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TYPES OF FUEL INJECTION SYSTEMS
MULTI-PORT FUEL INJECTION
In multi-port fuel injection systems, each cylinder has its own fuel injector.
When triggered, the fuel injector sprays a fine mist of fuel directly into the
intake manifold.
SINGLE POINT INJECTION
Called Throttle Body Injection (TBI) by GM and Chrysler and Central Fuel
Injection (CFI) by Ford, this system works more like a traditional carbure-
tor. One or two fuel injectors are mounted on the intake manifold. When
triggered, they spray fuel into the throttle body.
PRE-TEST INSPECTIONS
Before testing the fuel pressure, make a thorough inspection of the
engine to rule out other causes of poor performance. Finding and fixing
a bad connection saves you the trouble of testing the entire system.
1. Inspect electrical connectors and wiring harnesses for
corrosion or bad crimps. Check for bent pins or pins that are not
seated properly.
2. Check all hoses and wires to make sure they are attached in the
proper locations. Inspect hoses for kinks, cuts, splits or breaks.
Inspect wiring for breaks, burnt or damaged insulation, contact with
sharp edges or contact with hot surfaces.
3. Inspect actuators and sensors to make sure they are free from
damage.
4. Check ignition wires for breaks or kinks. Look for damaged termi-
nals. Make sure spark plug boots are not cracked or split.
5. Make sure the battery is fully charged.
6. Check all fuses.
7. On vehicles with an inertia switch (certain models of Ford/Lincoln/
Mercury), make sure that the switch has not been tripped. (Refer to
owner's manual for location of the inertia switch.)
8. Make sure the vehicle has enough fuel to complete the testing.
9. Check for fuel leaks and clean up any spills with a disposable rag or
shop towel right away.
10. Inspect gas cap and fuel vapor recovery system for wear or damage.
11. Inspect the ignition and car computer system. If the engine will not
start and no ignition spark is present, check the vehicle owner's
manual for solutions.
12. Make sure that the manifold vacuum is within the specifications
required by the manufacturer.
PREPARING FOR TESTING
READ ALL SAFETY INSTRUCTIONS
BEFORE BEGINNING!
Move vehicle to a safe, well-ventilated area. Turn off engine. Automatic
transmission cars should be left in PARK. Manual transmission cars
should be in NEUTRAL. Block the vehicle's wheels.
Inspect area for open flames (candles, pilot lights, etc.). Extinguish
all cigarettes.
Clean up any fuel spills. Have shop rags available for any fuel spilled
during testing.
Have a working fire extinguisher rated for chemical/fuel/
electrical fires on hand.
Make sure you have the following easily available:
The service manual for your vehicle (if you do not have one, check with
the library or the vehicle manufacturer to get a copy.)
The engine code for your vehicle (The engine code is part of your VIN
number. Check the service manual for the correct digit.)
The connection point, type of test adapter and fuel pressure number for
your vehicle. (Refer to service manual.)
Lightweight household oil that is safe for adapter seals and rubber
O-rings.
DISABLING THE FUEL PUMP(S)
Check service manual for the number of
fuel pumps in your vehicle. Some vehicles will have more than
one. Failure to disable all fuel pumps before testing can lead
to fire, vehicle damage, personal injury and/or death. If your
vehicle has multiple fuel pumps, be sure to remove the fuse/
relay for each one.
All fuel pumps must be disabled before testing can occur.
The most common way to disable the fuel pump is to remove
the fuel pump fuse/relay. The fuel pump fuse/relay is usually located
in a fuse box under the dashboard or under the hood. The correct fuse/
relay should be clearly marked.
On some vehicles, disabling the fuel
pump fuse/relay will also disable the ignition. If this is the case
with your vehicle, you will have to find another way to disable
the fuel pump(s).
DISCONNECTING THE FUEL PUMP CONNECTOR
It may be possible to disable the fuel pump by disconnecting
the fuel pump connector at the pump itself. Some vehicles will
have the fuel pump mounted on the outside of the fuel tank,
easily accessible. Others may have the fuel pump out of reach
inside the tank itself. Consult the service manual for the
location on your vehicle.
DISCONNECTING THE INERTIA SWITCH
Some Ford/Lincoln/Mercury vehicles are equipped with an iner-
tia switch which will disconnect power to the fuel pump(s) in
case of an accident. Disconnecting the inertia switch will
disable the fuel pump(s) in these vehicles.
2
If the inertia switch is not easily available (check owner's
manual for location), another method will have to be used to disable
the pump(s).
RELIEVING THE FUEL PRESSURE
Pressure will remain in the system even when the
vehicle engine has been shut off. Failure to relieve
the pressure can lead to serious injury.
The system cannot be opened and checked until all residual pressure
is released.
TO RELIEVE PRESSURE:
1. Turn the ignition OFF.
2. Release the fuel tank filler cap.
3. Disable the fuel pump.
4. Start the engine.
5. Run the engine until it stalls. If the engine does not stall, the
fuel pump is not properly disabled or there is a second fuel
pump that must be found and disabled.
6. Try to restart the engine for 3 to 6 seconds. Repeat until the
engine will not start.
7. Turn the ignition OFF.
When the engine will no longer start, the fuel pressure has been
relieved. It is now safe to begin the testing procedures.
CONNECTING THE TEST GAUGE
Always consult the ser-
vice manual for any procedures unique to your vehicle.
These are general instructions which will fit most vehicles
in the given categories, but they may not be correct for your
particular vehicle.
1. Read all Safety Warnings and perform all Pre-Test
Inspections.
2. On GM TBI vehicles, remove the air cleaner and plug the
thermal vacuum port on the throttle body with masking tape.
3. Apply lightweight household oil to test adapters and rubber
O-rings.
4. Make sure ignition is OFF.
5. Remove the cap on the test port.
6. Wrap shop rag around the test port to catch any leaks.
7. FOR CHRYSLER/GM VEHICLES WITH
TEST PORT: Screw test gauge hose direct-
ly to the fuel rail test port until finger tight.
8. FOR FORD/LINCOLN/MERCURY VEHICLES WITH
TEST PORT: Attach Ford test port adapter to the fuel
rail test port until finger tight. Screw Chrysler/GM test
port adapter to the Ford test port adapter until tight.
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