Bad fuel injector symptoms include a misfire on one cylinder, a rough idle, hesitation, a fuel smell and worse fuel economy. A clogged injector delivers too little fuel and makes that cylinder run lean; a leaking injector drips fuel and makes it run rich, can cause hard hot starts, and in severe cases washes oil off the cylinder wall.
Symptoms of a bad or clogged fuel injector
- A misfire code for one cylinder, such as P0301 to P0308; see engine misfire
- Rough idle and vibration
- Hesitation or stumbling under acceleration
- A raw fuel smell, or black smoke from a leaking injector
- Worse fuel economy
How injectors are tested
- Swap test: move a suspect injector to another cylinder and see if the misfire follows it.
- Resistance check: high-impedance (saturated) injectors typically read around 12 to 16 ohms and low-impedance (peak-and-hold) injectors around 2 to 3 ohms; compare with the specification.
- Balance or flow test: measures how much each injector delivers; specialist shops clean and flow-test injectors.
High vs low impedance injectors
Most factory gasoline injectors are high-impedance, driven directly by the engine computer. Low-impedance injectors draw more current and need a driver designed for them, so they are not a plug-in swap for a high-impedance system.
Sizing injectors for more power
A common rule of thumb for injector flow is: horsepower times brake specific fuel consumption, divided by the number of injectors times the maximum duty cycle. Gasoline engines are often estimated at about 0.5 lb per horsepower-hour naturally aspirated and around 0.6 to 0.65 with boost, with a duty cycle limit of about 80%; E85 needs substantially more flow. Treat the result as a starting point and size the fuel pump and calibration to match.
Cleaning vs replacing injectors
Mild deposits can respond to a quality fuel system cleaner. Badly clogged injectors should be ultrasonically cleaned and flow-tested by a specialist, who can also match flow across a set. Replace injectors that leak, fail an electrical test or will not clean up, and always fit new O-rings and seals when injectors come out.
Injectors we carry
For LS1 builds we carry a Fuel Injector Clinic set of eight 1200cc high-impedance injectors ($1,077.00), rated at 1200cc per minute (110 lb/hr) at 43.5 psi and about 130 lb/hr at the factory 58 psi, with plug-and-play adapters for the factory location. See the Fuel Injector Clinic page, fuel pump and filter symptoms and LS1 supercharger kits.
More guides: rough idle · fuel pressure regulator · fuel trim · fuel injector sizing · car smells like gas · bad diesel injector symptoms.
Trouble codes: P0172 System Too Rich Bank 1 · P0175 System Too Rich Bank 2 · P0300 Random/Multiple Misfire · P0201 Injector Circuit Cylinder 1 · P0202 Injector Circuit Cylinder 2 · P0200 Injector Circuit · P0203 Injector Circuit Cylinder 3 · P0204 Injector Circuit Cylinder 4 · P0205 Injector Circuit Cylinder 5 · P0206 Injector Circuit Cylinder 6 · P0207 Injector Circuit Cylinder 7 · P0208 Injector Circuit Cylinder 8 · P0261 Cylinder 1 Injector Circuit Low · P0262 Cylinder 1 Injector Circuit High · all OBD2 codes.
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