From time to time you run into a fuel injector that fails. It can fail and start leaking fuel externally, or the electronics can fail and it can stop working. It’s usually pretty easy to find a bad fuel injector if it’s leaking externally, just put your eyes on it. If you see fuel leaking out of it, replace it. The electronics can be a bit harder to track down. I normally find a bad fuel injector when doing a power balance test. If I find that I have spark but no fuel on a particular cylinder, the next place I look is to the fuel injector. If I suspect a bad fuel injector the first step I often take is to see if the injector is getting a signal to fire. If it’s not getting a signal, it’s not the fuel injectors fault. It could have a wiring or a driver problem that’s causing it not to fire and deliver fuel as it should. There is a tool for this test, it’s called a noid light.
Noid lights plug into the injector harness in place of the injector. When the injector is sent a trigger signal, the noid light will blink to indicate each time a signal is sent. If you have a fuel injector that’s not working and the noid light lights up when you run or crank the engine, you likely have a bad fuel injector. If however the noid light does not light up when you crank or run the engine, you likely have a signal problem and you should focus your search on the wiring and any components associated with delivering the signal to the injector. Some people use a test light to perform the same function as a noid light. I suppose it is possible to see an injector signal this way but I don’t think it’s as good or as accurate. Sometimes the test light will not light up enough for you to see the injector signal. If that’s the case it might lead to a misdiagnosis of the injector.
I haven’t had much luck checking injector resistance. Sure there are times when you find an open injector but there are other times where the same test can show a good injector but in fact the injector has failed. I won’t get into the specifics of why this is, just know that it’s a possibility. So if you’re checking an injectors resistance take the information with a grain of salt and see what you can do to confirm the injectors operation.
One last note on fuel injectors. Be careful of the seals. Fuel injectors often have 2 main seals. The upper seal is where the fuel injector attaches to the fuel rail. This seal keeps the fuel in the fuel rail and directed toward the injector. If this seal fails it can cause a fuel leak. The lower seal helps prevent vacuum leaks into the intake. If during your testing you damage one of these seals you can create a vacuum leak or a fuel leak. Either one is a problem that needs to be taken care of if you’re looking to solve a performance issue. I like to put a small amount of silicone paste on injector seals before I install them. This way I’m not installing them dry and in doing so I reduce the risk of damaging the seals.