Your car’s spark plugs contribute to your fuel mileage and engine performance. Each spark plug sees temperatures and pressures that can damage some metals.
They should last 75,000 to 100,000 miles, but outside factors can cause them to fail earlier than expected.
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When that happens, performance suffers, and you will start to notice problems popping up unexpectedly.
The most common problems with spark plugs can be masked by other engine problems, so we’ll go through each problem and its solution to help diagnose the root cause and quickly fix it.
#1 – Your Engine Has Trouble Starting
Your engine needs fuel, air, and spark to run, but sometimes it’s hard to know which is missing when it won’t start.
If the engine has trouble starting, it may be because the air filter is clogged, and the combustion process isn’t receiving the oxygen needed. It could also be because the fuel system filters are clogged with contaminants.
It may take two or three tries before the engine sputters to life with spark plugs that need to be checked.
How to Fix It
Knowing the combustion process needs three key ingredients for the engine to start, the first step in the diagnosis is to start eliminating the potential problems.
Start with eliminating the oxygen as the air filter is easy to remove from the intake system. If the air filter is a problem, removing it from the airbox should allow more air into the engine immediately. If you attempt to start the engine and nothing changes, the air filter isn’t the problem.
You can diagnose if the fuel system is the problem by spraying a starting fluid into the intake manifold. The spray will atomize and be distributed to each engine cylinder, and it should help the engine start immediately. If you spray starting fluid into the engine and it still won’t start, the fuel system isn’t likely to be contributing to the problem.
That will leave the spark as your likely contributing factor that won’t help the engine start. If the engine starts after a few attempts, all spark plugs may need to be checked or replaced.
If the engine stumbles and eventually roars to life, it could only be a portion of the spark plugs causing the poor-start condition.
At this point, you should commit to removing each plug one at a time and checking them. If they need to be replaced, remove each plug and replace it with a new one. Always replace every plug as a complete set rather than just one or two.
#2 – The Engine Starts But Idles Rough
If your car starts, but idles rough, it does indicate that you have all three key ingredients for the engine to run.
One or more of them are sporadic and is causing the engine to idle rough. Is it a filter or a spark plug causing the rough idle? That is what you must determine.
How to Fix It
Spark plugs fire hundreds of times per minute when the engine is idling, and when that is disrupted, it can cause surging, stumbling, and excess vibrations.
You can check if the air and/or fuel filters are contributing to the idling problem as noted with problem #1.
When you think it is the spark plugs, you need to remove each plug one at a time and check their condition.
If they are fouled with oil, coolant, or too much fuel, a new plug should be installed. Change one plug at a time when the engine is cold to prevent mixing up spark plug wires and installing them on the wrong cylinder.
#3 – Your Engine Starts But Eventually Stalls
When your car starts and idles, it uses the minimum required fuel, air, and spark needed to keep the engine running.
A cold engine will add extra fuel to the combustion chamber to help the engine warm up faster, and that extra fuel can mask a problem with the spark plugs. It may seem fine until the engine is warm, and then it can start stalling because of bad spark plugs.
How to Fix It
Diagnosing problems requires a bit of situational awareness and noting that the engine eventually stalls out when the car is warmed up is a clue about this exact problem.
The fuel system adds extra fuel into the combustion chamber until it detects that the engine is warm regardless of whether it has a carburetor or is controlled by a computer.
The amount of fuel is reduced when the engine is warm, and a weak spark can fail to ignite the reduced amount of fuel.
A weak spark can be caused by the coil, distributor, spark plug wires, or spark plugs. A service manual can help you determine if each component is within the manufacturer’s range, and if it isn’t it should be replaced with a new component.
#4 – Your Engine Has Reduced Power And Sputters
You may not realize there is a problem with your engine power until you step on the gas pedal. Your car may not show any indication of reduced power until you try to pass another car, and then it just doesn’t have the power to execute the pass quickly.
How to Fix It
Diagnosing reduced power is again going to take situational awareness to recognize that your car seems fine until you step on the gas pedal and need to make that pass.
Knowing that you need three key ingredients for combustion, you can quickly check the air filter to eliminate a dirty filter as a cause for the stumbling and power loss.
You would also see black smoke from the exhaust pipe because it would have more fuel than air causing it to run rich. If it was having trouble with fuel delivery, it would backfire and pop due to a low fuel issue.
If it just feels sluggish without smoking or misfiring, the next logical step is to check the ignition components like the spark plugs.
#5 – You Calculate A Significant Drop In Fuel Economy
After every fill-up, you should take a few seconds to determine the average fuel mileage or fuel economy over the last tank of fuel.
Keeping an eye on how efficient your car has been is one easy way to recognize when something significant has changed for the worse.
Dividing the distance driven by the quantity of fuel added to the tank will give you an approximate idea if your engine is running consistently and efficiently.
How to Fix It
Fuel economy will slowly get worse as the spark plugs wear, but at a certain point it will decrease significantly when all plugs need to be replaced.
It is an easy check if the air filter needs to be replaced, and the fuel filter should be replaced at 30,000-mile intervals. If those two items are good, the next is to assume the ignition system could need maintenance.
The spark plugs are easy to check and replace if they show significant wear or fouling due to oil, coolant, or fuel.
#6 – Your Exhaust Smells Like Raw Fuel
Worn spark plugs will cause the fuel economy to go down because your engine won’t burn all fuel effectively. When fuel remains after the combustion cycle, the exhaust gas exiting the tailpipe will smell like unburned raw fuel.
How to Fix It
If your car’s exhaust smells like fuel, you should immediately investigate why.
Worn spark plugs can leave unburned fuel in the engine, which is then sent to the catalytic converter in your exhaust system. These components can be damaged if they are exposed to raw fuel.
When you remove the spark plugs for review, you may see that they are wet with fuel. This fuel coating on the plug is a direct indication it is not burning all the fuel in the combustion chamber.
#7 – The Engine Starts Misfiring
Smelling fuel in the exhaust can be hazardous to your health, and the leftover fuel in the engine can ignite at the wrong time to cause a misfire condition.
How to Fix It
Misfiring can sound like a popping noise from the exhaust tailpipe or a backfire from the intake system.
It indicates that fuel in the combustion chamber is igniting at the wrong time, such as when an intake or exhaust valve is open rather than closed.
Misfires can cause significant damage to intake components, sensors, and catalytic converters.
If your engine is popping or sputtering from a misfire, immediately stop the engine and remove each spark plug separately after the engine has cooled. The cost of a new set of spark plugs is economical compared to the potential cost of damages they can contribute to.
#8 – Your Car Illuminates The Check Engine Light (CEL)
Most cars since the 1980s have a diagnostic system for the engine and its associated components.
When the system detects a potential problem, it will illuminate the Check Engine Light (CEL) to alert you that there may be a problem.
How to Fix It
Your Check Engine Light indicates that the control unit has detected a potential problem that you should be aware of.
Many times, the light coincides with a problem you hear and feel such as a misfire condition. But, subtle changes in engine performance don’t always have a physical manifestation you can detect.
If the Check Engine Light illuminates in your gauge cluster, you can connect a hand-held scanning tool to the computer and review the diagnostic trouble codes.
- Trouble code P03xx is associated with the spark plugs, and the ‘xx’ indicates which cylinder may be the problem.
- P0301 would be associated with cylinder #1, P0302 with cylinder #2, etc.
These codes can indicate the spark plugs, spark plug wires, or the ignition coil are faulty, but they will point you in the right direction to start your investigation.
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