For 1990-2009 cars

Cause | Correction
--- | ---
Abnormalities, severe cracking, bumps, or missing areas in the accessory drive belt Abnormalities in the accessory drive system and/or components may cause engine revolutions per minute (RPM) variations and lead to a misfire diagnostic trouble code (DTC). A misfire code may be present without an actual misfire condition. | Replace the drive belt.
Worn, damaged, or mis-aligned accessory drive components or excessive pulley runout may lead to a misfire DTC. A misfire code may be present without an actual misfire condition. | Inspect the components, and repair or replace as required.
A loose or improperly installed engine flywheel or crankshaft balancer A misfire code may be present without an actual misfire condition. | Repair or replace the flywheel and/or balancer as required.
Restricted exhaust system A severe restriction in the exhaust flow can cause significant loss of engine performance and may set a DTC. Possible causes of restrictions include collapsed or dented pipes or plugged mufflers and/or catalytic converters. | Repair or replace as required.
Improperly installed or damaged vacuum hoses | Repair or replace as required.
Improper sealing between the intake manifold and cylinder heads or throttle body | Replace the intake manifold, gaskets, cylinder heads, and/or throttle body as required.
Improperly installed or damaged manifold absolute pressure (MAP) sensor The sealing grommet of the MAP sensor should not be torn or damaged. | Repair or replace the MAP sensor as required.
Damage to the MAP sensor housing | Replace the intake manifold.
Worn or loose rocker arms The rocker arm bearing end caps and/or needle bearings should be intact and in the proper position. | Replace the valve rocker arms as required.
Stuck valves Carbon buildup on the valve stem can cause the valve not to close properly. | Repair or replace as required.
Excessively worn or mis-aligned timing chain | Replace the timing chain and sprockets as required.
Worn camshaft lobes | Replace the camshaft and valve lifters.
Excessive oil pressure A lubrication system with excessive oil pressure may lead to excessive valve lifter pump up and loss of compression. |

• | Perform an oil pressure test. Refer to Oil Pressure Diagnosis and Testing.

• | Repair or replace the oil pump as required.

Faulty cylinder head gaskets and/or cracking or other damage to the cylinder heads and engine block cooling system passages Coolant consumption may or may not cause the engine to overheat. |

• | Inspect for spark plugs saturated by coolant.

• | Inspect the cylinder heads, engine block, and/or head gaskets.

• | Repair or replace as required.

Worn piston rings Oil consumption may or may not cause the engine to misfire. |

• | Inspect the spark plugs for oil deposits.

• | Inspect the cylinders for a loss of compression. Refer to Engine Compression Test.

• | Perform cylinder leak down and compression testing to identify the cause.

• | Repair or replace as required.

A damaged crankshaft reluctor wheel A damaged crankshaft reluctor wheel can result in different symptoms depending on the severity and location of the damage.

• | Systems with electronic communications, DIS or coil per cylinder, and severe reluctor ring damage may exhibit periodic loss of crankshaft position, stop delivering a signal, and then re-sync the crankshaft position.

• | Systems with electronic communication, DIS or coil per cylinder, and slight reluctor ring damage may exhibit no loss of crankshaft position and no misfire may occur. However, a DTC P0300 may be set.

• | Systems with mechanical communications, high voltage switch, and severe reluctor ring damage may cause additional pulses and effect fuel and spark delivery to the point of generating a DTC P0300 or P0336.

Replace the sensor and/or crankshaft as required.