BMW fuel injectors connect to high-pressure fuel lines to deliver a fine mist of fuel for combustion. Injectors on some BMW engines may accommodate significantly higher fuel pressures than you would find in most other injectors. Regardless the fuel pressure used and the durability of the injector, the fuel injectors of a BMW operate in the same way as in other vehicles with fuel injection. The fuel injector solenoid, triggered by an electrical signal, opens a valve, allowing pressurised fuel to pass through nozzles to be injected into the cylinder.
Start the BMW's engine.
Hold the pad of a stethoscope against the top of the fuel injector. Listen through the stethoscope to the fuel injector. During normal operation, the fuel injector will make a clicking sound as the solenoid engages the valve.
Replace the fuel injector if the clicking sound is not present on any single injector.
Disconnect the vacuum line connected to the fuel pressure regulator.
Shine a light into the vacuum line connection hole. If fuel is present in the vacuum line the fuel pressure diaphragm is ruptured. Ruptured diaphragms may also result in gasoline fumes being drawn into the cabin of the vehicle.
Replace the regulator if the diaphragm has been ruptured.
Reconnect the vacuum line to the fuel pressure regulator.
Disconnect all fuel injector electrical plugs.
Set a multimeter to measure "Ohms." Touch both leads to the fuel injector's electrical plug terminal---one lead on each side. It does not matter which lead is connected to which side.
Read the multimeter output. Note the measure of resistance in Ohms. Repeat this process on all the fuel injectors.
Compare the resistance readings. All readings should be roughly equivalent. Fuel injector failure has occurred if one injector has a significantly higher or lower resistance than other injectors.
Replace injectors that fail the resistance test.
Rotate the ignition key to the "On" position. It is not necessary to start the BMW's engine to test the voltage being transferred to the fuel injector's electronics plug.
Pull the electronics plug from a fuel injector terminal.
Set the multimeter to measure "Volts." Insert both the red and black leads into the electronics plug leading removed from the fuel injector. Either lead may be inserted into either side of the plug.
Measure the incoming voltage. The incoming voltage should be approximately 12 volts.
Replace any injector if the lead is providing 12 volts but the injector is failing.
Disconnect all electrical leads from the fuel injectors.
Set a multimeter to "Volts." Connect the multimeter's red lead to any fuel injector plug lead. Connect the multimeter's black lead to the positive terminal of the BMW's battery.
Engage the starter of the motor. The engine will not start, but electricity will flow to the fuel injector leads. Monitor the voltage being measured in the multimeter. The voltage should alternate between 0 volts and approximately 12 volts. You may need a friend to assist you with the engine ignition while you monitor the voltage. Leave the multimeter leads connected for the next step.
Connect a fuel injector lead to the fuel injector. Engage the starter while monitoring the voltage on the multimeter. The voltage should continue to alternate. Repeat this step until only the original lead remains untested. If the voltage fails to alternate when a fuel injector is connected, the fuel injector has electrically shorted and must be replaced.
Test the final fuel injector lead. Disconnect any other tested fuel injector. Connect the originally used lead to the respective fuel injector. Retest for alternating voltage with the original lead connected.