What Causes a Misfire

Uh-oh. Your engine started stumbling, and there's a "check engine" light indicating a misfire. You shouldn't ignore engine misfires, but they can be time consuming and expensive for a shop to track down. We've covered the basics of what causes an engine misfire, but you might still be wondering where to get started fixing it. Here are some of the quickest and most affordable fixes for engine misfires. Hopefully, one of these cheap and easy solutions will get you back on the road.

Source | Chris Isherwood/Flickr

Understand the "check engine" light

That warning light on your gauge cluster is the vehicle's computer letting you know that it is seeing something wrong with your engine's operation. That light, or the code behind it, is a huge clue. You can head to any Advance Auto Parts for a code reading. It takes just a few minutes and can even pinpoint the exact issue, so you don't throw away money guessing at the problem. If you prefer the DIY route, you can purchase a code reader and run the scan yourself. Once the repair is complete, the same tool can clear the code, eliminating the "Check Engine" light.

Spark plugs

If your "Check Engine" light diagnosis shows a generic cylinder misfire code, you should start with the most likely culprits first. Fortunately, these are also the cheapest and easiest. Spark plugs are as low as $2 each, and can be swapped in about an hour with varying difficulty. Basically, it's just using a wrench or socket to remove the old spark plug for a new one, but there is some variability. Swapping plugs on a '90s Civic's inline four cylinder is easier than in a 1970 Corvette's narrow engine bay stuffed with an LS5 454. You'll just need a couple of basic tools to get started and our how-to guide. Even if the code reader shows only cylinder No. 1 is misfiring, replace all the spark plugs at the same time.

On the other hand, be mindful of the fact that some spark plugs now have a service life of about 80 to 100,000 miles. If the plugs are being carbon-fouled by a rich fuel mixture, for instance, you'll need to ascertain why that's happening instead of just replacing the plugs and going on your way.

Spark plug wires

Since you're replacing those old, worn-out spark plugs, it's a good idea to replace the spark plug wires as well, if equipped. Why? You're already working on the same area of the engine and have to remove the wires from the plugs to change the plugs, so replacing wires is only one additional step. Second, they are affordable too, with a set of spark plug wires averaging $30 to $80. Finally, with how long modern spark plugs last, by the time they have failed and are causing a misfire, the wires are probably due for replacement too. First read up on the fundamentals, then see the how-to guide to swapping spark plug wires.

Here's a shade-tree mechanic's way of checking the plug wires: Wait until it's dark, then drive to a parking lot or backroad where there's little light pollution. With the engine running, spray down the plug wires with mist of water from a spray bottle. Watch to see if any arcing occurs up and down the wires or if arcs jump from one wire to another.

Ignition coil

Definitely check out the ignition coil if your engine misfires are accompanied by backfires or stalls. If your vehicle was built before Michael Jordan retired, you likely have a single ignition coil. You can find its location by following the spark plug wires from the distributor. Just a couple of bolts and an electrical connection, and it pops right off. If your engine is from the LeBron James era, then you likely have the more precise coil-on-plug type of ignition coil. Those are affordable, too, but you will need more than one. Sitting directly on top of the spark plugs, these coils are easy to find and swap out.

Many times, failing coil-on-plug ignitions will show an erratic kind of miss and roughness. It might be fine at idle but could buck and jerk randomly when the engine is under load. If you ever experience this, you won't be able to miss it.

Vacuum leaks

If your engine is running lean (not enough gas in the air/fuel mixture), vehicle owners tend to blame a bad fuel injector. Before swapping those out, check under the hood for a vacuum leak that could cause a misfire. With the engine on, listen for a high-pitched hiss. That's a sign, but you likely won't be able to pinpoint the leak without some help. Pro mechanics use a smoke machine, and YouTube mechanics like using starting fluid, but you can use a cheap spray bottle filled with water and liquid dish soap to find a vacuum leak. Spray a cold engine along the vacuum lines, PCV valve, and along the intake manifold gasket. Start the engine and look for soap bubbles. Spray that area again, and you should hear the idle RPM change. That means there's a crack in the hose. Even if you can't see the damage, swap it out. Vacuum hoses are cheap. And please don't use the starter-fluid method, as that is dangerous, and the fluid damages the under-hood rubber and plastic.

Mechanical misfires

On a worn engine, the misfires can sometimes have a mechanical cause.

Timing chains are designed to last the entire life cycle of a vehicle, but a timing chain can still stretch enough over time to alter timing and cause a rough-running condition. Timing advance and retard is controlled by the engine computer, but you should still be able to diagnose a stretched timing chain or worn chain tensioner by using a timing light at idle speed. A stretched timing chain with an excessive amount of slop might also be audible with an engine stethoscope.

A regular miss can also come from a vacuum leak at the intake manifold, which will usually be accompanied by a loud, noticeable hiss at idle and can be pinned down by using the spray bottle/soapy water method discussed above. Bear in mind that some engines are particularly prone to these kinds of failures, such as the Ford 4.6 V8. In addition, some Ford engine families were equipped with aluminum heads and a tendency for the spark plugs to strip and be ejected right out of the head from the force of compression.

Diagnostic trouble codes (DTCs) are a real godsend when it comes to trying to pin down a problem, but they're not the final word. Remember that when you are looking at DTCs, you need to be able to read the signs and deduce what actually triggers that DTC in the engine computer. In other words, don't just presume that it's the sensor in question that needs to be replaced—there's a reason why that sensor is registering a reading that's out of spec.

Ever had a cheap fix for a misfire? Let us know what worked for you in the comments below.

Last updated August 25, 2021