Engine performance diagnosis procedures are covered in Engine Controls and should be consulted for diagnosis of any Driveability, Emissions, or Malfunctioning Indicator Lamp (MIL) concerns.

The following diagnosis covers common concerns and possible causes of mechanical misfires.

When the proper diagnosis is made, the concern should be corrected by adjustment, repair or replacement as required.

Refer to the appropriate section of the service manual for each specific procedure.

This diagnostic table will assist in engine misfire diagnosis due to a mechanical concern such as a faulty camshaft, or worn or damaged bearings.

This table will not isolate a crossed injector wire, faulty injector or any other driveability component failure that may cause a misfire.

The Powertrain On-Board Diagnostic System checks must be performed first.

When using this table to make a Base Engine Misfire diagnosis, begin with the preliminary information below and then proceed to the specific category.

Preliminary

1. Perform DTC P0300 engine performance diagnostic procedures found in the diagnostic table before proceeding with Base Engine Misfire Diagnosis information.

DTC P0300 will assist in determining which cylinder or cylinders are misfiring.

2. Perform a visual inspection of the following:

•	A loose or improperly installed engine flywheel or crankshaft balancer

•	Worn, damaged or misaligned accessory drive system components

3. Listen to the engine for any abnormal internal engine noises.
4. Inspect the engine for acceptable oil pressure.
5. Verify if the engine has excessive oil consumption.
6. Verify if the engine has excessive coolant consumption.
7. Perform a compression test on the engine.

Intake Manifold Leaks

An intake manifold that has a vacuum leak may cause a misfire.

Inspect for the following:

Coolant Consumption

Coolant consumption may or may not cause the engine to overheat.

Inspect for the following:

Oil Consumption

Oil consumption may or may not cause the engine to misfire.

1. Remove the spark plugs and inspect for an oil fouled spark plug.
2. Perform a cylinder compression test.
3. If the compression test indicates worn valves or valve guides, inspect the following:

•	Worn, brittle, or improperly installed valve stem oil seals

•	Worn valve guides

•	Worn valve stems

•	Worn or burnt valves or valve seats

4. If the compression test indicates worn or damaged piston rings, inspect the following:

•	Broken or improperly seated piston rings

•	Excessive piston ring end gap

•	Excessive cylinder bore wear or taper

•	Cylinder damage

•	Piston damage

Abnormal Internal Engine Noises

Important: A slight COLD knock or piston slapping noise could be considered normal if not present after the engine has reached normal operating temperatures.

1. Connect a timing light to the engine and run engine to determine if the noise is timed to camshaft speed or crankshaft speed. Two knocks per flash indicates crankshaft speed and one knock per flash indicates camshaft speed.
2. If the noise is timed to camshaft speed, inspect the following:

•	Missing or loose valve train components

•	Faulty valve springs

•	Worn lifters

•	Worn or damaged valve rotators

•	Bent or burnt valves

•	Worn camshaft lobes

•	Worn or damaged timing chain and/or sprockets

3. If the knock is timed to crankshaft speed, inspect the following:

•	Worn crankshaft main or connecting rod bearings

•	Piston or cylinder damage

•	Worn piston or piston pin

•	Faulty connecting rod

•	Excessive carbon build-up on the top of the piston

No Abnormal Internal Engine Noise

1. Inspect for a worn or improperly installed timing chain and/or sprockets
2. Remove the cylinder head cover and inspect for the following:

•	Loose or damaged valve train components

•	Faulty valve springs

•	Faulty valve lifters (bleeding down)

•	Worn or improperly seated valves

•	Worn camshaft lobes.

3. Inspect for a chipped or cracked reluctor ring