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 separate 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:
- Fuel Injector Tests.
- Confirm that the fuel injector is being activated or not with a Noid Light.
- Check that the fuel injector is not fried with a simple resistance test and then comparing its resistance value to the other four.
- Fuel Pump Tests.
- Check the fuel pressure with a fuel pressure gauge.
- 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.
- Fuel Pressure Regulator Tests.
- The most common failure is the fuel leaking out of the vacuum hose inlet nipple.
- 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.
Engine Mechanical: Misfire Troubleshooting
The mechanical health of the engine is one of the most overlooked causes of a misfire.
Recalling that all engine cylinders need air, fuel and spark, it's the pistons and cylinder head valves that are responsible for the 'Air' part of the equation.
So, if the piston rings and/or cylinder head valves are too worn out to completely compress the air, and take advantage of the combustion of fuel within the cylinder, you're gonna experience a bonafide misfire or rough idle condition.
Another engine mechanical condition that also causes misfires is when the engine is burning excessive amounts of engine oil. This causes Carbon to build up on the spark plug's center electrode, eventually stopping spark.
Other engine Mechanical problems are: vacuum leaks, dirty throttle plate and throttle bore. Here are some specific tests that you may have to look into and perform:
- Engine Compression Test.
- This will confirm if the misfire or rough idle is due to burned or severely worn cylinder head valves or Piston Rings.
- Vacuum Leak Tests.
- Physically check all vacuum hoses for dry rot, tears, and holes.
- Spraying carb. cleaner spray around the areas where the manifold bolts to the cylinder head (with a cold running engine) to pinpoint leaking intake manifold gaskets.
- Inspect and clean the throttle body.
- Carbon deposits can accumulate on the throttle plate and bore and cause idle quality problems.
- Cleaning should be done with a toothbrush and an approved throttle body Spray cleaner.
Related Articles
You can find a complete list of GM 4.8L, 5.3L, 6.0L test articles here: GM 4.8L, 5.3L, 6.0L Index Of Articles.
Here's a list of articles, you'll find there:
- How To Test The COP Coils (GM 4.8L, 5.3L, 6.0L, 8.1L).
- Blown Head Gasket Test (GM 4.8L, 5.3L, 6.0L).
- How To Do An Engine Compression Test (GM 4.8L, 5.3L, 6.0L).
- GM Electronic Throttle Body Circuit Descriptions And Testing Tips.
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