Engine Misfires Or Runs Rough

Misfire - Engine Miss

My Car Engine Misfires while starting or running
Vehicle Runs Rough

Helpful Information

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 issue.

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.

Step by step guide on how to troubleshoot and repair an automotive engine cylinder misfire, this information pertains to most non electric vehicles.

Difficulty Scale: 4 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.

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.

Infrared Temperature Meter

Step 3 - Start the engine cold, quicklytake 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.

Exhaust Temperature

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.

Fuel Injector Wire Removed

Step 5 - This can also be achieved by removing a coil wire connector (COP systems only - no plug wire attached ).

Remove Ignition Coil Connector

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.

Remove Spark Plug

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 orcatalytic converter, low compression, failed fuel pressure regulator and excessive fuel mixture.

Carbon Fouled Spark Plug

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.

Spark Plug Fouled with Gas or Oil

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.

Misadjusted Air Gap

Step 10 - Using a proper spark plug with a correct air gap will help ensure operation of the ignition system.

Correct Spark Plug

Step 11 - If the spark plug is wet with fuel or carbon fouled upon removal, a compression check is needed, perform a cylinder compression testto 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

FOR PART 2 FURTHER TROUBLESHOOTING CLICK THE LINK

ENGINE MISFIRE CAR RUNS ROUGH