Fuel injectors are essentially valves that open and close thousands of times per second. They connect to a line full of pressurised fuel; electric impulses from the car's computer cause the valve to open, allowing fuel to flow into your engine. Mechanical damage, electronic problems and debris caught between the valve and the orifice can all cause the valve to hang open and deliver a constant stream of fuel instead of closing like it should.
Black Smoke and Fuel Odor
An exhaust pipe that spews black, fuel-smelling smoke is a dead giveaway that your engine is receiving too much fuel. An leaking or stuck-open injector will cause your exhaust to emit a steady stream of fuel-smoke at idle, but this cloud may diminish with higher revolutions per minute (rpm). Repair or replace your injector immediately if it has begun to leak badly enough to cause fuel smoke, as such high concentrations of fuel in the exhaust can damage your catalytic converter.
A consistent, single-cylinder misfire at idle and possibly higher in the rpm range can indicate a leaking or stuck injector. The constant stream of fuel leaking into one cylinder throws the air/fuel ratio completely out of whack, making fuel ignition for that cylinder impossible. Note that a leaking injector will only affect its host cylinder; if you detect multiple or random cylinder misfires, the problem probably lies in your ignition system.
"Check Engine" Light
All modern cars have the ability to self-diagnose certain problems, including a bad air-to-fuel ratio. If your "Check Engine" light comes on, take the car to your dealer, trusted mechanic or local auto parts store to check the codes. The code may or may not come up specifically as a bad injector, depending on the sophistication of your engine. For most engines, you'll see a pair of codes: "Misfire, Cylinder #__" and "Fuel Rich Condition, Bank # (1 or 2)." These two codes could indicate either a leaking or stuck injector or an ignition system issue, but misfires in multiple cylinders usually indicate an ignition problem, clogged fuel filter, malfunctioning fuel pump or fuel pressure regulator.
Drop in Fuel Economy
A leaking injector, essentially a hole in your engine's pressurised fuel system, will have the same effect on fuel economy as a hole in your gas tank. The problem is compound, however; since the additional fuel going into the affected cylinder makes combustion impossible, you're wasting not only the amount of fuel leaked by the injector but also the amount that should go into powering that cylinder. A leaking injector can easily effect a 15 per cent drop in fuel economy for V-6 engines and a 25 per cent drop for four-cylinders engines.
Loss of Power
The same math that works for fuel economy also applies to horsepower production. If your engine has six cylinders and one of them isn't producing power, you can expect horsepower to drop by about one-sixth. Power loss might not always be so extreme, though; a very slightly leaking injector may only alter the air-to-fuel ratio in that cylinder enough to turn on the "Check Engine" light without significantly reducing power output.