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Squeezing every penny into the tank can cost big dollars down the road logo 2017-08-03 Brian Turner
When filling the tank, it's best to stop at the first "click" rather than keep forcing fuel into your car. © Joe Raedle, Getty Images When filling the tank, it's best to stop at the first "click" rather than keep forcing fuel into your car.

It’s  a habit many of us as drivers have and exercise every day; when refueling we keep clicking at the nozzle handle to round out our purchase or to squeeze every last kilometer possible out of tank-full.  But did you know that feeding your auto enough fuel to drive the level well into the filler neck can cause problems with your vehicle’s emission systems?

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Every vehicle on the road that meets current emission controls’ regulations has some type of vapour control system.  It’s there to capture raw fuel vapors from escaping into the atmosphere. With most gasoline powered autos this system usually involves some type of canister with a charcoal medium, sensors, valves, and lines. When fuel is removed from a station’s underground storage tank and pumped into the tank of a vehicle, the change in the temperatures it’s exposed to can cause the fuel to create non-liquid vapours.  The same thing can happen when the fuel in the vehicle is exposed to the normal sloshing from driving.

With some systems, any errant fuel vapours are simply controlled and piped forward to the engine’s intake to be burned in a normal combustion cycle. With others, these emissions are directed back into the tank. An air-tight canister does the job of temporary storage of these vapours.

So what happens when the tank is chock full of fuel? This can create enough positive pressure in the tank to cause the vapour control system to have problems due to the lack of expansion space in the tank. It can lead to failed valves and seals and, in rare cases, it may actually bulge the tank, rendering it in need of replacement. As a vehicle ages, the likelihood of these failures increase as our sometimes dry and salt-laden road environment takes its own toll on small-diameter hoses and plastic components.  

If your vehicle does develop a problem with this system, its onboard computer will illuminate the ‘check engine’ light on the dash. The trouble code that will be stored will often be titled ‘small evaporative leak’, which means a cumbersome diagnostic process to determine the exact location of the leak. Techs will often have to hook up a specialized piece of shop equipment to such a vehicle, which introduces a non-toxic visible ‘smoke’ vapour into the system that will help pinpoint the leak’s source. As you might suspect, this can lead to an hour or more of diagnostic time and its related fees, not to mention the cost of any replacement components and installation charges. Think invoices well over a $200 or more. This one trouble code consistently appears on Top Ten check engine light lists from firms such as CarMD.

Avoiding this trouble and its related expenses is as easy as shutting off the fuel station nozzle action at the first click when the tank is full. As most of us pay at the pumps with some sort of plastic, trying to round out the purchase is rather meaningless. And any money saved by trying to jam as much fuel as possible in when a particular station is offering a low price is often less than a few pennies and more than offset by any repair bills that might come up down the road.


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