Not only can a bad gas cap cause stalling. But it can also cause rough idling and bad fuel economy due to the gas sloshing out of the tank through the faulty gas cap. A gas cap is one of those critical components of a car that often goes overlooked.
Most fuel injected vehicles are pressurized. This pressure helps the fuel pump deliver fuel from the gas tank to the fuel injectors. A bad gas cap doesn’t just let gasoline out, it can also let pollutants, sediments and debris into the gas tank which is not good for engine performance. Here are a few signs of a bad gas cap.
1. Loose Gas Cap
A loose gas cap is almost always a sign of a bad gas cap which can cause stalling. If you’ve tightened your gas cap but you still feel it moving around. You can bet that it’s not doing much to pressurize the fuel tank.
That check engine light on the dashboard could very well be the result of something as simple as a loose gas cap. How does a gas cap become loose? It happens over time.
Most gas caps are made of polyurethane (plastic) and rubber which contracts and expands as the climate cools down and heats up. Over time, this can warp the gas cap. You’ll want to get the bad gas cap replaced as soon as possible.
2. Brittle or Missing Rubber Seal
The gas cap also plays an important role in a vehicle’s evaporative emissions system via the exhaust gas recirculation (EGR) valve. This system is designed to capture and reuse fuel vapors from the gas tank that would otherwise be released into the atmosphere.
A gas cap with a brittle or missing seal should be replaced right away. One way you’ll know if the rubber seal is missing, or brittle is that the gas cap won’t tighten securely.
Or when you twist off the gas, you can feel that it’s loose. While this may not seem like that big a deal. Without a proper seal, the entire fuel system is exposed to contamination.
3. Low Gas Mileage
My wife’s and my 2015 Ford Fiesta has a capless gas tank, while our 2021 Toyota Corolla does not. Ford pioneered the capless gas cap on the 2009 Explorer.
The great thing about a capless gas tank is since you’re not really touching any area where the gasoline goes in. There’s less chance of gasoline smell transferring onto your hands. Simply open the fuel door and slip the fuel nozzle into the gas tank without having to twist the gas cap off and on.
While going capless can speed things up at the pump. Unless your vehicle was manufactured with a capless gas tank, then going capless won’t give the gas tank a proper seal causing problems with fuel efficiency.
4. Petrol Odor
One of the more obvious signs of a bad gas cap which can cause stalling is the smell of gasoline. Think about it: how many times a year is a fuel filler cap removed and replaced to fill gas? And what about when you take your car into a garage. How often are mechanics opening and closing the gas cap to diagnose issues?
Over time, all that opening and closing leads to wear and tear and a gas cap that can no longer give the gas tank a proper seal. Although one of the purposes of a fuel filler cap is to keep contaminants out. If it can’t provide a proper seal, then it’s going to let fuel vapors out from the filler neck.
5. Check Engine Light
As said, that mysterious check engine light could be the result of a gas cap that’s failing to keep the fuel system pressurized. A cap that can’t seal properly may cause an evaporation system leak. When the ECU, which is your car’s computer detects the leak. The result is a CEL (check engine light).
But since the CEL can be set off for any number of reasons, you’ll want to scan for trouble codes that link to a bad gas cap. While a bad gas cap won’t necessarily cause performance issues. It will set off a check engine light preventing your car from passing emissions tests.
6. Cracked Gas Cap
A cracked gas cap also mean that the fuel tank isn’t getting a proper seal, which can cause your car to experience issues with stalling. One of the ways you can tell if you have a cracked gas cap is by inspecting it to look for any fissures in and around the gas cap. But sometimes a simple inspection won’t reveal any obvious cracks.
You can also try tightening the gas cap on the gas tank. Gas caps are designed so that there’s an audible clicking noise when the tank is fully tightened. But if there’s a crack in the cap you won’t hear this click, or the sound may be inconsistent. Sometimes you can see the cracks more clearly as you twist the gas cap onto the gas tank.
7. Rust and Corrosion
While not as big a deal for modern cars made of more rust resistant materials as it is for classic cars that were mostly made of metal.
Corrosion happens when materials deteriorate due to oxidation. This is caused by rusting which is a chemical reaction that happens when iron interacts with air and water. Rust doesn’t mean that corrosion is imminent. But it is a type of corrosion. Small amounts of rust can form under the gas cap and around the fuel filler neck.
This also creates an improper seal for the fuel system. Typically, it will cause some EVAP (evaporative emission system) codes and faults which cause your car to stall. EVAP repairs can cost you a little, or they can cost you a lot.
8. Bent Gas Cap
A bent gas cap won’t provide a proper seal either, resulting in engine codes that cause your car to stall. The fuel system is a closed system that requires a tight seal to work properly.
The EVAP system pulls small vacuum samples from the fuel tank and other components to test for leaks. This prevents fuel vapors from being released into the air.
A gas cap can become damaged by bumping against the side of your car if you’re prone to pulling away from a fuel stop and forgetting to replace the fuel filler cap. The gas cap can also become damaged or bent by getting dropped too many times.