Skip the auto shop - Our mechanics make house calls

Why Misfires are Often Misdiagnosed

Why Diagnosing a Misfire is Often Misdiagnosed

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

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.

Misfires are often misdiagnosed and should be scrutinized prior to any repair. If you need help doing so, ask one of our automotive specialists. YourMechanic is happy to help!

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
Icon-warranty_badge-02

Skip the repair shop, our top-rated mechanics come to you.

At your home or office

Choose from 600+ repair, maintenance & diagnostic services. Our top-rated mechanics bring all parts & tools to your location.

Fair & transparent pricing

See labor & parts costs upfront, so you can book with confidence.

12-month, 12,000-mile warranty

Our services are backed by a 12-month, 12,000-mile warranty for your peace of mind.

Get A Quote

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

More related articles

P0222 OBD-II Trouble Code: Throttle/Pedal Position Switch/Sensor B Circuit Low Input
P0222 code definition Throttle/Pedal Position Switch/Sensor B Circuit Low Input What the P0222 code means The manufacturer specifies a range of acceptable voltage to be produced from the throttle position...
How to Buy Fuel Treatments
Adding a fuel additive to your gas tank when you fill up is one way to clean vital engine parts of deposits, improve the performance of your engine,...
Insurance Requirements for Car Registration in Kentucky
The Kentucky Transportation Cabinet requires that all drivers in Kentucky carry liability automotive insurance, or “financial responsibility” in order to operate a vehicle legally and maintain vehicle...


Related questions

Q: Cylinder 2 injector control circuit Renault Scenic 2

Hi and thanks for contacting YourMechanic. The code P0202 is what shows for a cylinder that has lost connectivity to an injector. Either the injector has failed or the injector wiring harness has an issue. Check the cylinder 2...

Q: Does the OBD System Measure Gases in the Car's Exhaust?

The onboard diagnostic (OBD) system utilizes sensors mounted in the engine exhaust stream to measure the amount of oxygen that comes out of the engine after the combustion process. When an engine intakes a certain amount of air into the...

Q: Engine takes to long to warm up I've replaced thermostat already and it misses horrible till it warms up its a 4.9 engine

These engines are known to get real bad carbon build up on the back sides of the valves. If you engine has a hard time getting to operating temperature then it will also cause the engine to carbon up even...