4 Signs It’s Time For New Spark Plugs

Spark plugs play a key role in keeping your engine running, since it’s that little jolt of electricity that ignites the fuel/air mixture in a motor’s cylinders. And that little spark has to be delivered in large numbers as the plugs have to fire once for every two revolutions of the crankshaft. That means when an engine is spinning at 3,000 rpm, the spark plugs are firing 1,500 times every minute. They also have to stand up to extreme temperatures, and operate their best when their center electrodes reach 932 to 1,742 degrees Fahrenheit.

While spark plugs are engineered for long life, you will have to replace them eventually — in fact, you should change your spark plugs every 80,000 to 100,000 miles. There are many reasons oil can appear on your spark plugs, all of which can lead to early replacement. Carbon buildups and overheating can cause the same issue. Problems with the gap between the central electrode and the ground electrode — where the spark occurs — can lead to damage as well. The good news is that, generally speaking, you don’t need to adjust the gap on pre-set spark plugs.

Bad spark plugs will affect the combustion process, meaning your engine will tend to generate less power. You could notice worsening performance, worsening efficiency, and even some unexpected sounds from the improper fuel/air ignition. If you’re unsure whether your spark plugs need replacing, here are four signs to watch for.

Read more: Here Are The Worst Car Myths

Problems starting your engine

While spark plugs get your car going in the first place, they do need to get their electricity from somewhere. That’s a key role of the battery in typical cars today. In the old days, the electricity was provided from a magneto, a device that can generate electricity by moving a magnet through a special arrangement of metal wire. Early engines could be started with an electric starter, but they could also be hand-cranked, which would turn the magneto and send electricity to the antique-style spark plugs.

Bosch invented modern high-voltage spark plugs in 1902, but as we discovered when we asked our readers about starting dead cars, magneto ignition lasted for many more years. One commenter reported having to hand-crank his 1960 MGA coupe.

Anyways, the point is that a bad spark plug may not create enough of a spark to fully burn all the fuel and air in a cylinder. When that happens, the combusting materials don’t expand as fast or as strongly as they would from a full spark — and they may not deliver enough power to actually get the motor going on its own. At least not on the first try, and possibly not at all.

Your engine starts complaining

Sometimes, an engine with bad spark plugs will tell you there’s a problem, as long as you speak its language. For example, let’s say you hear a metallic knocking sound coming from under your hood. That could be your motor complaining about detonation, which can also occur if there isn’t enough spark for full combustion. If that happens, the unburned fuel and air that didn’t ignite from the weak spark can ignite on its own, with combustion coming just before the piston is supposed to start pushing exhaust gas out of the cylinder.

The collision between the two forces causes the knocking noise. It can wreak havoc in your engine as well, damaging and destroying pistons, cylinder walls, connecting rods, and more. This is also a good place to remind folks that what we’re talking about here, detonation, is not the same as pre-ignition. That term refers to early fuel/air combustion, from before the spark plug ignites. Pre-ignition can be the result of bad plugs as well, and it’s much more destructive than detonation because a full charge of fuel/air is being combusted, instead of just the leftover material from after a spark.

Your car becomes less efficient

When your spark plugs aren’t working right, any fuel that isn’t burned during the combustion stroke is wasted gas, pure and simple. Consider a car that gets 30 mpg, meaning it can go 30 miles on one gallon of gas. But it has bad plugs that prevent 10% of each gallon from igniting at the right time (during the combustion stroke). In that case, when you put a gallon of gas in the tank, only 90% of it is going to help motivate the vehicle. When you do the math, you see that the car is only achieving 27 mpg. You may as well have just poured that unused gas down the drain.

Of course, that’s going to harm the environment, but so will bad plugs. The fuel they don’t burn enters your exhaust system. There, it can put extra stress on the catalytic converter, which has to work harder to deal with the added emissions, and what isn’t caught by the cat can go right out into the air. However, that may not be the issue it once was now that automakers want the EPA to let them build dirty cars.

Your car doesn’t perform as well

Performance is sort of the other side of the coin here. On the one side, failing spark plugs can mean you have to use more gas to cover the same amount of distance as your car would go with new plugs. Yet for the same reason, it also results in having to use more gas to achieve the same rate of acceleration you’re used to.

Now, some people think you can push that logic to the extremes and raise a car’s base level of engine performance by upgrading the plugs. We don’t believe those so-called performance spark plugs add much horsepower, though — except when they’re replacing old plugs that have already been actively sapping your motors current power.

Using materials like iridium or platinum for the tip can produce a strong, more consistent spark for some boost to power and efficiency, but their real advantage comes through the ability of those materials to withstand the immense pressures and temperatures inside an engine’s cylinders. Beyond the electrode temperature mentioned above, the plug itself can be heated to 5,432 degrees during combustion and be subject to some 735 pounds of pressure per square inch.

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