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Fuel Injection 101: Understanding Your 1991-94 Mercury Capri (or any FI car)

Updated on May 7, 2011
1991 Mercury Capri XR2 Turbo engine
1991 Mercury Capri XR2 Turbo engine

Nearly all cars since 1980 have fuel injection. Most are Multiport injection, most have similar components to regulate the fuel and the use of an ECU. If you are experiencing fuel problems or issues, it helps when troubleshooting, to know the components that impact fuel injection for the problem may not be the fuel injector, but in one of the components that impacts it.

Usually, each cylinder has one FI, which is attached to a fuel rail. The electric fuel pump provides the fuel based upon the various readings the ECU obtains. The ECU will always try for the best fuel\air ratio from many sensors including: cold and hot start, acceleration, deceleration, run mode. The ECU counts on all of the sensors to be working, if one fails, you will have problems.

The Manifold Absolute Pressure (MAP) is usually a three wire sensor on the intake manifold to measure changes in the intake manifold air pressure and then generates an electric signal to reflect the changed pressure. It is used to: adjust fuel delivery, spark ignitions and barometric pressure readings upon starting the engine.

The Mass Air Flow (MAF) is usually in the air intake duct or manifold and measures the incoming air mass. When obtained, the ECU calculates the required fuel for the specific air mass flow rate.

The Throttle Position Sensor (TPS) is mounted on the throttle body assembly and is actuated by the throttle shaft. This sensor is really a variable resistor (potentiometer) that sends a voltage signal to the ECU that is proportional to the throttle shaft rotation. When the throttle is open, the sensor emits a high voltage signal. At idle, the signal is .45V, at wide open throttle, 4.5 to 5V.

The Throttle Air Bypass Valve is located on the throttle body of most engines. This solenoid valve allows additional bypass air when the engine is cold or load conditions.

The Air Charge Temp Sensor is also located in the engine air intake to sense the air induced into the engine manifold. The signal is sent to the ECU and adjusts fuel levels during a cold start, modifies the spark advance and regulates acceleration enrichment.

The Idle Air Control Valve is located in the throttle body of the fuel injector systems.The valve consists of a stepper motor that adjusts to vary bypass air during idle and non-idle conditions. The ECU uses this information constantly to adjust the bypass air around the throttle plates. The following input signals also impact this valve: throttle position sensor, engine load, battery  voltage, engine coolant temp.

The Coolant Temp Sensor is attached into the engine block and has direct contact with coolant. It sends the ECU data that adjusts fueling levels and idle speed adjustment, modify spark advance, electric cooling fan functions, activation of EGR.

The Oxygen sensor is attached to the exhaust manifold and measures O2 content in exhaust gases and sends the ECU the info, which then makes adjustments to maintain the optimum air-fuel mixture for fuel economy.

The In-Line Electric Fuel Pump delivers a pressurized fuel amount to the injector system. In a multiport system, it is usually 43 psi at the fuel rail. If your car is before 1987, the fuel pump is usually external. If after 1987, the fuel pump is inside the fuel tank. If the pump or fuel level sensor fails the entire unit must be replaced.

The Fuel Pressure Regulator maintains the correct fuel pressure in the car's system and has a direct impact on the fuel metering of the fuel injector. If the fuel pressure is not to spec, you will have fuel delivery issues.

As you can see, if one or more sensors fail, for whatever reason, finding it your biggest problem and until you do, the fuel delivery to your car is compromised.


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