Easy step by step guide on how to troubleshoot and repair an automotive engine cylinder misfire P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307 and P0308, though appearances may vary, the process is similar for most vehicles.
Difficulty Scale: 5 of 10
Begin with the vehicle on level ground engine "OFF" and the parking brake set, wear protective gloves and clothing for safety.
Step 1 - There are several combinations of misfire conditions, steady or random, at idle or under power, which may or may not be detected by the computer and trigger a check engine or service engine soon light, read trouble codes to help pinpoint the cylinder(s) in question and follow the repair guide below.
Step 2 - If no service light is triggered with
a steady misfire, use an infrared thermo gun to test the exhaust temperature of each cylinder.
Step 3 - Start the engine cold, quickly
take a reading
at the front of each cylinder's exhaust port on the manifold while maintaining similar
placement of the beam over each individual port, a misfiring cylinder will be considerably
colder than the remaining cylinders. Example: Three of the exhaust ports test at
190 degrees while one is at 81 degrees, the cylinder at 81 degrees is misfiring.
Step 4 - If no results are yet gleaned, start the
engine and allow to idle, remove the fuel injector electrical connector on each
cylinder one at a time while observing the engine performance, if no change is observed
at a particular cylinder, the misfiring cylinder has been located.
Step 5 - This can also be achieved by removing
a coil wire connector (COP systems only - no plug wire attached ).
Step 6 - Once the misfiring cylinder has been located,
remove the
spark plug for inspection, before removing the spark plugs, mark the plug wires
(if equipped) to identify their positions in the firing order, this will help for
proper reassembly.
Step 7 - When a cylinder(s) runs rich it will carbon
foul the spark plug causing it to short circuit. Malfunctions for this condition
include partially
plugged fuel
injector or
catalytic
converter,
low compression,
failed
fuel
pressure regulator and
excessive fuel mixture.
Step 8 - A wet spark plug with gas or oil can mean
a fuel injector is stuck open,
fuel
regulator has failed, no cylinder compression, broken piston oil control ring,
blown head gasket or excessive leakage from the valve stem seals.
Step 9 - A spark plug must be properly gapped with
a specific measurement from the electrode, a misadjusted air gap can result in a
misfire.
Step 10 - Using a proper spark plug with a correct
air gap will help ensure operation of the ignition system.
Step 11 - If the spark plug is wet with fuel or
carbon fouled upon removal, a compression check is needed, perform a
cylinder
compression test to locate a mechanical failure. If compression is low, it could
mean worn out or broken piston rings, flat camshaft, broken valve spring, burnt
or leaking intake or exhaust valves, dropped valve seat or blown head gasket.
Step 12 - If the compression test is okay along
with a wet spark plug, the
ignition
coil needs to be tested.
Step 13 - Spark plug wires are designed to transfer
an electrical charge from the coil to the spark plug, when these cables wear, they
can short circuit causing a misfire.
Step 14 - A fuel injector regulates the amount
of fuel which is consumed by the engine, if the
injector
operation has failed the cylinder will misfire.
Step 15 - A random misfire is can be associated
with broken or dilapidated vacuum hoses or tubes on and around the engine causing
a vacuum
leak, these hoses are typically connected to the engine intake manifold and
supply engine vacuum to various accessories such as the brake system. If an intake
gasket fails or a vacuum line that is close to an intake port it will cause a steady
misfire.
Step 16 - Fuel pressure must maintain a constant
pressure, if this flow is impeded by a
clogged fuel
filter or
weak fuel pump the engine can produce a random misfire.
Step 17 - A
mass air flow sensor sends feedback information to the computer, as this sensor
wears it can produce false data causing a misfire.
Step 18 - Each intake and exhaust valve utilizes
a spring which will return the valve to its original position (closed). If a valve
spring has broken or a cam lobe has worn down, it will cause the engine to misfire
and run rough due to the loss of compression. These conditions don't always show
up in a compression test,
because a compression test is performed at engine cranking speed. If a camshaft
lobe is halfway worn down, it will show up only when the engine is running and under
load.