Base Engine Misfire Diagnosis
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.
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, worn or damaged bearings or bent pushrod.
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
Perform DTC P0300 before proceeding with Base Engine Misfire Diagnosis information. DTC P0300 will assist in determining which cylinder or cylinders are misfiring.
Perform a visual inspection of the following:
Improper installation of the propeller shaft or a loose prop shaft hub clamp
A loose or improperly installed engine flywheel or crankshaft balancer
Worn, damaged, or misaligned accessory drive system components
Listen to the engine for any abnormal internal engine noises.
Inspect the engine for acceptable oil pressure. Refer to Oil Pressure Diagnosis and Testing .
Verify if the engine has excessive oil consumption. Refer to Oil Consumption Diagnosis .
Verify if the engine has coolant consumption.
Perform a compression test on the engine. Refer to Engine Compression Test .
Intake Manifold Leaks
An intake manifold that has a vacuum leak may cause a misfire.
Inspect for the following:
Improperly installed or damaged vacuum hoses
Faulty or improperly installed intake manifold and/or gaskets
Cracked or damaged intake manifold Inspect the areas between the intake runners
Improperly installed or damaged Manifold Absolute Pressure (MAP) sensor housing (located at the rear of the intake manifold)
Improperly installed or damaged MAP sensor housing O-ring seal
Improperly installed MAP sensor The sealing grommett of the MAP sensor should not be torn or damaged
Improperly installed throttle body or damaged gasket
Warped intake manifold
Warped or damaged cylinder head sealing surface
Coolant Consumption
Coolant consumption may or may not cause the engine to overheat.
Inspect for the following:
External coolant leaks
Faulty cylinder head gasket
Warped cylinder head
Cracked cylinder head
Damaged engine block
Oil Consumption
Oil consumption may or may not cause the engine to misfire.
Remove the spark plugs and inspect for an oil fouled spark plug.
Perform a cylinder compression test. Refer to Engine Compression Test .
If the compression test indicates worn valves or valve guides, inspect for the following:
Worn valve guides
Worn valve stems
Worn or burnt valves or valve seats
Worn, brittle, or improperly installed valve stem oil seals
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
With the engine running try and determine if the noise is timed to camshaft speed or crankshaft speed.
Using a timing light, two knocks per flash is crankshaft speed and one knock per flash is camshaft speed.
If the noise is timed to camshaft speed, inspect the following:
Missing or loose valve train components
Worn or loose valve rocker arms
Worn or bent pushrods
Faulty valve springs
Bent or burnt valves
Worn camshaft lobes
Worn or damaged timing chain and/or sprockets
Important
A slight COLD knock or piston slapping noise could be considered normal if not present after the engine has reached normal operating temperatures.
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
Damaged or faulty connecting rod
Excessive carbon build-up on the top of the piston
No Abnormal Internal Engine Noise
Inspect for a worn or improperly installed timing chain and/or sprockets.
Remove the valve rocker arm cover on the side of the engine with the cylinder that is misfiring.
Inspect for the following:
Loose valve rocker arm bolts
Bent push rods
Faulty valve springs
Faulty valve lifters (bleeding down)
Worn or improperly seated valves
Worn camshaft lobes
