P0300 Misfire Trouble Code

What is the P0300 code definition?

P0300 - Random or multiple cylinder misfire detected

What does the DTC P0300 mean?

The DTC P0300 sets when the engine control module (ECM) detects the engine has one or more cylinders misfiring. A misfire happens when the air/fuel mixture inside a cylinder fails to ignite properly, resulting in no combustion or incomplete combustion in that cylinder. The ECM watches each cylinder's contribution by monitoring changes in the crankshafts rotational speed. When it sees the crankshaft slow down on a cylinder's firing event it registers it as a misfire. If the number of misfires exceeds an acceptable limit the ECM will set a P0300.

P0300 Symptoms

• Check engine light (CEL) on or flashing

• Rough idle

• Hesitation

• Shaking or jerking during acceleration

• Hard start

• No start

• Excessive exhaust emissions and smell

  
  

What Causes P0300 to Set?

• Ignition system malfunction: The ignition system is used to create an electrical spark inside the engine's cylinders. This spark is used to ignite the air/fuel mixture in the cylinders starting the combustion process. If any part of the ignition system fails, it can cause one or more cylinders to have weak or no spark resulting in an engine misfire

• Fuel system issue: The fuel systems job is to provide the cylinders with the proper amount of fuel for each firing event. An issue with the fuel system can cause a cylinder or multiple cylinders to receive the incorrect amount of fuel. This results in an air/fuel mixture that is either too lean or too rich. If the air/fuel mixture becomes too far out of range an engine misfire will occur

• Incorrect air flow, air metering: Engines need air, or to be more precise oxygen to burn fuel. The engine control system also needs to be able to accurately measure the air entering the engine to calculate the correct air/fuel mixture. Any fault in the engine's ability to intake air or meter the air entering the engine can result in a misfire due to an incorrect air/fuel ratio

• Exhaust system restrictions: After a cylinder fires the exhaust gas needs to be expelled to make room for a fresh air/fuel charge. If the exhaust system becomes restricted, the exhaust gas will become trapped in the cylinder. This results in an exhaust back pressure condition that prevents cylinder breathing and creates an engine misfire

• Mechanical Engine failure: An engine is a complex system of moving parts that must work together to run properly. With the use of properly timed pistons and valves the engine works like an air pump. It pulls in air and fuel, compresses and ignites it, then pushes it out the exhaust. Any mechanical issue with an engine can affect this cycle and cause the engine to misfire

• Falsely detected misfires: The ECM uses the crankshaft position (CKP) sensor to detect engine misfires. Some vehicles calibrate the CKP sensor and crankshaft variation to detect misfires more accurately. If the CKP sensor was replaced or the ECM lost its calibration, it can cause the ECM to falsely detect misfires and set a P0300 code.

How to diagnose a P0300 DTC?

Recommended tools

• Scan tool (preferably one that shows data)

• Flashlight

• Hand tools

• Ignition spark tester

• Vacuum/pressure gauge

• Fuel pressure gauge

• Compression gauge

• Cylinder leak down tester

Diagnostic checks

• Determine which cylinder or cylinders are misfiring: Using a scan tool check the misfire data to determine which cylinder or cylinders are misfiring. If a scan tool is unavailable, start the engine and unplug the fuel injectors one at a time while observing the drop in engine RPM. If a cylinder is misfiring, unplugging its injector will not cause any noticeable drop in engine RPM. Once misfiring cylinders are identified move on to diagnosis

• Check ignition system: Inspect the spark plugs and related ignition components for any signs of damage or wear. Using a spark tester, check for spark on the misfiring cylinders. If any cylinders are missing spark, test the ignition coils and ignition control system for faults. If the vehicle has a coil on plug system, try swapping the coil with a known good cylinder and see if the misfire follows the coil.

• Check fuel system: Install a fuel pressure gauge and test the vehicles fuel pressure. If the vehicle has a gasoline direct injection (GDI) system, a scan tool will be needed to check the high side fuel pressure by reading the fuel rail pressure sensor data, this is due to the fuel pressure being too high to test with a fuel pressure gauge. Compare fuel pressure readings to the vehicles listed specifications. If ok, check the fuel injectors for proper operation. Due to the difference in fuel injector designs make sure to consult service information for the correct injector testing procedure

• Inspect intake / mass air flow sensor (MAF): Check the air filter and intake tubing for any restrictions and for proper installation. Closely examine the intake connections and boots for any tears or damage that could allow un-metered air in the engine. Start the engine and listen for any vacuum leaks around the intake manifold, look closely for any broken or split vacuum lines. Using a scan tool compare the MAF sensor readings with known good specifications. If MAF is out of spec, try it cleaning with a quality MAF sensor cleaner.

• Check exhaust for restrictions: Remove the oxygen or air fuel ratio sensor closest to the engine and install an exhaust back pressure gauge. Start the engine and read back pressure at idle, and at 2000 RPM. Normal specifications are 1.5 psi or less at idle and 3 PSI or less at 2000 RPM, but some vehicles will be slightly higher by design. If readings are high, take another reading from the oxygen or air/fuel ratio sensor downstream of the catalytic converter. If readings are now ok, suspect a plugged catalytic converter. If the readings are still high, inspect the rest of the exhaust to find the restriction.

• Test the engine for a mechanical issue: Install a vacuum gauge and measure engine vacuum at idle. Vacuum reading should be steady between 17-21 inches of mercury, a low or fluctuating reading may indicate a mechanical engine problem. Using a compression gauge, take compression readings of suspect cylinders and compare readings to the vehicles listed specification. If a specification is not listed compare readings to known good cylinders. If any cylinders have low compression, use a leak down tester to help pinpoint the cylinders sealing issue

• Relearn crankshaft position sensor variation: Follow service information to complete a crankshaft position sensor relearn or crankshaft pattern learn. This is commonly done using a scan tool, by going to the special functions menu and following the on-screen instructions.

How do I repair a P0300 DTC?

If a faulty component is found by following the testing above, replace the component and re-evaluate your engine misfire. If your P0300 is still present or was not diagnosed by following the diagnostic checks, more in-depth and intrusive testing will be needed to pin-point the cause. As listed above there are several complex systems involved in the operation of a modern engine and guessing could become costly.