Sensors Help Diagnose The Problem

In last month’s column, I extolled the virtues, but also highlighted the problems of using the 4.2L Ford fuel-injected engine in GSE equipment. This month, I want to briefly explain how this fuel-injection system works, but also offer more advice on how...


In last month’s column, I extolled the virtues, but also highlighted the problems of using the 4.2L Ford fuel-injected engine in GSE equipment. This month, I want to briefly explain how this fuel-injection system works, but also offer more advice on how the system’s various sensors operate.

As with any subsystem on a piece of GSE equipment, understanding how the system is designed to operate goes a long way to efficiently diagnose and repair that system. Without that knowledge, we are often just “shooting in the dark.”

The fuel-injection system is obviously designed to replace the carburetor. This system needs to perform several different functions. It needs to be able to provide cold enrichment (choke), cold start, high idle (high idle cam), consistent warm idle (base idle setting), and apply power based on throttle pedal position (accelerator linkage). In order to accomplish this, the sensors and fuel injectors replace the mechanical systems of the carburetor.

The amount of time that the injectors are turned on for each engine revolution determines the amount of fuel delivered to the engine. This amount of time is very small. At idle, the injectors may open for only 8 milliseconds (0.008 seconds). Under heavy load at wide-open throttle, this time could be just 40 milliseconds (0.04 seconds).

Engine Control Module

The Engine Control Module is able to react in milliseconds to varying circumstances, such as a change in throttle input or either an increase or decrease in engine load and engine temperature, to ensure it delivers the correct amount of fuel.

In order to accomplish this and make the calculations, the ECM needs input from its sensors:

The Engine Coolant Temperature Sensor tells the ECM the engine temperature so that it can provide the proper amount of cold enrichment. If this sensor does not work or is somehow inaccurate, the vehicle can run too rich or too lean.

The Throttle Position Sensor is built into the pedal and is actually two different sensors that work in conjunction with each other to tell the ECM what position the driver has the fuel pedal in as well as how rapidly the driver is changing the pedal position. A problem with one of the sensors in the pedal will usually result in a trouble code, and a vehicle that will only idle or barely creep along when the pedal is put to the floor.

The Manifold Absolute Pressure Sensor measures the amount of vacuum in the intake and is used as a measure of engine load. Remember that when the unit is at wide-open throttle and is pulling a load, the engine vacuum is around 6 inches of Hg; when it is decelerating, it can be as high as 22 inches of Hg.

The Oxygen Sensor is part of the emission control system built into the engine. It monitors the amount of oxygen left over in the engine’s exhaust system after combustion. The ECM uses it to determine if the engine is running rich or lean and adjust fuel delivery to fine-tune the air/fuel mixture and, hence, maintain the lowest emissions possible. Codes for the oxygen sensors are some of the most common. Much of the time, however, the code is not set because the sensor is bad. It is set because of a problem upstream that causes the engine to run too rich or lean, which causes the Oxygen Sensor to be stuck at a given voltage for an extended period of time.

Understanding the sensors is the first step in understanding how the system operates. Do not forget, however, that the trouble codes for these sensors are, in fact, trouble codes for these circuits. These trouble codes mean that the ECM is not seeing what it thinks it should from a given sensor.

This does not necessarily mean the sensor is bad, but it does mean there could be a problem with the circuit, such as a broken wire or poor connection. In future columns, I’ll discuss how to diagnose the specific codes related to these sensors and the types of drivability problems they cause.

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