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4. GENERAL INFORMATION
There are two basic types of fuel injection systems. Port Fuel Injection (PFI) uses separate injectors to supply the fuel to each cylinder.
Throttle Body Injection (TBI) injects fuel from a position above the throttle plate on the intake manifold. On both systems there is a supply side,
which brings fuel to the injectors, and a return side which brings unused fuel back to the tank.
Basically, there are three ways to check fuel pressure. First, many domestic cars with PFI are equipped with a special test port. Simply
connect the proper adaptor to the gauge assembly, thread the adaptor to the test port, and run the test (section 15). Second is an end of hose
connection. Some older PFI systems have a flexible hose connection at the cold start injector. Connect our single barb fitting (section 18) with a
hose clamp to run the test. Also, some systems have fuel bolt or banjo type fittings as an access point (section 18).
Third is in-line connecting. This means installing the proper adaptor(s) in series with the fuel line (sections 16, 17 & 19).
Unless a schrader-type test port is available, most manufacturers require that you relieve the fuel pressure before entering or leaving the system.
To relieve the pressure, it may be necessary to remove the fuel pump connector, relay, or fuse. Some models may have two fuel pumps - make
sure both are disabled. After the pump(s) are disabled, run the engine until it stalls, then try to restart it for five to ten seconds. The system is
now ready for testing. Once the proper adaptors and gauge are in place, reactivate the fuel pump and run tests.
When finished testing, repeat above procedure before removing tester.
5. BASIC DIAGNOSIS
Fuel injected engines require precise fuel pressure as well as adequate volume. Without the correct pressure and volume, performance and fuel
economy can suffer. Always consult the correct repair manual for accurate specifications.
When running tests, it may help to picture the fuel system as a circle. Fuel is pumped from the tank to the fuel regulator and injectors, and the
unused fuel is then returned to the tank. The fuel regulator serves as a divider between the supply side and the return side.
Lower than recommended pressure usually indicates a problem on the supply side of the circle. Likely causes would include a damaged or
restricted fuel line, clogged fuel filter, defective fuel pump or regulator, or improper tank ventilation. Higher than normal pressure usually indicates a
problem on the return side of the circle. Likely causes are a damaged or restricted fuel line, poor venting of the tank, or a defective fuel regulator.
You may be able to pinpoint problem areas on the return side by resetting. For example - by removing the return line near the fuel regulator and
putting the fuel line into a proper container, a retest that still shows a high reading would indicate a faulty regulator. If the reading would drop into
the normal range, you know that problem is further down the return line or the tank. Again, always consult the manufacturer of the vehicle, or a
good repair manual for specific trouble-shooting procedures. When testing is completed, make sure the fuel line is reassembled correctly. Replace
any o-rings or washers, and follow the manufacturer's recommendations for proper torque on any bolts or connections. Check the entire system
thoroughly for any leaks.
Following are some typical test procedures for different models.
6. FUEL PRESSURE TESTS
GENERAL MOTORS (TYPICAL)
CAUTION: Fuel system test begins with checking fuel injection system pressure. High fuel pressure may be present in fuel
lines and components. Relieve fuel pressure before disconnecting any fuel system components.
6.1.
FUEL PRESSURE CHECK FOR TBI
6.1.1.
Disconnect negative battery cable. Remove fuel filler cap. Since these TBI units contain an internal bleed-down feature, after a short
time, system fuel pressure should dissipate.
6.1.2.
Remove air cleaner and plug thermal vacuum port on throttle body. When removing fuel line, always use 2 wrenches. Install Fuel
Pressure Gauge and Adaptor in fuel system between steel line and flexible hose.
CAUTION: DO NOT pinch off fuel return line completely. DO NOT exceed pressure build-up of more than 13 psi. as regulator
may be damaged.
6.1.3.
Start engine and observe fuel pressure reading. Fuel pressure should read 9-13 psi. (if fuel pressure is okay, proceed to step 4). If fuel
pressure is low, gradually pinch off fuel return line to fuel tank. If pressure remains low, check and/or replace fuel filter or fuel pump. If
pressure increases to greater than 13 psi, replace fuel pressure regulator.
6.1.4.
Allow fuel pressure to dissipate. Remove pressure gauge and reconnect fuel line. Start engine and watch for fuel system leaks.
6.2.
FUEL PRESSURE CHECK OF PFI
Disconnect fuel pump at rear body connector (electrical). Start engine and run engine until it stalls. Crank starter for 3 seconds to
remove remaining fuel from fuel lines. Reconnect rear body connector.
Connect Fuel Pressure Gauge to fuel pressure fitting on fuel rail.
Reconnect fuel pump.
With gauge installed at fuel rail connector, turn ignition on. With ignition on and engine off, pressure should read within specifications.
Start engine. Pressure should drop 3-10 psi.
7. FUEL PRESSURE TEST
FORD (TYPICAL)
CAUTION: High fuel pressure may be present in fuel lines and component parts. Relieve pressure before attempting to open
system for testing or component replacement. DO NOT allow fuel to run onto engine or electrical parts or allow an open
flame in area while testing fuel system components.
7.1.
FUEL PRESSURE CHECK FOR PFI
7.1.1.
On 2.2L & 2.2L Turbo models, relieve fuel line pressure by disconnecting fuel pump relay and starting engine. After engine stalls, turn
ignition off. Reconnect fuel pump relay.
7.1.2.
On all other models, remove fuel tank cap. Using Fuel Pressure Gauge, release pressure from system at pressure relief valve
(Schrader valve) on fuel injection manifold rail.
7.1.3.
The fuel pump may be activated by grounding the fuel pump lead at the SELF-TEST connector. Use a jumper lead and ground the
"FP" terminal with ignition on. This activates the fuel pump.
CAUTION: Inspect fuel system for leaks or damage before testing fuel pump.
7.2.
FUEL PRESSURE TEST FOR TBI
7.2.1.
Disconnect wiring at inertia switch. Inertia switch is located behind trim panel in right rear side of cargo area on Sable and Taurus
station wagons and behind trim panel on left side of trunk on all other models. Crank engine for at least 15 seconds to reduce system
fuel pressure.
7.2.2.
Disconnect fuel supply line at throttle body. Install in-Line Adaptor and fuel Pressure gauge at fuel filter. Reconnect intertia switch and
start engine. Check fuel pressure at idle and when accelerating engine.
© Jack Sealey Limited
Original Language Version
VSE210 Issue 2 (HF)
08/09/21
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