Ignition System: Misfire Troubleshooting

How To Troubleshoot A Misfire (GM 4.8L, 5.3L, 6.0L)

The most logical starting point, when troubleshooting a misfire on your 4.8L, 5.3L or 6.0L pickup, van or SUV is testing the COP coils for spark.

Why? Because a failed COP coil is usually the reason behind the misfire in the majority of cases. Here are my specific recommendations:

  1. Check for misfire codes with a scan tool first.
    1. The misfire codes (other than a P0300) identify the engine cylinder that's missing or dead.
    2. If no misfire trouble codes are present or you have a P0300, then a cylinder balance test is recommended to find the misfiring cylinder.
      1. The cylinder balance test should be done by unplugging the electrical connector of the fuel injector or the COP Coil.
  2. Remove and Inspect:.
    1. Spark Plugs for:
      1. Cracks.
      2. Engine oil (from a leaking valve cover gasket).
      3. Fuel fouling.
      4. Center electrode capped/blocked with carbon.
      5. Check for carbon tracks on the ceramic insulator.
    2. COP Coils Spark Plug Wires for:
      1. Cracks or burns on the spark plug wire's rubber boot.
      2. Carbon tracks on the inside of the rubber boot.
  3. Test for Spark.
    1. Use a dedicated spark tester.
      1. Doing a spark test with anything other than a spark tester (like pulling the COP coil or spark plug wire from the spark plug) can and will damage the ignition coil.
      2. Using a timing light will only have you wasting your time and money since this is the MOST INACCURATE way to test for spark.

Fuel System: Misfire Troubleshooting

The previous generation of GM V8 engines were equipped with the infamous ‘Spider’ fuel injector assembly inside the intake manifold's plenum (and which was prone to fuel leakage failures). In the 4.8L, 5.3L, 6.0L engines, GM has done away with this type of fuel system and uses 8 seperate fuel injectors placed on a fuel rail on the outside of the plenum.

These fuel injectors can get dirty (internally) and/or clogged. When this happens, the fuel injectors will spray fuel in a jet instead of a fine atomized mist and/or no fuel at all. In extreme cases, this will soak and foul the spark plug with gasoline and the spark plug won't spark anymore.

Most of the time though, clogged fuel injectors create a rough idle and lack of power condition due to the lean Air/Fuel Mixtures they create in the cylinders.

Here are some of the tests you'll need to do:

  1. Fuel Injector Tests.
    1. Confirm that the fuel injector is being activated or not with a Noid Light.
    2. Check that the fuel injector is not fried with a simple resistance test and then comparing its resistance value to the other four.
  2. Fuel Pump Tests.
    1. Check the fuel pressure with a fuel pressure gauge.
    2. A fuel pump putting out a low fuel pressure (even if it keeps the engine running) will cause a lean condition that will cause misfires and/or a rough idle condition.
  3. Fuel Pressure Regulator Tests.
    1. The most common failure is the fuel leaking out of the vacuum hose inlet nipple.
    2. You can easily check this simply by removing the vacuum hose (that attaches to the regulator's vacuum inlet nipple) and check for fuel leaking out of the regulator.

Maintenance, like replacing the fuel filter, adding fuel injector cleaner to the gas tank are key to keeping the fuel injectors clean and happily injecting the correct amount of fuel.