Repair Info - Emissions Repair Explained

Overview | Component Testing - Doing It Right | Fuel System Testing

Fuel System Testing

To test the fuel system to determine if the correct air/fuel ratio is being delivered to the engine, a number of tests may come into play.

Older vehicles have carburetors and some parts of those carburetors are adjustable. If internal carburetor repairs are needed, some shops may prefer to replace the entire carburetor. However, this is usually not the most effective option from either an emissions perspective or a cost perspective. For best results, it is strongly recommended that your vehicle’s original carburetor be repaired rather than replaced.

Some vehicles built from 1980-1987 are equipped with feedback carburetors. This means that they have an oxygen sensor that measures the exhaust oxygen content and sends an electrical signal to an on-board computer, which continually adjusts the amount of fuel that is delivered by the carburetor. The oxygen sensor and the mixture control solenoid are electrical components that are critically important to correct operation and minimal emission levels.

For most vehicles built from 1988-on, a fuel injection system is used rather than a carburetor. All late model fuel injection systems are feedback controlled so that the proper air fuel ratio can be maintained for all conditions.

Carburetor Tests

The vast majority of CO problems on carbureted vehicles are caused by the carburetor itself. In most cases the problem can be remedied once it is pinpointed exactly where the fault lies.

There are quite a few things that can go wrong with carburetors. Therefore, there may be a lot of things that the technician will need to check:

incorrect adjustment of idle mixture
incorrect adjustment or operation of metering needle(s)
incorrect adjustment or operation of float
incorrect adjustment or operation of choke
operation of power valve relative to engine load
leakage of fuel from accelerator pump
leakage of fuel from casting plugs
leakage of vacuum from gaskets
restriction of air bleeds and emulsion tubes
wear of metering needle(s) and main metering jet(s)

A careful visual inspection is all that is required to check for most of these possibilities. However, performing a visual inspection may require disassembly of the carburetor.

Although there are a lot of things that can go wrong with carburetors, the technician can often eliminate some of the possibilities just by reviewing the inspection data and applying some logic.

Feedback Carburetor Tests

Many of the tests in the previous section also apply to feedback carbureted vehicles and, for the most part, the same logic can be used to narrow down the possibilities. However, the feedback system adds several more considerations.

If your vehicle has a feedback carburetor, the diagnostic technician will need to test the O2 Sensor and the mixture control device on the carburetor (usually an electrically actuated device). For more info regarding the testing of O2 Sensors see Oxygen Sensor Testing.

Fuel Injection System Tests

In most cases, fuel control problems on fuel injected vehicles are caused by a defective O2 sensor.

If O2 Sensor testing has been performed and the defect is still not evident, testing of the other fuel injection system components should be performed. An overview of some of these component tests follows.

Airflow Sensor

Many fuel injected engines use an airflow sensor to measure how much air is being drawn into the engine. The measured airflow is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired air/fuel ratio.

Problems with the airflow sensor or its electrical circuit result in incorrect fuel calculations by the vehicle's on-board computer.

Airflow sensors are tested by measuring the airflow indicated to the computer for various operating conditions and comparing to manufacturer's specifications. This is usually best done using an oscilloscope or voltmeter.

MAP Sensor

Many fuel injected engines use a manifold absolute pressure (MAP) sensor to measure air pressure in the engine's induction system. The measured vacuum is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired air/fuel ratio.

Problems with the MAP sensor or its electrical circuit result in incorrect fuel calculations by the vehicle's on-board computer.

MAP sensors are tested by measuring the vacuum indicated to the computer for various operating conditions and comparing to manufacturer's specifications. This is usually best done using an oscilloscope or voltmeter.

Coolant Temp Sensor

All fuel injected engines use a coolant temperature sensor (CTS) to measure the temperature of the engine. The measured temperature is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired smooth engine operation when the engine is cold and during warm-up.

Problems with the CTS or its electrical circuit result in incorrect fuel calculations by the vehicle's on-board computer.

Coolant temperature sensors are tested by measuring the coolant temperature indicated to the computer for various operating conditions and comparing to manufacturer's specifications. This is usually best done using a scan tool or voltmeter.

Intake Air Temp (IAT) Sensor

Many fuel injected engines use an air temperature sensor (ATS) to measure the temperature of the air being drawn into the engine. The measured air temperature is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired air/fuel ratio.

Problems with the air temperature sensor or its electrical circuit result in incorrect fuel calculations by the vehicle's on-board computer.

Air temperature sensors are tested by measuring the air temperature indicated to the computer for various operating conditions and comparing to manufacturer's specifications. This is usually best done using a scan tool or voltmeter.

Throttle Position Sensor

Many fuel-injected engines use a throttle position sensor to determine the driver's demand for power. The measured throttle (accelerator) position is indicated to the vehicle's on-board computer. This input is used to calculate how much fuel should be delivered to maintain the desired air/fuel ratio.

Problems with the throttle position sensor or its electrical circuit result in incorrect fuel calculations by the vehicle's on-board computer.

Throttle position sensors are tested by measuring the throttle position indicated to the computer for various operating conditions and comparing to manufacturer's specifications. This is usually best done using an oscilloscope or voltmeter.