Dear Ray & Car Talk, Thanks for your column, I enjoy it. I’ve wondered about my question for a long time. Is it harmful in any way to fill your gas tank to the brim,long after the pump has tripped? I don’t particularly enjoy gassing up, so to me it seems like the fuller you get it, the longer the tank will last. My '16 CR-V can hold about 3 1/2 gallons after the trip which should take me close to another 100 miles on a tankful. It irritated me when I bought it new & the dealer bragged it came with a full tank of gas, then I only drove about 40 miles before the gas needle started moving off full. Most cars I’ve ever owned stays on full for half the tank, then drops rapidly. I also have an 06 Tacoma, & I can only pack an extra gallon or so in it. I know, I’m the annoying guy at the pump who keeps squeezing in more till it’s at the top. Anything wrong with this practice besides being annoying to the guy behind me?
If you’ve read the CarTalk column or have been a regular CarTalk listener for a while you will have seen or heard this question asked and answered. The answer is, “Stop that practice, now!”
You have a system in your car that handles the gas vapors in your tank. In order for that system to work properly it has to be handling vapors, not liquid gasoline. By topping off the tank, you taking up the space that should be occupied by vapor, not by liquid. The pump shut off is there to protect that system as well as preventing a spill of gasoline.
My understanding is that diagnosing and repairing this system is a pain in the butt and can be quite expensive.
Seriously, would you want to be behind yourself in line at a busy station ? And Mr. Boiler is correct it is not a good practice to top off.
Over-filling beyond the click can push liquid gasoline into the charcoal canister, can dissolve the itsy-bitsy charcoal particles and cause the resultant debris to flow through the purge valve, possibly clogging it, or clogging the injectors. So the potential downside is the cost of a replacement canister, purge valve, and/or injectors.
Your gas tank must be able to freely breath in as the fuel is pumped out, and breath out to allow for volume changes as gas is added, fuel is agitated (agitated fluids expand), and temperature fluctuations expand the fluid. It does this through a bed of activated charcoal. The charcoal captures airborne hydrocarbon molecules to prevent them from escaping into the atmosphere.
If you “top off” your tank, you stand the risk of allowing fluid to saturate the charcoal bed. That’ll choke the tank’s breathing passage off. Try breathing through a water-saturated towel and you’ll get the point. It’ll also clean out your bank account to get the car running again. Replacing charcoal canisters is very expensive.
Let the pump’s design and your car’s design do their jobs. They’re working to your advantage. Stop filling when the pump handle clicks.