On The Road

Working under pressure to solve Subaru Forester's shut-off problem

Last month we had a relatively new customer tow in his 2008 Subaru Forester XT 2.5 Turbo. He said the car shut off while he was driving it.

We asked him what he felt before this happened, and he said the car had a slight hesitation before losing power.

After we pushed the car into the shop, we checked all the basics (fuel and spark). The ignition system had spark, but the fuel system had no pressure. We then sprayed a small amount of starting fluid into the intake, and the engine started. Next, we checked power and ground to the fuel pump; it had power and ground.

I called the customer and explained to him that we had to replace the fuel pump and fuel filter. The reason we always replace the fuel filter with the pump is that if there were a restriction in the fuel filter or dirt in the fuel lines, it could cause the new fuel pump to fail prematurely. I have replaced hundreds – if not thousands – of fuel pumps in my career, but what unfolded next I had never seen.

Most modern fuel pumps fit inside the fuel tank and come as an assembly. We primarily use original equipment supplier fuel pump assemblies because the aftermarket ones do not work well. After installing a factory fuel pump assembly, the car started up on the first turn of the key. We test-drove the car and all was good, so we called the customer to come pick it up.

Diagnosing the problem

The next day, the customer called to tell us the car died on the road again and was being towed to the shop – something I never want to hear. I began running through all of the possibilities on what could have happened, but I had to wait for the car to arrive. The first thing we checked for was fuel pressure – and there was none. We pulled out the fuel pump assembly and found something strange. The fuel pump motor had come apart. I had never seen one come apart like this, especially a factory one. That’s when I knew something else was happening.

We received another factory fuel pump and installed it. We then connected the fuel pressure gauge and saw that the fuel pressure was at 98 psi, which is way too high. At that moment, noting that there were no restrictions, we moved on to the fuel pressure regulator.

The fuel pressure regulator is installed in the fuel supply line. It is a small device about the size of two golf balls. It has a spring chamber and a fuel chamber. The spring chamber receives vacuum from the intake manifold. There’s a diaphragm at the end of the spring. As the engine vacuum is increased or decreased (acceleration or deceleration), the diaphragm moves back and forth. On the other side, the fuel chamber is connected to the fuel supply line and a fuel relief valve. The relief valve serves to bleed off unneeded fuel pressure. As the engine vacuum increases, the diaphragm pulls open to decrease the fuel pressure; when the engine vacuum decreases, the diaphragm is pushed to increase the fuel pressure. The reason for all of this is to keep the fuel pressure between 40 and 60 psi.

When I saw the 98 psi with a factory fuel pump, I realized that the fuel pressure regulator had failed and was delivering full fuel pressure through the relief valve back to the fuel pump. I called the customer and explained what we found. We then replaced the fuel pressure regulator and test-drove the car for more than 100 miles with the fuel pressure gauge connected. The system worked to specification.

Fuel-pressure regulators in general are robust and rarely cause problems. This was one of only a handful I had to replace over a long period of time. The other thing was that the old fuel pump did not come apart like the new one did. I feel this was because the old fuel pump had more than 200,000 miles on it and was weak. The old fuel pump could not build as much pressure to tear itself apart.

In the future, we plan to check the fuel pressure after we replace fuel pumps on Subarus with more than 100,000 miles on them.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email This email address is being protected from spambots. You need JavaScript enabled to view it. or visit sunnyvaleforeigncar.com.

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