• 2018

Too Much In The Tank

What to know when filling your fuel tank.

Putting too much fuel in a vehicle can be damaging. Let’s look at this task in a little more detail. Refer to Figure 1 for our discussion.

We’ll skip past the preliminaries at the pump and assume that the tank will be filled to Full. Open the fuel door, remove the gas cap and insert the gas pump nozzle into the fill tube (9) . Pull up on the handle and gasoline begins flowing into the fuel tank. Often, the nozzle handle is propped up with that small, metal lever. When the tank is full, a click is heard, the metal lever is released, the nozzle shuts off and the fl ow of gasoline stops. Or, if the handle is being held up by hand, the click will still be heard, and the nozzle still turns off.


The fuel system on all modern automotive vehicles is a closed system, preventing gasoline vapors from escaping into the atmosphere from anywhere within this system. It’s important to capture the fuel vapors when filling the tank.

In the old days of carburetors and mechanical fuel pumps, the fuel system stayed open. As you filled the gas tank, the air that was displaced in the fuel tank would escape into the atmosphere along with gasoline vapors. The system had to be open in order to allow fuel to be drawn down through the carburetor. The gas cap was vented, allowing the fuel system to breathe and vapors to escape naturally. So, in order to prevent air pollution, the closed fuel system was developed.


Figure 1 illustrates a basic closed fuel system. The fuel tank is vented through the check valve (5) . As gasoline flows into the fuel tank, the air in the tank has to go somewhere. In this system, the air and the fuel vapors flow through the vent valve, through the canister tube and are collected in the vapor canister (3) . The system is vented, but not in the same way as an open system. Fresh air enters the fuel system through the fresh air inlet (1) . This air enters the evaporative canister  and purges the fuel vapors. The vapors are routed to the intake manifold through the purge solenoid (8) to burn with the air/fuel mixture in the engine. The evaporative system integrity monitor (ESIM) (2) prevents fuel vapors from escaping through the air inlet.

The closed fuel system is comparable to a drinking straw. The top of the straw is the air inlet. Place the straw in a container of water, place your thumb over the end of the straw and remove it from the container. The liquid stays in the straw until your thumb is removed. In a car, opening the air inlet is the same as removing your thumb from the straw.


When a fuel tank is being filled, fuel flows down the fill pipe, creating an aspiration effect. This effect draws air into the fill tube along with the fuel.

As mentioned, the fuel tank vents the vapor canister as the incoming fuel displaces the air in the tank. When the tank is filled, (a) a float in the control valve rises to close the vent path to the canister, (b) the tank pressure increases, (c) the fuel rises up the fill tube and (d) covers the sensing port in the nozzle (a clicking sound is heard) and shuts off the fuel flow.

Many times, the vehicle owner will continue to add fuel into the tank (known as topping off). This is done for several reasons (e.g., to round off the price, fill the tank with as much fuel as possible, etc.). That’s where the problem can occur. In the old days, if you did that, fuel could run up the filler tube and spill out onto the car. In the modern vehicle, that fuel spills over into the overfl ow tube (the opening is just below the top of fill tube). And that can cause damage to the vehicle’s vapor collection system.

This excess fuel does not find its way into the fuel tank; rather, it backs up past the fuel tank pressure sensor (7) . If enough fuel flows into the overflow tube, it can back up all the way into the vapor canister. This excess gasoline cannot enter the fuel tank through the one-way control valve. Now a minor issue can become a huge headache.


The next time the vehicle is refueled, a hard to fill problem is likely to occur. The symptoms of this problem include (a) the fuel nozzle shutting off quickly after a small amount of fuel is dispensed, or (b) the fuel is dispensed at a very slow rate, much slower than normal. In addition, the check engine light might be illuminated. If you scan for Diagnostic Trouble Codes (DTCs), you are likely to see either P0440 (General Evap System Failure) or P0441 (Purge System Performance).

Here is the core of the problem. In the closed fuel system, the valves and surge solenoid must operate correctly and the vent lines cannot become clogged or blocked. If either of these occur, back pressure problems follow (such as the hard to fill condition). In extreme cases, when a signifi cant amount of fuel has backed into the vapor canister, a vapor lock condition can occur. In this situation, fuel cannot flow and the engine will become fuel starved. It’s not common, but this scenario has happened.

When a customer brings a vehicle into your shop with the hard to fill issue, or the MIL is illuminated and you detect one of the codes previously mentioned, the fi rst thing you should ask them is, “Do you top off your gas tank when refueling?” Their answer will be the basis of your diagnostic approach.

If you’re unable to ask this question, just start by checking the EVAP system. Look for any kinks in the vent hoses. Remove the fuel tank pressure sensor (7) . If there is gasoline in this valve, you can be sure your customer is topping off the fuel tank.

Also, check the vapor canister. Remove it from the vehicle (often this is mounted above one of the rear wheel wells). A strong odor of gasoline indicates liquid fuel is present. If you compare the weight of the canister to a new canister, the new canister will be noticeably lighter. In some cases, this excess fuel will damage the ESIM, requiring its replacement.

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