Engine Misfires, Car Runs Rough : Part 2

Engine Misfires
Car Runs Rough

For Part 1 Click The Link

Step 12 - If the compression test is okay along with a wet spark plug, the ignition coilneeds 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

Step 16 - Fuel pressure must maintain a constant pressure, if this flow is impeded by a clogged fuel filter or weak fuel pump the engine can produce a random misfire.

Fuel Pump Pressure Check

Step 17 - A mass air flow sensor sends feedback information to the computer, as this sensor wears it can produce false data causing a misfire.

Mass Air Flow Sensor

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, 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 load.

Flat Camshaft Lobe

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.

Valve Spring and Rocker Arm

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 cylinder misfire.

Related Engine Problems

• Vibration at idle
• Wont start
• Wont crank over