Electronic fuel injection systems are complicated yet thankfully are relatively simple to troubleshoot. Also potentially beneficial to the backyard mechanic is that fact that modern fuel injection system components are not capable of adjustment, thus eliminating the guesswork in a repair by allowing the simple replacement of defective or worn-out parts. Fuel injectors are essentially momentary valves that can be clogged in a variety of ways or may simply mechanically fail in the open or closed position.
If a fuel injector is truly "dead," it is either no longer opening or is clogged in such a way that no gasoline is able to get from the pressurised fuel line to the intake port. To successfully start a gasoline engine quickly, the fuel injectors run at 100 per cent duty cycle (maximum capacity), until the car's computer decides that it has warmed up sufficiently that less fuel can be used and the engine will still run smoothly. It is for this reason that the engine idles higher for the first few minutes of running while it is warming up. If one or more of the fuel injectors is dead or nearly clogged, the starter will engage properly and spin the engine as normal, but it will often take much longer for the engine to "catch," and run on its own via combustion of fuel.
Rough Running and Vibration
Most modern gasoline engines are of multiple-cylinder design, typically four to eight cylinders despite some exceptions. All multi-cylinder engines are designed with a counterweighted crankshaft, flywheel, and damper to smooth out the pulses of the firing of individual cylinders to a more constant rotational speed. If a fuel injector is dead and hence causing a dead or non-firing cylinder, the engine will essentially have a hiccup every time it completes a cycle, which results in stuttering or vibration that can be felt in the car or by opening the bonnet and looking at the engine. A further symptom of an engine running with a dead cylinder caused by a bad fuel injector is stumbling or delayed acceleration.
The symptoms of a stuck-open or incompletely closing fuel injector are quite different than an injector that is completely dead or partially clogged. In this case, there may be an odour of unburned fuel in the engine compartment, coming through the vents, or even out of the tail pipe. Also, it is common for the fuel economy of the vehicle to slowly worsen as a fuel injector starts to wear out and inject more fuel than desired. It is important to catch a leaky fuel injector early because if an excessive amount of unburned fuel is passed out of the cylinder and into the exhaust, it can overheat and permanently damage the catalytic converter.
Trust the Dash Warning Lights
Even if none of the above symptoms are noticed, especially on cars with greater than 6 cylinders, there may still be a problem lurking, and the first symptom may come from the engine's computer in the form of a CEL or "check engine light." The engine uses a long list of pressure, vibration, temperature, and gas composition sensors to determine the health of the electronic fuel injection system. Many modern vehicles are able to identify misfiring cylinders long before the typical owner would notice any changes in the performance of the car. When a check engine light appears, always have it checked. There is no need to bring the car into a service station, as most local auto parts chain stores will happily use their electronic code-reading tool to determine the issue and offer to sell you the appropriate replacement part.