Last Updated: 07.04.2022

The spark plug is one of the most essential components in the engine. Since it’s responsible for the functioning of the engine (combustion), its health has a direct impact on the performance of a vehicle. And that’s not all: a bad spark plug can have adverse effects on the vehicle as well. We take a look at the effects of a bad spark plug on a car or truck. Please note that this is valid only for gas-powered cars; diesel engines don’t have a spark plug.

Before going further, let’s first recapitulate what a spark plug does and why it’s important. In an internal combustion gas engine, the spark plug ignites the air-fuel mixture. This process, in which electricity is transferred between the two electrodes of a spark plug to create controlled combustion, is responsible for developing power. This happens thousands of times in a minute, and a spark plug failure means the engine will suffer both in the way it delivers power and also the way it consumes fuel. 

Drop in Performance

In a multi-cylinder vehicle (like all cars and trucks), if a spark plug fails, it essentially renders that cylinder useless. This means the engine’s effective capacity goes down, and the power it generates is reduced by a substantial percentage. Do keep in mind that the drop in performance can also be an issue with the ignition system. 

The reduction in performance is easily felt when you want to accelerate (both from a standstill and in motion). It can be a bit unnerving especially in an emergency when you have to make a quick maneuver.

Rough idling and inconsistent power

A bad spark plug can cause misfires. Misfire is basically the engine missing a beat on one of its cylinders. That means its power delivery won’t be smooth — and it’s most likely to cause more vibrations. That can easily be felt during idling — that is when the engine at its lowest revs and the car not in motion. Misfires can occur due to faulty spark plugs.

Due to misfires, power delivery is bound to be inconsistent. So while there will most likely be a drop in power, you might witness a surge of power as well. This anomaly can lead to the car being uncontrollable.

Bad fuel economy

Another negative about having a bad spark plug is the reduction in fuel economy. Since the engine is far from its optimal condition, it will burn more gas to make the car move. Now that’s unlikely to be noticed until you realize that the trips to the gas station have become more frequent.

Apart from the economical and slight effect on the environment, this is dangerous because if users don’t pay attention, it’s easy to ignore this sign. But if you pay attention to how much fuel your car is guzzling, this won’t get ignored. And it will probably ensure that you don’t run into any of the other troubles mentioned here.

Inability to start

Imagine you have to go somewhere and the car just wouldn’t start. The battery is all right, and so is everything else including the fuel as well. So what could it be? The spark plugs. It’s a possibility that bad spark plugs can make the car inoperable. A faulty spark plug is one of the things that can lead to this, but you can’t be absolutely sure until you check. Having said that, spark plugs have more chances of wearing out (since they have a limited life), hence there’s a solid reason why we say that.

Component Failure

A bad spark plug can have adverse effects on other components as well. For instance, if you continue to drive the truck with a faulty plug and it shows hesitation in starting up, it’ll keep burdening the onboard battery.

Another component that can have a direct effect due to the faulty plug is the catalytic converter. In the case of misfires (common if a spark plug fails), the exhaust system — mainly the catalytic converter — can be affected. And that’s bound to be a bigger expense.

But is there a way you can ensure that none of these happen to you?

If you stick close to the manufacturer-specified intervals for changing spark plugs, there are fewer chances of it happening to you. Keep a close eye on how your car behaves as well. If you suspect that it’s losing power, or is irregular in producing power, you must slow down and get it inspected as soon as possible.

Routine check-ups do help a lot. And even if you can’t check the spark plug gap, you can always ask your trusted mechanic to do that. In addition to letting you know about the condition of the spark plug itself, the plug can actually tell you a lot about the engine’s health as well. For instance, a slightly dirty plug is normal but if it’s too white or the deposits are glossy, then you are looking at other issues. The worst of these can be the presence of oil on the plug because it’s not supposed to get oil on it.

At the time of replacement, you can choose spark plugs that last longer. These plugs, with Iridium or Platinum tips, are slightly more expensive than the conventional (nickel) ones, but with a longer life, you save up on labor charges — if you were to get the plugs installed at a mechanic’s. There are quite a few nice spark-plug brands that you can choose from. Check out how to change spark plugs for F150 4.6 L engine

A spark plug’s performance is largely dependent on its gap. Do check if the ones you have need to have the gap set, before replacing them. This needs to be kept in mind for new ones as well. All plugs ideally come pre-gapped, but you must stick to the manufacturer-specified gap. Otherwise, you can run into a variety of problems like the above-mentioned ones. So it’s a good idea to have the gap checked on the new plugs — and if needed, have the gap set by a mechanic (or if you can do it yourself — needs some equipment for that).