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What Problems Can a Loose Gas Cap Cause? A Small Twist with Big Consequences

Typical gas cap

Few people expect that something as simple as a loose gas cap could trigger a check engine light and lead to system-wide problems.

On the surface, a loose gas cap doesn't seem like that big of a deal, but it can affect your vehicle's emissions system, confuse the ECU, and lead to idling problems, misfires, stalling, and a handful of other more serious issues.

Therefore, tighten your gas cap properly and replace it if it does not create a complete seal.

In this article, we'll quickly run through a host of problems that can be caused by a simple loose gas cap and explain why they happen.

Table Of Contents [show]

    Typical gas cap
    Typical gas cap

    What Problems Can a Faulty Gas Cap Cause?

    Initially, a loose gas cap won't have much of an effect on your vehicle.

    At most, it may trigger a check engine light.

    However, suppose you leave the problem for too long. In that case, your vehicle will begin emitting gas vapors that can harm the environment, affect your vehicle's fuel economy, and even damage the emissions system.

    In time, this can cause the engine to rough idle, misfire, or even stall out. Therefore, always be sure to tighten your gas cap completely and replace a damaged gas cap as soon as possible.

    Can Loose Gas Cap Cause Rough Idling?

    Most modern automobiles use a fuel injection system to bring gasoline into the combustion chamber.

    This system relies on a specific amount of pressure from a combination of gas vapors and internal controls.

    Part of these calculations includes the presence of a properly sealed gas tank.

    If your gas cap is loose, the fuel injection system cannot create the necessary pressure, and your engine will begin to run lean.

    You'll notice the difference as your vehicle begins to rev inappropriately and idles rough.

    Additionally, if your vehicle's gas cap is not properly tightened, dust and debris can fall into the gas tank and get sucked into the injection system.

    Dirty fuel injectors can become clogged and sticky, eventually failing to produce a balanced mixture of air and fuel.

    In time, this could also lead to idling problems.

    Does the Gas Cap Affect the Engine?

    Although the gas cap doesn't directly have anything to do with your vehicle's engine, it does play a vital role in trapping gas vapors that affect the fuel injection system.

    If gas vapors are allowed to escape from the tank, they cannot create the balanced internal pressures needed to inject fuel, and the engine will eventually run lean.

    Can a Loose Gas Cap Cause Engine Misfires?

    A misfire happens when your vehicle's engine doesn't create the necessary combustion needed to drive a cylinder.

    This can cause your engine to suddenly hiccup and lose RPMs.

    Even if it misfires just once, it's a sign that something is wrong with the engine and that you need to get it checked.

    Since a loose gas cap affects the internal pressure inside the fuel injection system, it can also cause the engine to misfire.

    Suppose the fuel injector cannot properly create a balanced mixture of air and gasoline. In that case, your engine will not be able to produce the necessary combustion needed to drive the cylinders, and your car may misfire.

    Although this is possible, a host of other problems can cause your engine to misfire, and you'll likely catch a loose gas cap before this happens.

    Instead, your vehicle is probably misfiring due to:

    If a loose gas cap causes the problem, you'll be able to detect other tell-tale signs to diagnose the problem better.

    Can a Loose Gas Cap Cause Backfires?

    Although the terms sound similar, a misfire and a backfire are caused by very different issues.

    A backfire happens when a small amount of gasoline remains in the combustion chamber and sparks during the exhaust cycle, causing a miniature explosion within the engine.

    The explosion escapes through the exhaust, creating a crack we associate with backfires.

    If your gas cap is not properly tightened, it can cause your engine to burn lean, meaning that the fuel/air mixture is overly saturated with oxygen.

    A lean mixture burns more slowly, causing a small amount of fuel to linger in the combustion chamber.

    The next time the injector sends fuel into the engine, it can cause a sudden backfire.

    Can a Loose Gas Cap Cause Stalling?

    A loose gas cap will likely not cause an engine to stall, although it could cause an engine to idle rough.

    Does a Bad Gas Cap Affect Mileage?

    Driving with a faulty gas cap can affect your vehicle's fuel economy.

    Gasoline is a highly volatile liquid that readily transforms into a gas, even at temperatures below its boiling point.

    This means that gasoline gives off a vapor as it sits inside your vehicle.

    If your car's gas cap is loose or cannot create a tight seal, the vapors will escape from your tank and into the atmosphere.

    Not only will this affect your vehicle's ability to maintain internal pressure, but you won't get as much bang for your buck.

    As your car starts burning through liquid gasoline, it won't have extra vapors to use in combustion.

    Therefore, you won't get as many miles per gallon.

    Loose Gas Cap Causes Car to Shake?

    If your vehicle is shaking, you're probably experiencing a rough idle.

    If it feels as if the engine is struggling to function as you approach a stop but picks back up whenever you hit the accelerator, this is a case of rough idling.

    As mentioned above, a loose gas cap can cause your vehicle to rough idle.

    Without necessary gasoline vapors, your vehicle will struggle to maintain internal pressure, and the fuel injection system will not be able to create a balance of gasoline and air.

    Your vehicle will begin to shake and even misfire when you press the accelerator.

    Foul Gasoline Smell While Driving

    If you smell gasoline while driving, this could be caused by a loose gas cap, but it's not the only reason.

    If the gas cap is loose, you may be smelling the vapors escaping from the tank and being sucked into the air vents.

    However, you may also have a faulty fuel line or a blown seal.

    In this case, your vehicle could be leaking gasoline, which is incredibly dangerous.

    We recommend stopping, checking your gas cap, and then driving to the nearest mechanic as soon as possible to check the seals and fuel lines.

    Not only is gasoline a highly flammable compound, but its fumes can cause nausea, dizziness, and brain damage.

    Continuing to drive despite the smells can cause you to lose control of the vehicle and lead to a crash.

    Fuel Contamination

    A loose fuel cap may allow moisture and other contaminants into the fuel tank, leading to problems like a rough engine idle.

    If you suspect your engine problems are caused by contaminated fuel, you may add a gas treatment, preferably to a near-empty tank, which may fix your issues.

    Problems Tightening the Cap

    If you've checked the gas cap and discovered that it wouldn't tighten, you'll have to replace the part as soon as possible.

    Fortunately, gas caps are relatively easy to remove and replace, and you should be able to take care of the problem on your own.

    Gas caps can degrade over time and struggle to tighten due to the following reasons:

    • A loose or damaged O-ring - The O-ring is a small, circular piece of rubber that sits inside the gas cap and helps create a tight seal. If it is old, dried out, or cracked, it may not create a tight seal and cause the cap to remain permanently loose.

    • A faulty pressure release valve - The gas cap does more than just seal the gas tank. It also helps regulate internal pressure by releasing small amounts of vapor whenever necessary. Unfortunately, modern gas caps have a pressure release valve, and the gap will not create a complete seal if damaged.

    • The filler housing is damaged - If you cannot find any problems with the gas cap itself, there could be a problem with the filler housing. If the housing is dented or stripped, the cap will not fit into the tank, and your vehicle won't be able to maintain internal pressure.

    If the cap itself is the problem, continue down this article to learn how to replace a broken cap.

    If the housing is damaged, this is a far more serious problem, and you'll have to take your vehicle to a mechanic to get it fixed.

    Is a Loose Gas Cap Dangerous?

    Ultimately, a loose or damaged gas cap may not seem like a serious problem, but it is.

    You might not notice the difference at first, but if left unchecked, it can lead to many other problems, all of which can be pretty serious.

    These problems can extend from mechanical to health-related and even environmental issues:

    • Mechanic Issues - A loose gas cap can damage your vehicle's fuel injection and emissions systems over time. If your vehicle picks up incorrect emissions readings, it will try to adjust using the wrong data. This can damage hoses, sensors, and other essential parts.

    • Health Issues - As noted above, gasoline is a highly volatile compound that emits toxic fumes. These fumes can lead to long-term brain damage and heart disease if inhaled for a prolonged period.

    • Environmental Issues - Vehicles are outfitted with emissions systems and catalytic converters to manage the fumes and toxic compounds released into the atmosphere. If gasoline vapors and not controlled and allowed to escape from the gas tank, these fumes can lead to air pollution and environmental damage.

    To better manage these potential risks, always tighten your gas cap until it clicks and replace any cap that refuses to tighten.

    What Code Does a Bad Gas Cap Throw?

    If you fail to tighten your gas cap, your ECU will issue a check engine light within 5 to 10 miles.

    This light is commonly referred to as a “gas cap light,” but you may always receive a few additional codes if you leave the cap for too long.

    For example, you may receive a code P0455 for a large EVAP system leak.

    This indicates that your engine's fuel injector system is struggling to maintain pressure, and you should pull over to tighten the cap as soon as possible.

    A check engine light could mean you have a problem with your gas cap
    A check engine light could mean you have a problem with your gas cap

    Can a Bad Gas Cap Cause a Lean Code?

    The longer you leave your gas cap unattended, the more likely you will receive a lean code.

    This code indicates that your engine is running lean or burning a fuel mixture that contains too much oxygen.

    Lean codes can be caused by a problem with the air intake system, but if your gas cap is loose, they can also indicate a problem with the fuel injection system.

    The code simply means that your engine is burning too much air, regardless of whether it's caused by too much air intake or too little fuel injection.

    A lean code does not necessarily mean that your gas cap is loose.

    If you receive a lean code without an EVAP system leak or gas cap code, you should take your vehicle to a mechanic and have the other systems checked for damage.

    How Do You Reset Check Engine Light After Loose Gas Cap?

    If you simply forgot to tighten your gas cap all the way and your ECU has issued a check engine light, pull over and tighten the cap all the way.

    You will not have to take your vehicle to a mechanic to disengage the light.

    Instead, just keep driving.

    As your vehicle's sensors detect the cap and restore internal pressure, the ECU automatically disengages the check engine light within 10 to 15 miles.

    If not, take your vehicle to a mechanic and ask them to check the codes.

    If nothing else comes up, the mechanic can disengage the check engine light for you.

    How Much Does a Gas Cap Cost?

    Compared to other parts, a gas cap is inexpensive, but you may be surprised by the bill depending on the make and model of your vehicle.

    Standard four-door sedans usually use inexpensive gas caps that range from $15 to $30.

    However, if you drive a luxury vehicle, you could pay quite a bit more for a simple gas cap.

    For example, a new gas cap for a Porsche Cayman will set you back around $200.

    On the other hand, a Range Rover uses a $50 gas cap.

    If you take your vehicle to a mechanic to change the gas cap, you'll also spend around $100 on labor.

    You can save on these costs by installing the gas cap yourself.

    How to Fix a Bad Gas Cap?

    If you're in the process of replacing your gas cap with a new one, you're probably wondering how to install it.

    Here are some helpful instructions:

    1. Start by loosening the bolts or clamps holding the old gas cap in place. You may need a wrench or pliers to do this.

    2. Once the bolts or clamps are removed, you should be able to pull the old gas cap off simply.

    3. Take a look at the new gas cap to familiarize yourself with how it works. A threaded portion will usually need to be screwed onto the fuel tank.

    4. Begin screwing the new gas cap into place. Make sure that it is screwed on tightly so that it doesn't come loose while you're driving.

    5. Once the new gas cap is in place, you can reattach any bolts or clamps holding the old one on.

    If you have any questions, be sure to ask a professional for assistance.


    As you can see, there are quite a few problems that a loose gas cap can cause.

    From idling and stalling issues to health risks and environmental concerns, it's important to ensure that your gas cap is tightened every time you fill-up.

    If you're having trouble tightening the gas cap or it won't stay tight, you may need to replace it with a new one.

    Gas caps are relatively inexpensive and easy to install, so there's no need to wait to fix this problem.

    Leaving any longer than necessary can cause your vehicle to idle rough, stall out, misfire, or even backfire.

    If you ever smell gasoline while driving, stop, check your gas cap, and then drive to the nearest mechanic to check the fuel lines and seals.

    Frequently Asked Questions?

    Do you still have more questions? Then, keep reading below to find the answers to some of the most commonly asked questions we receive about gas caps.

    If we don't answer your questions here, let us know, and we'll see what we can do.

    Should There Be Pressure When Removing a Gas Cap?

    Modern gas caps are designed to seal the gas tank, so it makes sense that you might wonder if they should release the pressure every time you remove the gas cap.

    However, if your gas tank is sighing out vapors every time you open it, this is a sign that your EVAP system has a problem.

    Any vapors released from gasoline within the tank must be rerouted throughout the engine to create necessary pressure in the fuel injection system.

    If the vapors are escaping every time you open the gas tank, the system is failing and should be checked.

    Your gas cap's pressure release valve should already dissipate any excess vapors that the gas in the tank could release.

    If not, the cap will not tighten properly, and you'll receive a gas cap light on your dashboard.

    How Do I Know if My Gas Cap is Bad and How Can I Test It?

    The first and most likely reason for a faulty gas cap is physical damage to the O-ring.

    The O-ring acts as a gasket that seals the cap into place, but if old or damaged, it cannot create a tight seal.

    To check the gas cap for damage:

    • First, look for any signs of drying or cracking around the rubber seal.

    • If the O-ring is coated in dirt or debris, it won't seal in place and may need replacing.

    • Next, check to make sure the gasket has rolled back or become pinched, preventing it from sealing in place.

    • Next, check for broken ridges around the O-ring that could be preventing it from sealing.

    • If you cannot find any physical damage to the O-ring itself, look around the filler housing for any left behind the rubber.

    If the seal seems ok, you can then move on to check the pressure release valve.

    The pressure release valve is a small valve located on the gas cap that releases pressure when the cap is removed.

    To check the pressure release valve:

    • With the cap off, put your finger over the valve and see if any air is released.

    • If air is not released, the valve may be clogged or damaged and need replacement.

    • If air is released, the valve is not the issue.

    If any of these issues prove to be the problem, order a new gas cap or pick one up from your nearest auto parts store and replace the cap as soon as possible.

    How Long Do Gas Caps Last?

    An OEM-certified gas cap should last most of your vehicle's life.

    This isn't to say that they always will, though.

    After the 50,000-mile mark, you should start paying closer attention to the condition of your gas cap.

    Because gas caps are made almost entirely from plastic, they can dry out and become brittle over time.

    This makes them more susceptible to damage, and you may have to replace them long before your vehicle meets its demise.

    How Long Can You Drive Without a Gas Cap?

    In short, as long as it takes you to get to a safe place to replace your gas cap, you can drive without one.

    However, we highly recommend replacing it as soon as possible as driving without a gas cap can cause several problems.

    Also, remember that your car will burn through gasoline faster, so we don't recommend driving too far from a gas station without a gas cap.