How to perform fuel pressure tests, a must read!

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How to perform fuel pressure tests, a must read!

Postby K MANIAC » Mon Oct 31, 2016 12:26 am

Our Lincolns, like all cars, need proper fuel delivery and proper spark in order to run effectively and efficiently. Drivability issues like stalling, surging, and failure to start can be caused by failures of ignition components and/or fuel system components. The symptoms of both ignition system failure and fuel system failure are the same, so before we start replacing parts, we need to first find out which system is failing. Working from the simple to the complex, the first system that should be analyzed is the fuel system.

Our Lincoln Mark VII's all use some form of electronic fuel injection. It is a simple system to analyze when we all understand how it works and how to properly test the system.

Our fuel injection systems contain the following parts:
An electric fuel pump inside the fuel tank, equipped with a filter screen on the fuel pick up (suction) end of the pump.
A fuel filter on the underside of the car just ahead of the passenger side rear tire.
A fuel rail on top of the engine that feeds the eight fuel injectors.
A pressure regulator on the passenger side of the fuel rail towards the rear of the engine

The fuel pump suck fuel from the tank through the filter screen and pumps the fuel first to the fuel filter. From the fuel filter, the fuel passes to the fuel rail. The fuel pressure regulator maintains a variable pressure on the fuel rail. Fuel exits the fuel rail at the fuel injectors. Excess fuel passes through the fuel pressure regulator and returns to the fuel tank through the fuel return hose. The fuel rail is equipped with a Schrader valve test port where a fuel pressure gauge can be attached to measure fuel pressure when the engine is running or when the fuel system primed without starting.

The fuel system can be primed without starting the engine by turning the ignition key on without starting. Every time the ignition key is turned on, the computer activates the fuel pump for two seconds, then shuts it off.

To demonstrate the fuel pressure tests, I decided to use my newest acquisition, which I have named Sandy, a 1989 Mark VII LSC. Sandy has not displayed any drivability issues, but since I have no service record and planned to replace the fuel filter out of general principle, I took the opportunity to run the pressure tests.

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The tool needed to perform fuel pressure tests in a fuel pressure test kit. The kit comes with several parts that can be mixed and matched to perform various tests.

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The first test to perform is the pressure test at the fuel rail test port. The fuel test port on a 1988 and newer HO's is located on the steel fuel line behind the alternator. On earlier cars, like my 1986 Continental, it may be on the rear of the engine by the fuel pressure regulator. The fuel test port is a Schrader valve and should have a black plastic cap.

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The cap should be removed.

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Attach the pressure gauge to the pressure hose in the kit and connect the second pressure hose with the Schrader valve connection.

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Then attach the hose assembly to the test port. If the engine does not start, then perform five "Key on/Key off's" to prime the system. If the engine is able to start and run, start the engine and disconnect the vacuum hose from the fuel pressure regulator. A proper functioning fuel system will generate 39 psi +/-2 psi of fuel pressure.

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As you can see, Sandy has a proper functioning fuel system. But if your fuel pressure is less than specified, a "Dead Head Pressure" test of the fuel pump is required to verify the condition of the fuel pump. This test is accomplished by installing the single nipple metal gauge connector into one end of a rubber fuel line hose with a hose clamp, attaching the pressure gauge to the nipple, then attaching the other end of the rubber hose to the metal line coming out of the fuel tank.

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These are the parts needed for the dead head pressure test.

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This picture shows a fully assembled fuel filter. The filter has two flexible lines attached. The end of the flexible line in the top of the picture is attached to the fuel line from the pump where the test line must be attached. The other end of this line is attached to the back of the filter, out of view. The flexible line attached to the front of the filter here is the second line that connects to the metal fuel line going to the engine.

The flexible lines are attached using plastic clips. Remove the plastic clips to detach the flexible lines.

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Once the filter and flexible rubber lines are removed, attach the free end of the rubber test line to the fuel pump steel line and secure with the hose clamp.

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Once this line is secured in place, perform five "key on/key off's" to prime the pump. A good working pump should generate 50-65 psi of fuel pressure into the pressure gauge.

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Wow, looks like Sandy has a good strong fuel pump. Should you measure a dead head pressure less than 50 psi at first, perform five more pump primes. If the dead head pressure is still less than 50 psi, it's time to consider replacing the fuel pump.

NEVER, EVER REPLACE A FUEL PUMP WITHOUT FIRST PROVING THE FUEL PUMP IS WEAK OR BAD WITH A DEAD HEAD PRESSURE TEST!

Proper diagnostic techniques will help keep our cars running and keep us from wasting time and money trying to guess which parts are bad.
"This car may be old, but it will still climb Kirker Pass at 110!"- quote of the original owner of my green 1964 Chrysler 300-K

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1986 Continental
1989 Mark VII Bill Blass (Oxford White)
1989 Mark VII LSC (Pewter Metallic)
1989 Mark VII LSC (Sandstone Metallic)
1/2 of a 1992 Town Car Signature (my guaranteed inheritance)
K MANIAC
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