|Trouble Code||Fault Location||Probable Cause|
|P0301||Cylinder 1 -misfire detected||Engine mechanical fault, wiring, ignition/fuel system, injector, ECT/MAF sensor, ECM|
What Does P0301 Mean?
A misfire is just what it sounds like; a cylinder that is not firing as it should. The P0301 code is caused by a misfire on the number one cylinder. The Misfire monitor diagnostic is based on variation in crankshaft velocity. The PCM determines this information using the crankshaft position sensor and camshaft position sensor. A misfiring cylinder slows down momentarily, so by monitoring the cam and crank sensor signals, the PCM can calculate when a misfire occurs. A P0301 code tells you only that a misfire has been detected, not WHY the cylinder is misfiring. That’s why with this code, a thorough diagnosis is mandatory.
Common Causes of P0301
A single cylinder misfire is typically caused by one of the following:
(1) Spark problem
(2) Compression problem
(3) Fuel injection problem
1. Spark Problem
The ignition system converts battery voltage into the high voltage used to ignite the cylinder air/fuel mixture. The low voltage side of the system is referred to as primary circuit. It includes the battery, ignition switch, primary coil winding, a triggering mechanism (such as a crankshaft sensor) and switching device (such as an ignition module or ECU). The high voltage side of the system is called the secondary circuit. It includes the secondary coil winding, spark plugs and spark plug wires. In a system that uses a distributor, the secondary circuit also includes the cap and rotor.
Diagram of a simple distributor-less ignition system
It’s fairly easy to check if spark is being delivered to the plugs. All you need is an inline spark tester. The tester is installed with one end connected to the plug wire and the other to a good ground. If the spark tester lights up, the primary side of your ignition circuit is good, along with the plug wires and/or coil.
What the spark tester doesn’t reveal is whether the spark plug itself is bad. The best way to test the entire ignition system, including the plugs, is with an oscilloscope. Attaching the scope ignition lead to one of the spark plug wires will provide you with a trace of the entire ignition waveform pattern. Then, you can compare a known good waveform pattern to the waveform pattern of the vehicle being tested. Using a scope, you can properly diagnosis everything from a bad coil pack to a worn spark plug.
Oscilloscope ignition waveform pattern
Of course, if you don’t have access to a scope, you can test the spark being delivered to the plug using a spark tester. Then pull the park plug out and inspect it for wear or damage.
2. Compression Problem
Mechanical issues can also cause a cylinder misfire. These issues can be pinpointed by performing a compression and/or leak down test.
Testing the compression of a cylinder is easy. Attach your pressure gauge to the test cylinder and obtain a pressure reading by starting the engine. A good cylinder reading is generally around 140 PSI. Make sure you disable the ignition coil, fuel injectors, and spark plugs before testing the cylinder so that the car does not start during the test. Also, make sure your battery is fully charged or the pressure readings will be low. For more information, Linn Benton Community college has an excellent article on compression testing: http://www.linnbenton.edu/auto/perform/compress.html
A relative compression test should be performed on the engine to verify what a good cylinder reading is for your car. The relative compression test is performed by testing one of the non-misfiring cylinders. The low compression reading for the two cylinders should be within 70% of the high reading.
If a cylinder is found to have low compression, perform a “wet compression” test by squirting a small amount of oil into the cylinder. If the new reading increases significantly over the initial reading, the piston rings are worn. If the results don’t change, you should move on to performing a cylinder leak down test.
Before performing a leak down test, ensure the cylinder is at top dead center on the compression stroke. Next, use the compression tester to fill the cylinder with air while monitoring the gauges and listening for air leaks. Air heard coming from the oil fill area with the cap removed indicates excessive ring leakage. Air escaping out of the tailpipe points to a leaking exhaust valve and air leaking from the throttle body indicates a leaking intake valve. Finally, remove the radiator cap to check for air bubbles in the coolant, indicating a bad head gasket or cracked head. Here’s a video on performing a leak down test: https://www.youtube.com/watch?v=ofSiTGeLXvc
How to hook up a leak down tester
4. Fuel Injection Problem
In the past, most fuel injected engine used a continuous return system. However, more and more modern vehicles are using a returnless system.
With the continuous system, fuel is delivered from the fuel pump to a fuel meter assembly. The pump is designed to deliver more fuel than the engine requires. A regulator maintains the required fuel pressure by routing the excess fuel bank to the tank via a return line.
Continuous return system
A returnless system on the other hand, has no return line. Instead, excess fuel is returned to the tank through a passage in the tank-mounted regulator. This system is widely used on newer vehicles because it reduces emissions.
returnless fuel system
Port injection and direct injection are the two fuel injection methods in use today. With port injection, fuel is sprayed into the intake ports. With direct injection, fuel is sprayed directly into the combustion chamber.
When the solenoid inside a fuel injector is energized, it lifts a valve off inside the injector and fuel is sprayed into the engine. Typically, there is always battery power at one of the two injector terminals. When fuel injection is required, the ECU grounds the other terminal. This completes the circuit and fuel is sprayed into the cylinder.
If an injector is bad either electrically or mechanically, it can cause a cylinder misfire. The easiest way to check if the control circuit to the injector is good is by using a noid light. You simply disconnect the injector and insert the light into connector. If the noid light flashes when cranking then engine, the control circuit is functioning properly.
Testing with a noid light
To test the electronic portion of the injector, measure the resistance between its two terminals. The manufactures resistance value specifications can be found in the vehicle repair manual. Of course, an over limit reading (OL) on your meter, indicates there is an open circuit and your injector requires replacement.
Checking the injector with a digital multimeter
Sometimes, however, a static resistance test is not enough to pinpoint a bad injector. The best tool for determining injector health is a digital oscilloscope. Using a scope, you can view a waveform pattern of the injector circuit operation. By comparing the waveform on the screen to that of a known good injector, you can quickly pinpoint injector circuit faults.
Oscilloscope injector waveform pattern
In addition to electrical problems, injectors can experience mechanical failure and become clogged. To determine if the injectors are restricted, an injector balance test should be performed using a dedicated tester. Here’s is a good video showing how to perform an injector balance test: https://www.youtube.com/watch?v=K7STocr9FBI
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