To Diagnose:
Throttle body injection:Take a light and as the car runs look at the spray pattern from the injector, it should be cone shaped and the same all the way around, any breaks or streams rather than an even spray means the injector is faulty. Port injection:Figure out what cylinder has the problem, I usually inspect the spark plugs first, then do an ignition short test. The spark plugs may have one or more showing a distinct difference in the colour of the deposits.Insure the computer and wiring are firing the injector, I have a set of noid lights for this, available from most automotive tool suppliers.
Insure there are no vacuum leaks, spraying WD40 or similar around works really well.
Remove and if possible bench test (at this point if I think I have a bad one I spend the average $35.00 (average) and replace the bloody thing.)
pull the fuel injectors out and disconnect all electronics but one and put a plastic bottel on it now run the starter, if it is a nice mist you are ok, if it sprays then they are bad.
Bad fuel pump, clogged or broken fuel lines, or if it is fuel injected it could be bad fuel injectors.
Bad fuel line? Bad fuel filter? Bad fuel tank? Bad throttle body? Bad injectors?
Possible reasons the fuel injectors are not working: Faulty ECM (engine control module, computer) Faulty fuel injector(s). Fuel injectors clogged. Insufficient fuel pressure. Bad Ground. Faulty wiring otherwise.
you may have bad injectors, or not enough fuel pressure
Bad fuel filter or clogged fuel injectors
Bad as in dirty would simply supply unfiltered fuel to the injectors resulting in misfiring,hard starting and uneven and generally bad acceleration.Bad as in faulty could lead to fuel starvation,motor won't start, rev,cut out etc. no, supply unfiltered gas to the injectors? its a restriction, meaning no or very little gas to injectors, low or no fuel pressure
bad fuel injectors, bad timing, bad spark plugs and wires, clogged catalytic converter.
Check the fuel injectors . one or more are bad or going bad.
Need to know what engine and what year and if it's gasoline or diesel to answer that. Some gasoline injectors are held in place by the fuel rail.
Your check engine light should come on. You will have poor performance from the engine, no get up and go.
Switch the electrical connections between the two injectors and see if the same one is still bad or if the problem switched to the other one. If the same one is bad, you have a bad injector.