Rick's Free Auto Repair Advice

Random Misfires — How to diagnose and fix

How to diagnose random misfires

The #1 cause of random misfires is worn, faulty, improperly installed or incorrect spark plugs

If you’ve recently installed new spark plugs and are now getting a P0300 with random misfires, and you didn’t use a torque wrench to install the new plugs, that may be the cause. Or you may have installed the wrong heat range plug or the wrong type of plug.

If you’re overdue for spark plug replacement and are now getting a random misfire P0300 code, that’s a sign you’re due for new spark plugs.

What makes a spark plug misfire?

• The gap has worn and is now too large
• The center electrode is rounded requiring a much higher firing voltage that results in a weak spark with too short a duration

worn versus new spark plug 2

The #2 cause of random misfires — fuel pressure and volume issues

Keep in mind that a P0300 affects all cylinders, so the root cause would have to be a failure that affects all cylinders

• Improper fuel pressure — fuel pressure is too low, causing a lean misfire or too high, causing a rich misfire
• Clogged fuel filter — A clogged fuel filter would affect fuel volume and pressure
• Low fuel volume — The pump can output the correct pressure but not enough volume to keep up with engine demands. Low fuel volume would starve all the cylinders, mostly at higher speeds or higher loads
• Low fuel quality — Yeah, here we’re talking about bad gas that would affect all cylinder.

The #3 cause of random misfires — air leaks

An air, vacuum, or exhaust leak that causes an improper

torn air duct

Torn intake air duct

air/fuel ratio or skews oxygen sensor readings will affect all cylinders and cause random misfires. A P0300 caused by an air leak will most likely have other sensor-related or fuel-trim related trouble codes listed.

The #4 cause of random misfires — mechanical issues

• improper valve timing due to clogged or worn VVT mechanisms
• Timing chain wear or timing chain/ belt tensioner issues that throw off valve timing

What things would NOT cause random misfires or trouble code P0300

Bad fuel injector

A bad fuel injector would only affect a single cylinder. The likelihood of having all or multiple bad fuel injectors is almost zero.

Bad ignition coil

A bad ignition coil would only affect a single cylinder. The likelihood of having all or multiple bad ignition coils is almost zero.

Bad head gasket

A bad head gasket would affect combustion in just the affected cylinder, not all the cylinders

Bad camshaft, push rods or lifters

A bad bad camshaft, push rods or lifters would affect combustion in just the affected cylinder, not all the cylinders


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.

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.

©, 2021 Rick Muscoplat



Posted on by Rick Muscoplat

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