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Engine misfire diagnostics

Engine misfire diagnostics

The most common causes of engine misfires are worn or faulty spark plugs and faulty ignition coils. For more information misfires, see these posts first. This post picks up after those and covers other engine misfire diagnostics.

How spark plugs work

Diagnose misfires

Other common causes of engine misfires

The wrong spark plugs

• improper cylinder pressure caused by ring or valve wear
• high or low ambient temperature
• improper air/fuel ratio
• improper valve timing
• fuel quality
• carbon deposits.

Let’s examine each of these.

The Wrong spark plugs can cause misfires

The proper spark plug heat range is critical

Each carmaker recommends a spark plug with a specified heat range. If you install a spark plug with a different heat range, the plug can run too hot or too cold, causing misfires. In addition, changing the type of spark plug can cause misfires.

The proper electrode material is important

You can upgrade from a platinum plug to an iridium plug. But you should never downgrade from a precious metal style plug to a copper plug. On older vehicles with point ignitions systems, you should NOT upgrade to platinum or iridium. Those plugs require higher firing voltages; voltages that may stress your point style ignition system.

Switching ground electrode design can cause misfires

Some spark plug manufacturers promote an ‘open” multiple ground electrode design, as opposed to a “closed” “J” style side electrode. The open design supposedly projects the flame kernel downward into the combustion chamber for a more efficient and more powerful burn. That sounds good in theory as long as the engine was designed for that type of flame spread. But most carmakers design their engines to provide a swirl airflow pattern into the cylinder. The swirl is designed to properly ignite a lean mixture. In those designs, the swirl passes past the flame kernel inside the “J” channel and propagates from there. Changing from a “J” side electrode to an open multiple side electrode design can cause the flame kernel to disrupt the swirl, causing a misfire that actually decreases performance.

In other words, if the recommended spark plug for your engine is a single “J” style side electrode and you switch to a different style plug with multiple side electrodes, you can cause a misfire.

High cylinder pressure can cause misfires

As cylinder pressure increases, so does the voltage need to fire the spark plug. What causes cylinder pressure to increase? Putting the engine under load: climbing a hill, hauling a heavy load, putting the “pedal-to-the-metal” from a stop.

If your spark plugs are worn, placing the engine under heavy load will increase cylinder pressure, increase resistance across the spark plug gap and increase the needed firing voltage to jump the gap. That’s why a spark plug may perform just fine during cruising, but misfire when you accelerate harder.

This phenomenon is also why testing a spark plug outside the engine is not a valid test. A spark plug and ignition coil combination can fire perfectly fine when grounded to the engine in the open air, but fail when installed in the engine.

High cylinder pressure and worn spark plugs cause coil damage

Putting an engine under load when the spark plugs are worn forces the coils to output higher than normal firing voltages. That overheats the coil and degrades the insulation on the windings. The result is misfire and coil failure.

Low cylinder pressure causes poor combustion and misfire

Worn piston rings and worn valves prevent the engine from developing proper compression. Lower compression lowers spark plug firing voltage, but the lower compression also reduces combustion efficiency, resulting in incomplete combustion induced misfires.

Cold temperatures increase firing voltage needs

Cold air and cold fuel are harder to ignite. When you combine cold air and fuel with a worn spark plug, you get misfires and failure to start. That is why worn spark plugs may perform fine in early Fall but misfire or fail to start the engine in winter.

High heat and humidity increases firing voltage needs

Hot humid air is less dense and is harder to ignite, requiring higher than normal firing voltage. If the spark plugs are worn or the coil windings are degraded, you’ll get poor performance and misfires in hot humid conditions.

Clogged fuel injectors cause misfires

Clogged fuel injectors result in less fuel entering the combustion chamber. Most modern engines already run on lean mixtures to meet emissions standards. But when the injectors are clogged, the air/fuel mixture is so lean that it can’t ignite or propagate properly, resulting in misfires.

Low fuel pressure causes misfires

Just like clogged fuel injectors, low fuel pressure or low fuel volume causes misfires because it results in a lean mixture.

Improper valve timing causes misfires

Most modern engines incorporate variable valve timing (VVT) mechanisms. Those systems change camshaft and valve timing by pulsing high-pressure engine oil into advance/retard chambers that rotate the camshaft in either an advance or retard position. Oil change neglect or using the wrong oil viscosity can greatly affect how these VVT systems work, causing the valves to open too soon or too late, resulting in misfires.

Fuel quality affects ignition performance and can cause misfires

Low-quality fuel can cause excessive carbon buildup, clogged fuel injectors and air/fuel problems.

Carbon deposits cause misfires

Carbon deposits on the spark plug, valves, cylinder head, or piston crown can cause pre-ignition

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