Rick's Free Auto Repair Advice

Diagnose misfires

Diagnose misfires

If you own a vehicle with coil packs and spark plug wires or a coil-in-plug setup, you may see these misfire codes. This post will help you diagnose misfires.

Diagnose misfires — is it the spark plug or the ignition coil?

Whether your engine is equipped with coil packs and spark plug wires or coil-on-plugs, always diagnose misfires by checking the spark plugs. Why? Because a worn or damaged plug can degrade your spark plug wires and even damage the ignition coil. If you replace the wires or coil without dealing with the root cause of a bad spark plug, you’ll just damage the new components.

How to check spark plug condition

How many miles are on the plugs?

When DIS and Coil-on-Plug ignition systems came out and carmakers switched to platinum spark plugs, spark plug change intervals jumped to as high as 100,000 miles. Then, when iridium plugs were introduced, intervals jumped again to as high as 125,000. Then carmakers introduced turbochargers and that changed everything. Many turbo equipped engines require new spark plugs every 30,000 miles. 

So you MUST diagnose misfires by first checked when the plugs were last replaced. Check the carmakers recommended spark plug change intervals for the specific year, make, model, and engine you’re working on.

Check spark plug gap

Use a wire style gap gauge to check the gap. If the gap is greater than the carmaker’s specs, install new plugs. Never close the gap on used plugs with excessive gaps just to save money.

worn spark plug gap

These spark plugs have excessive gaps.

Check the center and side electrode condition

The spark always jumps from a sharp edge on the center electrode to a sharp edge on the side electrode. If the center electrode has rounded edges, it’s automatically requiring a much higher firing voltage. Replace it.

Check the spark plug color

Compare your spark plugs to the conditions on this chart supplied by Autolite Spark Plugs.

spark plug color chart

Click on the image to open a PDF of the Autolite Spark Plug Color Chart

Diagnose misfires caused by the ignition coil

Ignition coil factoid: Coil packs and coil-on-plug ignition coils usually don’t go bad on their own unless they’re sprayed with water when hot. The best way to fry a coil pack is to run the engine with worn or damaged spark plugs or worn spark plug wires. Here’s why.

#1 Cause of ignition coil failure — worn spark plugs and/or spark plug wires

Excessive firing voltage caused by worn spark plugs (the excessive spark plug gap), excessive resistance or open in the spark plug wires. The higher than normal firing voltages raise coil temperatures and degrade coil windings.

#2 Cause of ignition coil failure — air/fuel-related problems creating a lean mixture

Lean air/fuel ratios caused by clogged fuel injectors or vacuum leaks that require much hiring firing voltages to establish the spark. The higher than normal firing voltages raise coil temperatures and degrade coil windings.

#3 Cause of ignition coil failure — winter temperatures and worn spark plugs

It takes a much higher firing voltage to ignite cold fuel and cold air. If the spark plugs are worn, it takes even higher voltage, and that damages the ignition coil.

How an ignition coil is damaged

In cases where the ignition coil must constantly generate a higher than normal firing voltage due to worn plugs, spark plug wires, or a lean air/fuel condition, the higher voltage causes excess heat which degrades the coil winding insulation, causing coil failure over time.

Misfire code definitions

P0300 – Random/Multiple Cylinder Misfire Detected
P0301 – Cylinder 1 Misfire Detected
P0302 – Cylinder 2 Misfire Detected
P0303 – Cylinder 3 Misfire Detected
P0304 – Cylinder 4 Misfire Detected
P0305 – Cylinder 5 Misfire Detected
P0306 – Cylinder 6 Misfire Detected
P0307 – Cylinder 7 Misfire Detected
P0308 – Cylinder 8 Misfire Detected
P0309 – Cylinder 9 Misfire Detected
P0310 – Cylinder 10 Misfire Detected
P0311 – Cylinder 11 Misfire Detected
P0312 – Cylinder 12 Misfire Detected

© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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