, sometimes with a simple filter, to equalize the pressure in the tank with atmospheric pressure. Naturally, no hissing when removing gas cap, except that the ventilation channel was completely clogged.

In these cases, fortunately quite rare, the cars worked wonders stalling unpredictably and suddenly draining the tanks, which, after checking, turned out to be the result of flattening and loss of capacity. Now everything has changed, ventilation began to comply with strict environmental standards.

hissing when removing gas cap

causes of hissing when removing gas cap

Hissing when opening the gas tank cap can occur for various reasons. Most of them are pretty harmless and easily fixable. But sometimes an unpleasant sound occurs due to more serious malfunctions that cause some discomfort when driving. At the same time, you should not be afraid if the cover is unscrewed with a sound, that this will lead to an explosion of the fuel tank or other dangerous consequences. All this is nothing more than a myth. And in order to understand why such an unpleasant sound occurs, it is worth considering in detail the main reasons for its appearance.

In most cases, the hissing sound when opening the fuel tank is not dangerous and occurs for quite natural reasons.

It’s very hot outside

Hissing when opening the gas tank cap often occurs in the heat. Hot weather with no precipitation leads to the fact that the temperature of gasoline or diesel in the tank begins to rise noticeably. This releases a large amount of volatile chemicals.

Since the tank is sealed, the gases in it create increased pressure. This allows you to easily start the engine. But at the same time, air often comes out of the gas tank with a characteristic sound. Therefore, if it hisses only in the heat and no sounds are observed in cooler weather, you should not worry. The hissing will disappear along with a decrease in the temperature outside.

little fuel in the tank

An empty tank can also cause a hiss when the fuel cap is opened. If there is very little or practically no fuel in the gas tank, its vapors from the fuel neck come out. This is what causes the sound. After refueling the car, it will definitely disappear. Therefore, in this case, you need to think, not why it hisses, but whether you have enough gasoline or diesel fuelto get to the gas station on your own.

Design features

In some cars, air may come out with a hiss due to design features. Most often it occurs in modern models equipped with a plastic gas tank. You can learn about this feature when buying a car from a dealer or in the owner’s manual. This topic will definitely meet on the forum dedicated to this model. In this case, you will most likely always have to open the fuel tank cap with a sound. For some machines, it may appear periodically. But it will probably not be possible to completely get rid of it.

If the first three reasons are not dangerous and do not require the intervention of the car owner, then the following problems need to be addressed.

Fuel cap

The fuel filler cap may be loose or very tight. It can also cause hissing when it is opened.

Ventilation may be impaired due to a defective cover. The gas tank cap of a modern car has a complex design. It has a special valve that regulates the fuel pressure in the system. When it fails or becomes dirty, this sound occurs.

Vapor tube

A problem with the gravity duct or its hoses often results in hissing sounds when the fuel filler flap is opened. The hoses or channel can become dirty over time, which impairs their functionality. The tube stops effectively removing vapors from the tank.

Problems with the adsorber

An adsorber is a carbon filter that prevents the penetration of harmful fuel vapors into the atmosphere. Today it is installed on almost all modern car models. Over time, the element may lose its properties and become contaminated. Therefore, it can no longer perform its function. Because of this, the gas tank flap can also hiss.

Is it normal for gas tank to hiss?

Problems will arise in case of malfunctions in ventilation. An increase in pressure to a dangerous value is unlikely, for this the tank would have to be artificially boiled, but the fall will occur for quite natural reasons.

An electric fuel pump is installed in the tank, constantly pumping out part of the fuel to power the car’s engine.

If you do not ventilate the tank, that is, communicate it with the atmosphere, then such a vacuum is formed that the tank will lose its shape, it will be squeezed by the environment with a force of up to 1 kilogram per square centimeter.

Really much less, but enough to ruin an expensive part.

How to fix hissing when removing gas cap?

How to fix hissing when removing gas cap?

If you can hear air escaping from the fuel filler flap, it is not always necessary to fix the problem. In some cases, it may disappear on its own or remain forever if the reason is the features of this car model. But sometimes a strong sound needs to be eliminated by yourself or in a car service. This usually requires:

  • Replace or clean the fuel tank door if it is damaged or dirty.
  • Clean the outlet tube or gravity channel.
  • Change adsorber.

The last procedure should not be done without a thorough diagnosis of the car, since this element is not cheap for many models. And air can often come out for other reasons. If self-checking is difficult, it is advisable to contact a car service for masters who inspire confidence. They will quickly find the cause and fix it for a reasonable fee.

And, of course, you should not drive with an almost empty tank. And in hot weather, you can recommend to put the car in the shade.

How are gasoline vapors removed?

The tank ventilation system with the introduction of environmental standards has become very complicated. An adsorber was introduced into it a device for collecting gasoline vapors from gases exchanged with the atmosphere.

Along the way, several nodes serving its work appeared. Particularly advanced systems even have a pressure sensor in the fuel tank, which is quite logical from the point of view of the theory of automatic electronic control but looks like overkill for mass designs.

Previously, the so-called two-way valves, which open at low pressures in both directions, to the inlet and outlet of gas, did quite well.

Since it is impossible to simply dump the excess into the atmosphere, it is first necessary to select gasoline vapors from them, that is, the gas phase of the fuel. To do this, the tank cavity first communicates with a separator – this is a tank where gasoline foam remains, that is, not quite gas, and then with an adsorber. It contains activated carbon, which successfully separates hydrocarbons from atmospheric air.

It is impossible to accumulate gasoline vapors forever, as well as to achieve their condensation and drain, so the adsorber is cleaned in the purge mode.

The electronics switch the corresponding valves, and the coal filling is blown with outboard-filtered air, after which it, already saturated with fuel, enters the intake manifold through the throttle.

Gasoline will be used strictly for its intended purpose, a rare case when the interests of the economy and the environment are carried out simultaneously.

is it dangerous to drive without a gas cap?

If the gas cap sizzles when you open it, you’re good to go. Usually, this sound is not accompanied by serious violations in the operation of the car. But with some malfunctions, the following problems may occur:

  • Increase in fuel consumption.
  • Reduced motor power.
  • Deterioration of the dynamics of the car.
  • The appearance of errors on the dashboard.
  • Difficulty opening or closing the fuel filler door.

If these problems are present, it is necessary to determine the cause of the malfunction within a reasonable time and eliminate it. If the hissing is caused by hot weather or vehicle design, then there is nothing to worry about. And driving with an almost empty tank is dangerous not only by the appearance of an unpleasant sound. Due to the lack of fuel, you can stall on the highway at the most inopportune moment, when the nearest gas station is very far away. In addition, because of this, the fuel pump may fail. But usually, the part breaks if driving with an empty gas tank has become a habit for the driver.


Until now, there is no consensus on whether the hissing when removing gas cap is a malfunction or a normal phenomenon. In the presence of such sounds from the gas tank, the car owner should monitor the car, and if it has increased fuel consumption or there is a persistent smell of gasoline in the cabin, then in such a case, the problem is most likely in the adsorber, which will require an appropriate replacement. It is necessary to closely monitor the condition of the gas tank, preventing it from leaking since in such a case there are certain problems with the operation of the car.

Leave a Comment