Overfilled gas tank-what effects?


Can you remind me what harm is done to a car when the gas tank is overfilled?

I remember once hearing a Car Talk caller who had problems due to overfilling, and so I have been careful to buy only the amount of gas I judge I need by my gas gauge, and not let station attendants “top it off”.

My husband tells me this is ridiculous. But yesterday he pumped gas for me and actually let the tank overflow because the pump’s stop-mechanism wasn’t working. He says that no harm will be done if it happens only once, but I don’t believe him.

My husband says he overfills the tank on his truck often, and it does no harm. But his truck has $0 resale value, while my car is pretty valuable. Who is right, and what remedial care do I need for my car after the overfilling?


The only possible damage that this can make to your car is that the fuel gauge will be damage or become unaccurate besides that nothing else no damage to any pumps , hoses or anything else i recommend that if you want to have your fuel gauge running smoth and accurate for several years doent overfill the tank


The biggest problem is that it can wreck the evap system. This collects gasoline fumes from the tank and routes them to the engine for combustion. Overfilling could force raw gas to enter the system and prevent it from working right. It could also cause a condition where you have a hard time filling the tank, as the system can’t properly relieve pressure from the tank and the gas pump’s automatic shutoff keeps engaging.

Of course, another reason is that it’s wasting gas. Why allow gas to spill out with the price that it is?


What car do you have? Some are supposed to be sensitive to this (gas gets in the vapor canister), but many have no problems at all with overfilling.


It’s a 2003 Subaru Forester


Subarus, like Toyotas, are very sensitive to overfilling of the gas tank. Hopefully this one incident will not destroy the carbon canister that is the heart of your evaporative emissions system, but I suppose that it is possible for one incident to do it in.

If it is never again overfilled (or topped off), hopefully you can avoid the expense of replacing that carbon canister. The last time that I checked, replacement of that part cost somewhere north of $400.00, so you might want to tell your husband to start putting money aside for this repair in case it is needed.


Some cars (most of them) are totally immune to any harmful effects of overfilling. Their owners do it at every fill-up and have done so for years. A few cars suffer. Dunno about the Forester.

You will find out when you next fill your tank. If the procedure is normal, no damage has been done. Forget about it. If you can only squeeze in a bit at a time, the nozzle clicking off, then a problem has occurred. As for immediate remedial care, there isn’t any. What happened has already happened. Drive normally without worries. Most likely nothing untoward has occurred.


Back in the 70’s that would have been true, but unlikely to damage the gauge. Today it is totally wrong. Overfilling the tank can damage the vent system saturating the charcoal canister and requiring a fairly expensive repair. That is why most cars today have a warning about that on a sticker somewhere and in the owner’s manual.


Actually, if you did saturate the charcoal bed you may find out before the next time you fill up. As the gas is pumped out of the tank the tank breaths in through that charcoal bed that everyone is alluding to. That’s its only source of air to replace the gas removed. If the bed is saturated, a vacuum may form in the air space and make it difficult for the pump to pump gas out to the engine.

If after your tank gets below 1/2 full you begin to have loss of power or operating problems, remove and replace the gas cap. If you hear a whooshing and the problem goes away, it’s a safe bet that you’ve saturated the charcoal and the tank can’t breath in.


If the stopping mechanism didn’t click off like it should have, you should let the attendant at the station know about this. While no harm may have been done th is time, but how many people stop at that station? How many cars are going to overflow and spill gas all over the ground? It could be a possible environmental issue if this isn’t put in check. If the attendant doesn’t do anything about it, call the local EPA and inform them of this problem and let them know you told the attendant at the station, but he did nothing about it.