Engine misfiring? How to tell bad spark plugs from a bad ignition coil

Signs of a misfire

• The engine shakes or runs rough at a light or in Park.
• A stumble, jerk or hesitation when you accelerate, often worse going uphill or merging.
• Less power and worse fuel economy than usual.
• Harder starting, especially in cold or wet weather.
• The check engine light comes on, or flashes while the engine is shaking.
• A raw fuel smell from the exhaust.

If the check engine light is flashing

The EPA explains that a blinking check engine light means a severe engine problem, such as a misfire that can damage the catalytic converter, is happening right now, and that it should be dealt with as soon as possible. Keep driving to a minimum and avoid high speeds and heavy loads. Unburned fuel can overheat the converter, which is a far bigger repair than a plug or a coil.

Spark plugs or ignition coil: how they differ

• Worn spark plugs: symptoms usually build slowly over weeks or months, and often affect several cylinders. The plugs may be past their replacement interval.
• A failing ignition coil: often comes on suddenly and affects one cylinder. It can be worse under load, when the engine is hot, or in damp weather.
• Trouble codes help narrow it down. A code like P0300 means a random or multiple-cylinder misfire; P0301 to P0308 point to a specific cylinder (cylinder 1 to 8).
• On engines with one coil per cylinder, a technician can swap the suspect coil to another cylinder. If the misfire follows the coil, the coil is the problem; if it stays put, look at the plug, the fuel injector or the engine itself.
• Other causes that look the same: a clogged fuel injector, a vacuum leak, a damaged plug wire, oil in the spark plug wells, or low compression in one cylinder.

When spark plugs are due

The replacement interval is in your owner’s manual and varies a lot with the type of plug: from roughly 30,000 miles for traditional copper plugs to 100,000 miles or more for many platinum and iridium plugs. Plugs are usually replaced as a full set, and worn plugs that are overdue also put extra strain on the coils.

If the plugs come out wet with oil, the seals around the spark plug tubes or the valve cover gasket are likely leaking. Oil in the plug wells can cause misfires and ruin new coils, so it should be fixed at the same time.

Misfires and NYS inspection

An active check engine light fails the OBD-II emissions part of the New York State inspection. Clearing the code just before inspection doesn’t help: it also resets the readiness monitors, and the car can fail as "not ready".

How we handle it here

• We read the codes and the live misfire data to see which cylinder is affected and when, then test the coil, plug and injector instead of guessing.
• We replace spark plugs and ignition coils in our shop, and check for oil in the plug wells while we are in there.
• We give you the price before any work starts, and nothing is done without your approval.
• We are open 24 hours a day, 7 days a week, and we speak English and Spanish.