It is a significant safety risk, but a fire is not guaranteed. The primary danger is the potential for a fire or explosion due to igniting gasoline vapors. Modern vehicles have safety systems that make a catastrophic event less likely than in older cars, but the risk is real and serious. The core issue is that a running engine produces extreme heat from the exhaust system and contains electrical components that can create sparks. Meanwhile, gasoline vapors, which are highly flammable, are present around the fuel filler neck. Combining these elements creates a major hazard.
The most common ignition source isn't a large flame but something less obvious: static electricity. When you get in and out of your car, you can build up a static charge. If you then touch the metal gas nozzle before dispelling that charge, a tiny spark can occur, which is enough to ignite vapors. This risk exists even with the engine off, which is why touching the metal to discharge static is always recommended. A running engine simply adds more potential ignition points.
While the fuel tank itself is sealed and the evaporative emission control system (EVAP) is designed to capture vapors, this system can be overwhelmed during refueling. The main safety protocols exist for this exact scenario. If you realize you've done this, do not panic and pull the nozzle out. Finish fueling, carefully remove the nozzle, and shut off the engine immediately. The chances of an incident are low if the process is completed without spillage, but it's a violation of safety codes at every gas station for a critical reason.
| Factor Increasing Risk | Mitigating Factor in Modern Cars | Why it Matters |
|---|
| Hot exhaust components (manifold, catalytic converter) | Sealed fuel systems with rollover valves | Prevents large spills, but vapors are the main concern. |
| Electrical sparks from engine bay | Evaporative (EVAP) charcoal canister | Captures fuel vapors, but can be saturated during refueling. |
| Static electricity from the driver | Bonded fuel nozzles and stationary guides | Helps, but the driver is still a primary source of static. |
| Potential for fuel spillage | Automatic shut-off nozzles | Reduces, but does not eliminate, spill risk. |
| Overall Probability of Fire | Considered low, but the potential consequences are severe. |
