Rick's Free Auto Repair Advice

How to Diagnose and Fix Engine Misfire Code

What is an engine misfire code?

Engine misfire code can pertain to a single cylinder, partner cylinders or random cylinders. Here are the codes and what they mean

What Causes a P0300 Random misfire?

The computer is detecting engine misfires but can’t nail down the misfire pattern to a single cylinder. Since the problem is random, think of a system wide air/fuel related problem like:

• worn spark plugs. If you’re overdue for new spark plugs, your old

Worn spark plug

Worn spark plug

ones may cause a P0300

• Worn spark plug wires—if your vehicle uses spark plug wires

• vacuum leak. This should cause a lean code to be set in the computer, but may cause a P0300 code to appear first. The easiest way to check this is to connect a scan tool with live date and check the short and long term fuel trims.

• unmetered air caused by a tear or break in the air duct after the MAF sensor but before the throttle body. This should cause a lean code to be set in the computer, but may cause a P0300 code to appear first. The easiest way to check this is to connect a scan tool with live date and check the short and long term fuel trims.

fix engine misfire

Tear in air intake hose that allows unmetered air into the engine

• fuel supply problems like low fuel pressure or bad fuel pressure regulator. Check fuel pressure and perform a fuel volume test

What causes a single cylinder engine misfire code?

If you have a single cylinder misfire like: P0301, P0302, P0304, P0305, P0306, P0307, P0308, follow the testing in the order shown below.

The above codes indicate a misfire problem associated with a single cylinder. When a misfire is associated with a single cylinder, the cause is usually:

• bad spark plug

• bad spark plug wire(s)

• bad COP ignition coil

• clogged fuel injector in that cylinder

• worn or sticking intake or exhaust valve

How to diagnose a single cylinder misfire

1 If the engine has spark plug wires: Use a spray bottle to

engine misfire

Arcing from deteriorated spark plug wires

mist the spark plug wires and check for arcing on the plug wire to the misfiring cylinder. If you find arcing, replace the entire set of spark plug wires.

!A If the engine uses coil-on-plug ignition coils, examine the coil from the misfiring cylinder. Check for evidence of fire-through. If you can’t see any failure signs, swap the coil from the misfiring cylinder with a coil from another cylinder. If the coil is bad, the misfire code will show on the different cylinder.

coil on plug ignition coil comparison

Misfiring coil on plug ignition coil. Notice the arcing on the bottom coil. The spark has been shooting righ through the side of the coil above the boot.

 

2 Remove the spark plug from the cylinder and check the spark plug gap. Check the porcelain for signs of carbon tracking. Even if the plug appears good, replace it and apply dielectric grease to the porcelain. Then retest

3 Perform a cylinder leakdown test to check the condition of the valve and valve seats. If leakdown is 10% or greater, you have a wear problem that requires a valve job.

fix engine misfire

Cylinder leakdown tester

4 If the leakdown test shows less than 10%, swap the fuel injector with another cylinder and see if the misfire follows to the new cylinder

What causes a two cylinder engine misfire code?

Determine the type of ignition system. If you have a

DIS coil pack

DIS coil pack. Each coil powers two cylinders

distributorless ignition system (DIS), you’ll see individual coil packs or one large coil with spark plug wire towers for each cylinder. In a DIS ignition system, each coil pack provides ignition for two cylinders. Here’s how it works: Two pistons reach top dead center at the same time. One cylinder is at the top dead center of the compression stroke and the partner cylinder is at top dead center of the exhaust stroke. The ignition call sends high voltage to the  center electrode of the compression spark plug. The

Ignition coil

DIS Ignition coil

spark jumps the gap to the side electrode, producing a hot spark to ignite the air/fuel mixture. The voltage flows through the engine block and jumps from the side electrode to the center electrode on the spark plug of the exhaust stroke cylinder. The voltage then flows up through the spark plug wire and back to the coil pack. So a fault anywhere in that system will result in TWO misfire codes.

How to diagnose two engine misfire codes in a DIS ignition system

1 If the engine has spark plug wires: Use a spray bottle to mist the spark plug wires and check for arcing on the plug wire to the misfiring cylinder. If you find arcing, replace the entire set of spark plug wires.

2 If the coils are separate, swap the coils and see if the misfires move to the new location.

3 Remove the spark plugs and wires and check for spark plug gap, wire connection. Replace and check for misfire codes again.

4 Perform a cylinder leakdown test to check the condition of the valve and valve seats. If leakdown is 10% or greater, you have a wear problem that requires a valve job.

Diagnose engine misfires when cold

This gets a bit tricky because engines often rotate at a slower speed when cold and a slower rotation can cause misfires. Some of the same checks listed above apply. But follow these checks in order.

1 Check oil level and viscosity. If you’re using the wrong oil viscosity (thick oil), it can slow engine cranking to the point of causing cold misfires

2 Check last oil change. If you’ve exceeded the recommended oil change interval, the oil may be degraded to the point where it is too thick

2 If the engine has spark plug wires: Use a spray bottle to mist the spark plug wires and check for arcing on the plug wire to the misfiring cylinder. If you find arcing, replace the entire set of spark plug wires.

Multiple engine misfires or no start

The engine cranks but will not fire up. The causes can be:

• no spark caused by bad spark plugs, bad wires, bad ignition coil, bad crankshaft or camshaft position sensor

• no fuel caused by bad fuel pump, clogged fuel injectors, no fuel injector signal due to bad crankshaft or camshaft position sensor. Use scan tool to check for short and long term fuel trim and crank and camshaft signals.

For more tips on diagnosing engine misfires, read this post

©, 2017 Rick Muscoplat

 

 

 

 

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