The root cause is the S-197 Onboard Refueling Vapor Recovery System (ORVR) is not sufficient to provide proper venting of the fuel tank while refueling under the variety of conditions encountered.
What is actually happening in the Mustang's fuel system at the time of this frustrating fuel cutoff is the fuel tank pressure rapidly increases preventing fuel from flowing down the filler neck and causes the filling station pump to shutoff. Besides personally owning two of these S-197 Mustangs, I see them in my shop all the time. With patience and a good scan tool, you can see the problem. Note the Fuel Tank Pressure (FTP) spike to 4.38 Hg and Fuel Tank Pressure Voltage FTP(V) for to 3.24V in the graphs during re-fueling:
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Ford is well aware this is a design problem with the Onboard Refueling Vapor Recovery System (ORVR) but will not retrofit these vehicles. The Ford Technical Service Bulletins (TSBs) do not provide an effective solution that will allow the fuel tank to be filled every time. With all due respect to Ford, the language in the TSBs describes a vehicle that cannot be reliably re-fueled and the blame is pointed at the same filling station pumps that are refueling every other vehicle that pulls up. If the filling station pumps work properly for other vehicles on the market, they should work for these Mustangs.
The On-board Refueling and Vapor Recovery (ORVR) system has a fair number of parts, lots of plumbing, and electronically activated valves. Please excuse the hand drawn diagram (I barely passed 1st grade art class), but the service data diagrams do not show the entire system and its interaction with the fuel, EVAP, and engine management systems. Pay close attention to the location of the atmospheric vent on the right side of the tank and the fuel filler pipe on the left side of the tank. These will come up several times in the discussion of why the fuel pump cutoff occurs.
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The parts of the ORVR system that contribute to the fuel tank pressure spikes include:
- Poor fuel filler design. The filler pipe is narrow (5/8" to 3/4") with a severe S-bend before it reaches the tank. There is a spring-loaded plug at the top of tank built into the fill neck that closes the fill pipe until fuel pushes the plug down to open the fuel tank. A filling station fuel pump with high pressure and flow can easily overwhelm the small volume of the filler pipe and cause the fuel pump to automatically shut off or cause fuel spillage out the filler neck.
- Poor fuel tank design. The saddle style tank has two sections that "saddle bag" over the drive shaft and the opening between the two tanks is very narrow and at the top of the tank. The fill pipe enters the left side of the tank and the tank vent is located on the right side of the tank. As fuel fills the left side of the tank and begins to flow over the narrow top hump, the venting crossing from the right side of the tank to left is reduced causing the pressure to rise quickly inside the tank on the left side. At this point the only way to vent the left side of the tank is back up the filler pipe and against the flow of fuel from the filling station pump.
- Insufficient tank venting system. The vent hose from the right side of the tank is about ½" in diameter and must pass through the charcoal cannister to get to the canister vent valve (CV). The cannister vent valve further reduces the vent size and connects to a dust filter that has a ½" hose that winds up and around to get to the atmosphere to externally vent the right side of the tank. The capacity of this vent system is not sufficient to vent the fuel tank during all refueling conditions.
In summary, the OVRP design on the 2005 to 2007 Mustangs is not sufficient to prevent excessive fuel tank pressure build-up during refueling. Poor design of the double-hump fuel tank, small diameter fill tube, canister vent valve (CV) size, and narrow, twisting tubes/pipes all contribute to excessive fuel tank pressure during refueling causing the filling station fuel pump to automatically shut-off.
The 2008 and later S-197 Mustangs are much less likely to experience this issue because additional venting was added to the OVRP system in those vehicles.
Unfortunately, I only know of two 100% solutions to the fuel fill cutoff problem:
- Retrofit the entire ORVR system with the redesigned 2008 forward parts. This will be prohibitively expensive and the parts are no longer readily available because of the age of these vehicles.
- Add an additional fuel tank vent that can be opened during refueling to counter-act all the design flaws and allow the fuel tank to be quickly filled to capacity. After filling, the valve can be closed and the EVAP part of the system will perform as designed. If Ford does this, it is OK. If you perform the modification yourself, it could be considered "tampering" and possibly illegal by the EPA.
In a just and fair world, the multitude of 2005 to 2007 Mustang owners who complained to the NHTSA and EPA about this fueling problem should have resulted in a Ford recall to retrofit these vehicles with the 2008 and newer ORVR system. Ford is only required to keep parts inventory to repair vehicles for 10 years and these vehicles are now in the 20-year age group, so parts are going to be hard to locate. It is also going to be expensive and Ford isn't going to do it voluntarily.
How do I know the additional tank vent will work? That is a fair question given I'm just an independent technician, not a Ford engineering insider. After suffering with this issue for 19 years from the second week I owned my brand-new Mustang and trying every recommendation in all the Ford TSBs as well as some internet fixes, I was disgusted enough to consider getting rid of the car. But, in spite of this problem and a few others (broken spark plugs, water leaks into the cabin, sloppy shifter, etc.), these are really good cars and I love Mustangs. During a slow time in the shop, I made it my personal mission to find the root cause of this problem and come up with a solution.
If you look back at the ORVR diagram above, putting a temporary vent in the vapor line between the Fuel Tank Pressure sensor and the Charcoal Canister will give the fuel tank plenty of venting during refueling to prevent fuel tank pressure build-up on the right side of the tank.
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Above: Temporarily splicing a Y-Connector into fuel tank vapor hose between the FTP sensor and the Charcoal cannister in the ORVR system.
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Above: Temporary auxiliary manual fuel tank vent valve installed in 2006 Mustang GT. This allows testing extra fuel tank venting to prevent filling station pump cut out during re-fueling.
The scan tool screen shot below shows filling the fuel tank on a hot car with the temporary auxiliary vent open. Look at the Fuel Tank Pressure (FTP) and Fuel Tank Pressure Voltage FTP(V) graphs and values.
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Above: After opening the 5/8" manual auxiliary fuel tank vent valve, the pressure spikes to 2.0 Hg and the extra fuel tank venting immediately reduces fuel tank pressure to less than 2.0 Hg to allow the tank to be filled with fill handle at full pressure.
The extra fuel tank venting allows easy, no-hassle filling of the 2005–2007 Mustang fuel tank under almost all conditions: after hot idling for 15 minutes, during 98°F summer weather, ¼ full fuel tank, winter weather, etc. The only exception is beginning fueling with a greater than ¾ full fuel tank. At this level, the back pressure from a hot fuel tank can slow the transfer of fuel between the left to right side of the tank enough to cause a fuel pump shutoff. This can usually be overcome by reducing the flow rate from the filler hose. If the auxiliary vent also went to the right side of the fuel tank, the greater than ¾ full fuel tank fill-up would not be a problem because both sides of the tank would have sufficient venting.
I cannot recommend owners cut into the ORVR vapor line to install their own manual vents, but the above temporary modification is provided with the scan tool results to prove the appropriate fuel tank venting in the ORVR system will reduce fuel tank pressure and support refueling. I do know this is a solution to the problem and as loyal Mustang owners, we deserve a solution that works 100% of the time. I recommend you continue to report your refueling issues to the EPA and the NHTSA and ask them to take action on this problem.
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