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Subaru Fuel Pumps

Lately I have been writing articles on things that I have never seen before in my 28 years of working on cars and this article will be no exception. About a month ago we had a relatively new customer tow in his 2008 Subaru Forester XT 2.5 Turbo.   The customer stated that he was driving and the car shut off.  We asked the customer what he felt and he stated that the car had a slight hesitation and then just lost power. 

After we pushed the car into the shop we started to check 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 ran. We then checked power and ground to the fuel pump. The fuel pump had power and ground.

I called the customer and explained to him 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 was 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 as the aftermarket ones do not work well. We installed a factory fuel pump assembly and the car started up on the first turn of the key. We test drove the car and all was good. Called the customer and he came and picked it up. 

The next day the customer called in an told us that the car had died on the road again and it was being towed to the shop, which is never something I want to hear!  I began running through all of the possibilities on what would've happened but I had to wait for the car to arrive. The first thing we check for was fuel pressure and yep -  there was none. We pulled out the fuel pump assembly and what we found was strange. The fuel pump motor itself had come apart. Now I have seen hundreds if not thousands of fuel pumps but I had never seen one come apart like this,  especially a factory one. I then knew that something else was happening.  

We received another factory fuel pump and installed it. We then connected the fuel pressure gauge and saw the the fuel pressure was at 98 psi which is way too high.  At that moment I knew we had to check for a restriction and we also had to check the fuel pressure regulator. After seeing 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 the intake manifold. At the end of the spring is a diaphragm. As the engine vacuum is increased or decreased (acceleration or deceleration) the diaphragm will move 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 is to bleed off unneeded fuel pressure.  As the engine vacuum increases the diaphragm is pulled open to decrease the fuel pressure and when the engine vacuum decrease the diaphragm is pushed to increase the the fuel pressure.  The reason for all of this is to keep the fuel pressure between 40 to 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 the the fuel pump.  I called the customer and explained what we found.  We then replace the fuel pressure regulator and test drove the car over 100 miles with the fuel pressure gauge connected. The system worked to specification.  


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

In the future we plan on checking the fuel pressure after we replace fuel pumps on Subarus with more than 100,000 miles on them.  Thanks for reading.

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