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.
- 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.
Check Engine Light
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
Infrared Temperature Meter
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 ).
Fuel Injector Wire Removed
Remove Ignition Coil Connector
Step 6 - Once the misfiring cylinder has been located,
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
Step 7 - When a cylinder(s) runs rich it will carbon
foul the spark plug causing it to short circuit. Malfunctions for this condition
Remove Spark Plug
excessive fuel mixture
Step 8 - A wet spark plug with gas or oil can mean
a fuel injector is stuck open,
Carbon Fouled Spark Plug
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
Spark Plug Fouled with Gas or Oil
Step 10 - Using a proper spark plug with a correct
air gap will help ensure operation of the ignition system.
Misadjusted Air Gap
Step 11 - If the spark plug is wet with fuel or
carbon fouled upon removal, a compression check is needed, perform a
Correct Spark Plug
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
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.
Test Ignition Coil Output
Step 14 - A fuel injector regulates the amount
of fuel which is consumed by the engine, if the
Shorted Spark Plug Wires
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
Test Fuel Injector Operation
, 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
Step 16 - Fuel pressure must maintain a constant
pressure, if this flow is impeded by a
Repair Vacuum Leak
weak fuel pump
the engine can produce a random misfire.
Step 17 - A
mass air flow sensor
Fuel Pump Pressure Check
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
Mass Air Flow Sensor
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
Step 19 - In some conditions a broken valve spring
can effect different aspects of engine performance for example: If just a small
part of a coil breaks off, the spring can still close the valve but only at low
RPM's, causing a high RPM misfire. If the spring breaks somewhere in the middle
it will affect both idle and power conditions. To test for this condition, remove
ignition coil connector or ignition system or fuel pump fuse to disable the engine
from starting. Remove valve cover(s) to gain access to visually inspect the valve
train. Inspect the condition of the valve springs, use a flashlight and small mirror
to aid in the inspection. Have a helper crank the engine over while observing rocker
arms and cam lobes, confirming full movement, if one or more lobes are traveling
less than the others the camshaft has a flattened lobe and replacement is required.
Flat Camshaft Lobe
Step 20 - A head gasket is used to seal the cylinder
head to the engine block. The cylinder head is fastened to the engine block using
head bolts that are tightened to a specific torque. When a head gasket wears (blows)
it can allow coolant or exhaust gasses to enter the combustion chamber causing a
An engine tune up is necessary at manufacturers recommended intervals to maintain
proper engine run-ability. A misfiring engine will cause, low gas mileage, low power
output, increased emissions and possible internal engine damage if left untreated.
Typically an engine misfire under load, will be a problem with the ignition system,
while low engine speed problems will be a vacuum, fuel management or compression
When retrieving trouble codes, only consider related codes to a specific misfire
problem, like an ignition coil failure code. If a trouble code is present and doesn't
pertain to the immediate problem like an EVAP code, ignore it until a later time,
when the engine is running properly these codes could cycle out.
Related Engine Problems
Valve Spring and Rocker Arm
Article first published 2016-12-06