Why does the gas pump shut off before the tank is full?
When the gas pump shuts off before the tank is full, the problem is most often caused by a failure in the vehicle’s evaporative emissions system (EVAP). The EVAP system is designed to prevent gas vapors from venting into the atmosphere during fill-up. Instead, the gas vapor is pushed into a canister filled with activated charcoal. The charcoal adsorbs the gas vapor. It’s designed to adsorb the vapor from a typical fill-up. Once the engine starts, the system purges the gas vapor from the canister, emptying it and allowing it to adsorb fuel again the next time you fill-up.
Here’s how the EVAP system works during fillup
Here’s how the EVAP system works during the purge cycle
What goes wrong with the EVAP system?
The vent line or vent solenoid fails
That depends on the year, make, and model of the vehicle. In GM vehicles, for example, the most common problem is a clogged vent line or clogged vent solenoid. Spiders are attracted to the scent of fuel and build nests in the vent line, which eventually clogs the line or the solenoid, preventing fresh air from entering the canister during the purge cycle.
The purge valve fails or the purge line gets clogged with charcoal particles.
On many Japanese vehicles, the purge valve fails, preventing engine vacuum from sucking vapor out of the canister. In some cases, the charcoal canister fails, allowing charcoal particles to get sucked into the purge line and purge valve, also preventing proper purge operation.
One other cause of gas pump shuts off before tank if full
Every gas tank filler neck includes a vent tube. If the vent tube collapses or corrodes, that can cause the gas pump to shut off before the tank is full.
How to check your EVAP system
1). Disconnect the small vacuum hose going to the purge valve. Check for charcoal particles. If you find any, replace the charcoal canister and purge valve and flush the line to remove all particles.
2) Check the vent line for signs of spider nests, dirt or debris. If you find any, clean out the line and the vent valve.
3) Using a handheld vacuum pump, apply vacuum to a closed purge or vent valve to see if it holds vacuum. If so, apply battery voltage and ground to the solenoid to operate it and check for proper opening and closing.
©, 2020 Rick Muscoplat
Posted on by Rick Muscoplat