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2005 Mustang 4.6L, P0456 code, gas won't fill

My 2005 Mustang w/ 4.6L is giving a P0456 code and also won’t let me fill it with gas completely. I think I’ve narrowed it down to the canister vent or purge valve. Looking for help narrowing down which would cause the gas fill issue.

There is a TSB on this car

It refers to a fuel pump orifice as a culprit BUT I don’t think that would affect filling the tank. I’d think purge valve

Since the purge valve is in the engine compartment in fairly easy reach, you can test its function. Just see if is sealed now and then power the pis w 12v and see if it opens. If not, that’s likely your problem.

Apparently the early S197 Mustangs had a lot of P0456 codes. Sometimes just opening the gas cap is enough to make it go away. Some have had fuel pumps replaced per the TSB.

P0456 is “small leak detected”. I believe this is only secondarily related to the difficulty filling the tank.

Define “won’t let me fill with gas completely”. Are you trying to “top off” the tank after the pump handles clicks off? If so, the system is operating properly. It’s designed to prevent you from “topping off” the tank. By doing so, it prevents the gasoline from contaminating the charcoal bed that allows the tank to vent without allowing hydrocarbon molecules to escape.

The purge valve allows hydrocarbon molecules captured by the charcoal bed while filling the tank or during normal operation to be “purged” (drawn) into the engine’s inductions system with the air that’s getting sucked in. It will not affect the ability to fill the tank.

However, if the charcoal bed is saturated with liquid gas, which usually happens from “topping off” the tank, IT will prevent you from being able to fill the tank properly by preventing the tank from being able to expel its air.

In short, you’re describing classic symptoms of a saturated charcoal bed (in the canister), with the exception of the P0456 code, which may simply be a gas cap leaking air.

Post back with your thoughts on this. It isn’t my intention to find fault, only to help solve the malfunction(s). If my assumption about “topping off” is incorrect, I’d like to continue to try to help. I can offer an in-depth description of the system if that will help.

Now worries, thanks for the feedback. By filling completely I mean I can only put about half of a tank in from empty.

I took it to Auto Zone for the free code scan and it gave that code with the suggestion to replace the purge control valve. Could an always closed purge valve cause the charcoal to get saturated and also cause a small leak in the evap system? I’ve seen reports of clogged vent valves stopping gas from filling in but I’m not sure that would give an error code for a leak.

As you pour gasoline into the gas tank the same volume of air has to exit the tank. Otherwise the pump will shut off. Pollution laws require it exits via the canister. The canister charcoal bed grabs hold of the gasoline molecules as it passes by, and lets the clean air exit to the atmosphere. But if the canister is saturated with gasoline it blocks off the exit path. To prevent this from happening, the canister is supposed to have its stored gasoline purged into the engine when the engine is running via the purge valve.

Another possibility, there’s a problem from the top of the gasoline tank to the canister. There’s a valve at the top of the gas tank on some vehicles that can fail.

How all this works exactly varies car to car. The following link might be helpful, check on the left for links under “you may also like” too.

It could cause saturation of the charcoal and subsequent inability of the vent line to allow the gas fumes to relieve the pressure caused by filling the tank, thereby preventing proper fill. Purging of the fumes, allowed by the valve, keeps the charcoal dry.

I don’t see any way it could cause the small leak.

The canister purge valve should run around $60, and unless I miss my guess it’ll be located under the hood and attached to its hoses with clamps and its solenoid with a plug. That’ll make it readily replaceable without special tooling or expertise. You might even be able to remove it and bench test it. The solenoid that opens and close the valve should operate with 12VDC and when activated the passage between the inlet and outlet should be open.

Stop by the Ford dealer’s parts window and ask nicely if they’ll print you an under-the-hood drawing showing the location of the canister purge valve and a bench test procedure for it. In my experience most parts window guys are happy to do this if you ask nicely.

Post back with your progress.

It is very unlikely that the purge valve is the problem. The purge valve is normally closed when refueling and not a factor. If the purge valve never opens during engine operation there will be a purge flow fault. If the line to the purge valve is disconnected there will be a purge flow fault, this system is monitored by the PCM.

Focus on the small leak and venting at the fuel tank for the fill problem.