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Gas nozzle shuts off before tank is full — most common cause

Gas nozzle shuts off before tank is full — Most common cause

Diagnose and fix a gas Gas nozzle shuts off before tank is full condition

If you’re having trouble filling your tank with gas, this might be the article for you. If the gas nozzle shuts off before the tank is full, chances are you have a problem with the evaporative emissions system in your vehicle. The problem shows up as the fuel nozzle clicks off before the tank is full. You try again and the fuel nozzle clicks off again. Each time you can only add a gallon or two before the gas nozzle shuts off.

The evaporative emissions system is responsible for absorbing fuel vapors during fill up to prevent those polluting vapors from escaping the tank. The evaporative emissions system consists of a charcoal filled canister, a purge valve, and a ventilation valve. In short, the activated charcoal in the canister absorbs fuel vapors as you fill the tank. Once you start the vehicle, a purge valve located in the engine compartment opens, allow the engine vacuum to begin sucking the vapors from the charcoal canister. At the same time, a ventilation solenoid valve opens, allowing fresh outside air into the canister. During this “purge” cycle, the engine is sucking the vapors out of the canister and burning them in the engine.

What goes wrong with the evaporative emissions system to cause a gas nozzle to shut off before the tank is full situation?

Charcoal canister failure

There’s a screen inside the charcoal canister that prevents charcoal particles from leaving the canister and entering the engine. If the screen ruptures, charcoal particles can get sucked into the purge valve and clog it. That prevents a canister from purging itself of fuel vapor. When this happens, the canister and purge valve must be replaced and the purge hose from the canister to the purge valve must be flushed.

Purge valve failure

The purge valve is a solenoid-operated valve. It can fail in the closed or open position. In many cases, it fails in the fully closed position, prevent the purge cycle. When that happens, the charcoal canister remains filled with fuel. So when you try to add fuel to the tank, the vapors have nowhere to go so they back up the filler neck, causing backpressure that shuts off the fuel nozzle.

Vent valve failure

If the vent valve fails to open or opens but is clogged, the canister cannot complete the purge cycle. The failure can be the result of a failed solenoid, clogged valve seat or obstruction in the vent hose to the atmosphere.

How to diagnose a gas nozzle shut off problem

Always start by checking the condition of the rubber hose that runs between the filler neck and the tank. Don’t limit your inspection to just the exterior of the hose. Tech are finding obstructions inside hoses that look perfectly fine from the outside. Any obstruction that restricts flow will cause enough back pressure to shut off the fuel nozzle. You must examine the hose through it’s entire length. If you find any obstructions, replace the hose.

Next, check for obstructions in the vent valve hose. Spiders are attracted to the scent of fuel and tend to build nests inside the vent hose. If you see any buildup, clean the vent hose. Check the vent valve for signs of spider webs of dirt.

Finally, power the vent and purge valve to see if the open and close properly

©, 2018 Rick Muscopl

Posted on by Rick Muscoplat

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