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.


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