For 1990-2009 cars

Cause | Correction
--- | ---
Fuel injector harness connectors are connected to the incorrect fuel injectors/cylinders | Relocate the fuel injector harness connectors, as necessary.
Abnormalities, such as severe cracking, bumps, or missing areas in the accessory drive belt Abnormalities in the accessory drive system and/or components may cause engine 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. Refer to Drive Belt Replacement - Accessory .
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.
Loose or improperly installed engine flex plate or crankshaft balancer A misfire code may be present without an actual misfire condition. | Repair or replace the flex plate and/or balancer, as required. Refer to Automatic Transmission Flex Plate Replacement , or Crankshaft Balancer Replacement .
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.
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.
Worn or bent pushrods |

• | Replace the pushrods.

• | Inspect the top of the pistons for valve contact. If the top of the piston shows valve contact, replace the piston and pin assembly.

Stuck valves Carbon buildup on the valve stem can cause the valve to not 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. |

1. Perform an oil pressure test. Refer to Oil Pressure Diagnosis and Testing .
2. 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. |

1. Inspect for spark plugs saturated by coolant. Refer to Spark Plug Inspection .
2. Inspect the cylinder heads, engine block, and/or head gaskets. Refer to Coolant in Combustion Chamber .
3. Repair or replace, as required.

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

1. Inspect the spark plugs for oil deposits. Refer to Spark Plug Inspection .
2. Inspect the cylinders for a loss of compression. Refer to Engine Compression Test .
3. Perform cylinder leak down and compression testing to identify the cause. Refer to Cylinder Leakage Test .
4. 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 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 P0300 DTC 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 P0300 DTC or P0336.

Replace the sensor and/or crankshaft, as required.