How to Test Fuel Injectors on a Mazda 626

Fuel injectors are close to the spark plugs in each cylinder.

Fuel injectors in the Mazda 626 vehicles connect to pressurized fuel lines to spray fuel into the combustion chamber. A solenoid is charged with electricity to briefly open a valve allowing the fuel to pass through the injector nozzles into the engine for combustion. The Mazda 626, first introduced in 1979, was equipped with electronic fuel injection in 1988. All Mazda 626 models have been equipped with fuel injectors since 1988. With the exception of OBD-II port reading -- available starting in 1996 -- the steps to test the fuel injectors remains the same for all models produced since 1988.

Things You'll Need

• Long-handled flat-head screwdriver
• Multimeter
• OBD-II reader

Instructions

Detecting Fuel Injector Pulse

1. Start the engine. Open the hood and locate the top of the fuel injectors. The fuel injectors will be connected to the fuel rail and in close proximity to the spark plugs.

2. Press the tip of a long-handled screwdriver against the top of the fuel injector.

3. Detect the presence of the valve and solenoid operation. The operation creates a ticking feeling or sound through the injector. Hold the screwdriver handle against your ear to amplify the sound. If the ticking is not present, the solenoid and valve operation is not functioning. Check other injectors to ensure you are accurately diagnosing the failure.

Voltage Detection

1. Remove the electrical plug from the fuel injector. Turn the ignition key to the start position but do not start the car.

2. Turn the multimeter to measure voltage. Touch the red multimeter probe to one side of the injector plug connection points -- there are two -- and the black probe to the remaining point.

3. Read the voltage value displayed on the multimeter readout. The correct voltage should be 12 volts. Replace injectors that are failing but receiving 12 volts of incoming current.

Resistance Test

1. Remove all the electrical plugs from the fuel injectors. Move the plugs away from the fuel injector to allow access to the blades on the inside of the injector's plug connector

2. Set the multimeter to measure "Ohm," which is a measure of resistance. Ohm measurement may be indicated by the Greek symbol Omega, which looks like an upside down horseshoe with two legs that jut out either side.

3. Connect the red multimeter probe to one of the blades on the fuel injector plug connection. Connect the black multimeter probe to the remaining blade. Write down the resistance measure displayed on the multimeter tool. Repeat this step on all remaining fuel injectors

4. Compare the resistance measurements of all the injector plugs. Any obvious resistance deviations indicate a failed or failing injector. The resistance measurements should be very close on all injectors. Replace failed or failing injectors.

Electrical Short Test

1. Disconnect all the electrical leads from the fuel injectors. Set the multimeter tool to measure voltage.

2. Connect the black multimeter probe to the negative terminal on the vehicle's battery. Connect the red multimeter probe to any fuel injector's electrical plug. Do not connect the red probe to the injector.

3. Have an assistant engage the starter motor for a few seconds. The disconnected fuel injector plugs prevent the engine from starting but allow the electrical current to pass through to the fuel injector's electrical plug. Monitor the voltage. The voltage should alternate between 0 volts and approximately 12 volts. Leave the multimeter leads connected to the battery and this plug for the next step.

4. Connect one of the remaining electrical plugs to the associated fuel injector. Have your assistant attempt to start the engine for several seconds. Monitor the voltage to ensure the original plug alternates between 0 volts and approximately 12 volts. If a fuel injector has an electrical short, when it is connected to the electrical system the original plug will show no alternation of current. Repeat this step until all the remaining electrical plugs, except the original being used for the test, are connected. To test the last injector, connect the electrical plug to the injector, then remove any other plug and test it as you did the original.

5. Replace any injector that causes electrical shorting.

Scan Tool

1. Connect an OBD-II reader to the OBD-II port. The OBD-II port is below the dash on the driver's side.

2. Turn on the reader. Turn the ignition key to the "Start" position. Follow the menu instructions on the reader to retrieve any diagnostic codes being stored by the on-board computer.

3. Interpret the diagnostic code. Many readers provide the interpretation on the menu screen. Others require interpretation through the user's manual or the OBD website. Failed injectors may be indicated by codes describing lean or rich conditions in specific cylinders. Other codes may specify injector failure. Test and replace any failed injectors.

Tips & Warnings

• Commonly, a "check engine" light is triggered by failed injectors. Diagnose check engine lights with an OBD II reader.

• Check for fuel leaks at the fuel rail before conducting electrical tests. Sparks can cause fires. Fuel leaks are caused by failed O-rings securing the injector to the fuel rail.

References

• Ohm Check: Troubleshoot Fuel Injectors
• Edmunds.com: Mazda 6 & 626 History
• OBD Clearinghouse: Vehicle OEM Database - 1996 Mazda 626

Resources

• RepairPal: Fuel Injector Replacement
• ThirdGen.org: Injector Swap
• OBD-II On-Board Diagnostic System: OBD-II Trouble Code Lookup