Misfire - Engine Miss

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.

Compression Test

Step 12 - If the compression test is okay along with a wet spark plug, the ignition coil needs to be tested.

Test Ignition Coil Output

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.

Shorted Spark Plug Wires

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.

Test Fuel Injector Operation

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.

Repair Vacuum Leak


Written by
Co-Founder and CEO of
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.

Article first published (Updated 2015-01-09)