A misfire occurs in an engine due to an electrical malfunction or a fuel delivery issue. Most of the time misfires occur when a spark plug, ignition wire, or coil pack fails. There are lots of possibilities that contribute to a misfire other than just ignition.
Symptoms of misfire
- Failed spark plug
- Burned or broken spark plug wire
- Failed ignition coil
- Plugged fuel injector
- Bent or broken intake or exhaust valve
- Plugged exhaust port
- Internal coolant leak
- Excessive oil consumption
Most of the time when diagnosing a misfire becomes misdiagnosed. Often the spark plugs and spark plug wires are replaced in the event of a misfire. This may or may not fix the problem. When replacing a component and it does not fix the situation, the best thing to do is rule out all of the possibilities that can disrupt the current situation.
It would be best to check all of the spark plugs and wires before replacing them. Check the fuel delivery system for proper operation. For electronic injectors, one injector can be disconnected at a time to determine if the injector has indeed failed or is plugged. For a bent or broken intake or exhaust valve, plugged exhaust port, or excessive oil consumption, it would be best to perform a compression test and a leak down test to verify the condition before jumping to conclusions.
If the engine has an internal coolant leak, using a block tester with the engine at operating temperature will determine if the combustion gases are escaping into the cooling system.
- Warning: Testing a cooling system with a block heater can be dangerous. Note that the radiator cap or reservoir cap will be off during this test. Do not shut the engine off at operating temperature with the cap removed. This will result in a flash boil and spray the coolant everywhere.
Bent or broken intake valves will cause the engine to have a misfire and create a popping sound in the intake. This can damage any sensor located in the intake system and burn the air filter. Plus, a lot of people don’t know that when a misfire occurs in the intake, some of the flame is sent through the ventilation tube into the valve cover and beginning to burn the oil and oil guide seals on the valve train. This can lead to pre varnished oil and oil consumption.
Bent or broken exhaust valves will cause the engine to have a misfire and create a popping sound in the exhaust. This can damage the air fuel ratio sensors (or Oxygen sensors) and damage the catalytic converter. Plus, when there is a misfire without a flash flame, the unburnt fuel can be collected inside the catalytic converter causing the catalytic converter to burn hotter. The popping sound, commonly know as a backfire, can also damage the engine by deteriorating the other exhaust valves with no issues.
Exhaust with ports that are plugged will create excessive heat and cause the pyrometer temperature to exceed the limits to melting point. This can cause serious damage to the engine.
Excessive oil consumption can cause a misfire by heating up the cylinder to a hotter temperature than normal creating a preignition symptom. This will cause the engine to slow down and damage the crankshaft bearings. One misdiagnosed problem with having excessive oil consumption is having a positive crankcase valve, or PCV, stuck shut causing the crankcase to build too much pressure forcing oil into the combustion chamber.
This article originally appeared on YourMechanic.com as Why Misfires are Often Misdiagnosed and was authored by Marvin Sunderland.