The engine still misfires after changing the spark plugs. Here’s why

So your changed spark plugs still misfiring? Sorry about the frustration. One of the common ways to fix a misfiring engine is to change the spark plugs.

However, sometimes the problem may not go away even after replacing the plugs, begging the question;

Why does the engine still misfire after changing the spark plugs?

Basically misfiring after changing spark plugs will generally occur as a result of a few issues here and there in the engine. These include but are not limited to the presence of carbon deposits and buildup in the engine, a fuel injector problem, and bad intake manifold gaskets.

Possible causes why the engine is still misfiring after replacing the spark plugs

There’s a Buildup of Carbon Deposits Within Crucial Areas of the Engine

Carbon is a by-product of incomplete fuel combustion in the engine. Unfortunately, this substance will deposit and harden on your crucial engine components like the injector nozzles, cylinder walls, and intake valves.

Over time, it will build up to a large extent, which can be problematic despite the engine having new and well-functioning replacement spark plugs.

To be precise, it can result in cold start misfires when you start the engine when it’s cold, contrary to its normal operating temperature, perhaps due to extreme cold conditions.

A possible solution: Getting rid of the carbon deposits and buildup can help fix the problem. You can do that by cleaning the engine with a special solvent in conjunction with fine steel wool or wire brush.

There’s a Problem With The Fuel Injectors

Your car has several fuel injectors, one injector per cylinder. These tools inject fuel into the intake manifold at a very precise angle, making sure the cylinders fire as expected.

Now, if there’s a problem with an injector, that can cause a misfire in the affected cylinder. Actually, if a misfiring is occurring in only one or two as opposed to all cylinders, that’s a strong indication that it’s a fuel injector issue.

A possible solution: Determining and fixing the injector problem may help make the misfiring issue go away.

Intake Manifold Gaskets Are Bad

Actually, one of the most common symptoms of a bad intake manifold gasket(s) is unexplained misfires or misfires that occur despite the engine having healthy and functional spark plugs.

You see, as the vehicle clocks a higher mileage, the intake manifold gaskets may wear out. Consequently, these components may lose their ability to seal the engine’s vacuum and pressure effectively.

In case you didn’t know, a vacuum leak can significantly disrupt the engine’s air-fuel ratio, causing misfires among other engine problems.

A possible solution: If you determine that it’s indeed bad intake manifold gaskets that are causing misfires, replacing those gaskets can help resolve the problem.

Does The Wrong Replacement of Spark Plugs Still Cause Misfires?

Yes. In fact, replacing spark plugs wrongly can be another major cause of misfires besides the above causes. When you install spark plugs badly, there’s going to be incomplete combustion as the spark plugs won’t be able to fully ignite the engine’s air/fuel mixture.

However, it’s hard to install spark plugs incorrectly unless you under-tighten or over-tighten them. Also, cross-threading them installs them incorrectly and so is installing the spark plug wires(if applicable) improperly.

Can Wrong Spark Plugs Still Cause Misfires After a Change?

The short answer is yes. After all, not all spark plugs are universal. You’ll find that only a specific spark plug model(s) suits your car’s engine. Otherwise, using the wrong set could affect your engine’s normal performance, causing misfires and a myriad of other issues.


If your engine is misfiring, the first step to take is to examine and replace your spark plugs if you think bad spark plugs are what’s causing the problem.

But if the problem doesn’t go away even after changing the spark plugs, check if there are carbon deposits and buildup on your cylinders and other crucial components or areas of your engine because that could also be another reason for the misfires.

If there’s none, check if all of your fuel injectors are working correctly because a bad injector can also be another cause of this problem.

If satisfied that all the injectors are OK, check your intake manifold gaskets, keeping in mind that bad intake MGs can also be another cause of misfires.

Other issues that can cause your engine to misfire include replacing your spark plugs with the wrong spark plugs or installing your replacement spark plugs incorrectly.

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