EVAP System Vent Problems – Gas Won’t Go Into TankDecember 24, 2010 2:30 pm Check Engine Light, DIY, EVAP, How To Auto Repair
The black specs in the picture are charcoal pellets that came from the charcoal canister.
© DenLorsTools.com Summary: DenLors auto repair blog covering a common EVAP (Evaporative Emission Control) system problem. EVAP charcoal canisters can contaminate the system causing flow and vent issues. Sometimes problems with venting can make it almost impossible to fill up at the gas pump. A very frustrating ordeal at the pump is to have the pump shut-off while putting gas in BEFORE the tank is full. Some vehicles even have the fuel splash back out due to improper vent tube operation. This car repair article points out several things to look for when charcoal from an EVAP canister has broken loose and has entered the EVAP lines causing a blockage.
EVAP system faults are a leading cause of CEL’s (Check Engine Lights) to come on, i.e. Low Purge Flow (Code P0497). Out of all of the EVAP system faults, LEAKS are the most common cause of problems. For repair tips and advice on EVAP system leaks see our previous car repair article on Common EVAP Codes which covers pinpointing EVAP system leaks. This automotive repair article today covers another problem with EVAP systems, which are flow and vent issues caused by the EVAP canisters coming apart. When filling up at the gas pump, many people can’t resist squeezing an extra .50 or .75 cents after the pump initially shuts off. Little do they know that overfilling the fuel tank could cost them a lot of time and trouble in the long run. When putting too much gas in the tank, fuel can end up where it shouldn’t be… in the charcoal canister. Inside the charcoal canister, there are tiny pellets of charcoal. The normal function of the EVAP charcoal canister is to temporarily store gasoline FUMES (not liquid gasoline) that evaporate within the fuel supply system (mainly from the fuel tank). When car owners overfill the fuel tank, the EVAP charcoal canister can be saturated with raw fuel instead of just fumes as it was designed. The liquid fuel can float the pellets of charcoal out of the canister and into the EVAP hoses. If pellets enter solenoids or orifices, they can be plugged and restrict the proper flow of vapors within the system. The fuel tank needs to breathe. Anything causing a restriction will prevent it from breathing properly reducing efficiency and sometimes causing early shutoff at the gas pump during fill-up.
Hyundai Elantra charcoal canister is located at the left rear of car.
Easy Check of an EVAP Canister
One easy way to look for a problem with the charcoal canister is to remove the EVAP hoses that attach to it. Sometimes the pellets will fall out of the hoses revealing the problem. Any solenoids in close proximity of the released pellets will more than likely be affected. Hoses will need to be blown out with compressed air. By blowing air through the hoses, venting can be checked. Make sure not to blow too much pressure into the tank since a leak from the fuel pump seal or from some other component could be created. If the tank is not venting properly it may actually swell when blowing air in through the hoses. It may be normal for the tank to expand a little and quickly contract when the air exits the tank. Feel and squeeze rubber hoses to check for orifices. The Hyundai Elantra in the picture had a plastic orifice which acted as a filter, capturing some of the pellets and preventing them from traveling further into the EVAP system. The canister and the purge solenoid was replaced and the orifice was cleaned and re-installed. The venting was restored to normal. The customer was advised NOT to overfill the tank in the future to prevent the same problem from re-occurring.
No Charcoal Pellets Found in Hoses
If experiencing “early pump shut-off” and there are no charcoal pellets found in the hoses there could be a problem in the fill neck itself or from a built in valve in the tank. Sometimes the problem can be pinpointed to a check ball sticking or a vent valve not opening properly. It’s always a good idea to check the service repair manual for specific vehicle information. Frequently there’s a known issue which is called out in a TSB (Technical Service Bulletin). Other times if a TSB is not put out by the manufacturer, a dealer tech may be able to help identify common problems with certain makes and models. If the tips laid out in this auto repair article did not help lead to the problem with your car, check out our other articles.
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