BEWARE OF OVERFILLING YOUR CAR’S FUEL TANK
Does the gas station pump keep cutting off when you’re filling up your car? We explain why that happens
We’ve all seen this happen, or done it from time to time — when refueling, we keeping clicking the gas pump handle to either round off the price, or make sure we have it filled to the brim and sometimes get frustrated when the handle keeps shutting off before we think we’re done. This is normal, and it’s known as “spit-back” — but what if it happens when you just begin to refill a low tank?
If you’ve ever had this failure or witness someone at the gas station experiencing it, you know it can turn a three-minute exercise into a half-hour (or longer) affair, depending on the tank size and the operator’s patience.
In these cases, the main culprit is usually a blocked or defective fuel tank vent system. As fuel enters a tank, it displaces any air inside and it must find an escape route. As air in any fuel tank is considered an environmental hazard, automakers use systems to control and capture these fumes to keep them from being expelled into the atmosphere.
These systems are commonly known as evaporative emissions controls. They usually consist of an air-tight canister mounted under the vehicle near the tank, with an electric pump which can create a vacuum to draw fuel vapors into the canister, where they collect into liquid form to be returned into the tank.
When these systems fail, they can pressurize the tank, keeping fuel from easily entering via the filler neck. If you’ve ever removed a gas cap and gotten a substantial rush of air outward, this system may need attention.
Another common cause is a plugged vent line on the fuel filler tube itself. The tube the gas cap sits on runs to the tank, and often has some type of vent hose on it to allow for a quick flow of fuel at the pumps. With age, these flexible lines can deteriorate and become plugged with deposits or debris.
They’re often hard to access, but sometimes can be cleared with a gentle shot of compressed air. In certain vehicles this venting system is built into the tank itself or exists as a replaceable vent valve.
There’s very little we can do to maintain these components or prevent their failures, with one exception — don’t cram an excessive amount of fuel into a tank, especially on a hot day, so it overloads the vapour control systems.
When fuel travels from the underground storage tank to your vehicle’s tank, the temperature can rise dramatically and this increase will cause more pressure from evaporative fumes.
The canisters that hold these vapours have a limited amount of space, and if they can’t complete their task, they may trigger a ‘check engine’ light — or even pop off a vent line. Few of these lines are ever clamped down, because they’re not expected to be subjected to higher pressures.
Bottom line: When the pump handle clicks the first time, you’re done refueling.