How Do I Know If I Have a Bad Fuel Injector?


Fuel injectors are relatively simple devices that have gotten very reliable.  Like any new technology, fuel injectors were plagued with problems when they were first developed but today’s injectors can be reliable for hundreds of thousands of miles so it’s unusual to have a bad fuel injector.  If you do have a bad fuel injector the most likely symptom you will notice is a rough idle and an engine code for a misfire.

A fuel injector is simply a electronic valve that opens and closes at the demands of your engine’s fuel management computer.  Your vehicle’s fuel pump maintains pressurized fuel on top of the valve so when the injector opens the fuel sprays into the intake manifold.  

Having a bad fuel injector can either happen mechanically or electronically:

Electronically Failed Fuel Injector

A fuel injector can fail electronically either from a bad connection, a broken wire or the valve itself failing.  If the signal isn’t getting from your engine’s computer to the valve to open the fuel injector will fail to open. You can use a noid light to test to make sure the injector is getting the signal to fire.  If there isn’t a signal present, you can check the wiring from the computer to the injector clip.  

A bad injector can also be due to the injector itself failing internally.  You can check for this by checking the resistance of the injector at the electrical connectors. The resistance should either be 2-4 ohms or 10-12 ohms depending on if you have low or high impedance injectors.  If the resistance is significantly higher or shows an open circuit all together then you’re going to need a new injector.

Mechanically Failed Fuel Injector

A fuel injector can mechanically fail either partially or fully.  Fuel injectors are designed to turn the fuel into a fine mist where it can mix completely with the intake air charge.  If the tip fails or gets clogged the fuel spray can be more of a drip which will burn slowly and incompletely. Of coarse the injector can also fail mechanically to the point where the valve doesn’t open at all and fuel never enters your engine.

How to Diagnose a Bad Fuel Injector

  1. If a misfire is present, make sure your spark plugs and ignition system are functioning properly first.
  2. Check for signal to the fuel injector using a noid light for the cylinder registering the misfire.
  3. Check the resistance of the fuel injector.
  4. If you have the ability consider performing an cylinder power balance to determine if 1 cylinder isn’t pulling it’s weight.

If everything looks good to this point, it is most likely that your injector is simply clogged due to dirty fuel and needs to be cleaned.  Using BlueDevil Complete Fuel System Cleaner can unclog dirty fuel injectors restoring a smooth idle and helping your engine run properly.

You can pick up BlueDevil Complete Fuel System Cleaner at one of your local auto parts stores like:

  • AutoZone
  • Advance Auto Parts
  • Bennett Auto Supply
  • CarQuest Auto Parts
  • NAPA Auto Parts
  • O’Reilly Auto Parts
  • Pep Boys
  • Fast Track
  • Bumper to Bumper Auto Parts Specialists
  • S&E Quick Lube Distributor
  • DYK Automotive
  • Fisher Auto Parts stores
  • Auto Plus Auto Parts stores
  • Hovis Auto & Truck Supply stores
  • Salvo Auto Parts
  • Advantage Auto Stores
  • Genuine Auto Parts stores
  • Bond Auto Parts stores
  • Tidewater Fleet Supply
  • Bumper to Bumper Auto Parts
  • Any Part Auto Parts
  • Consumer Auto Parts

Pictures Provided By:

bad_fuel_injector.jpg – By Kirilllutz – Licensed By Getty Images – Original Link

2 responses to "How Do I Know If I Have a Bad Fuel Injector?"


  1. Jim on July 23, 2020 at 8:31 am

    I out reman in truck getting cide 300 part store said misfire I put new plugs wires my question is after 10
    Miles I pulled out one plug its black is that a sign of injector bad

    • BlueDevil Pro on July 24, 2020 at 8:41 am


      If you are getting carbon deposits on the spark plug, it may be an indication that you have an overly rich fuel mixture. This would explain the misfires and the code you were getting. The overly rich fuel mixture could be attributed to a leaky injector, so you may want to have the vehicle looked at by an ASE certified mechanic for a proper diagnosis.

      Thank you!


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