Service Information

Base Engine Misfire without Internal Engine Noises

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 inlet manifold and cylinder heads or throttle body.

Replace the inlet 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 inlet 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 tappets.

Excessive oil pressure

A lubrication system with excessive oil pressure may lead to excessive tappet 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.