Tis the season for tune-ups, my friends! Spark plugs are a popular item to change, and it is important to know what a good spark plug vs. a bad one looks like. There are tons of different types of spark plugs (iridium, platinum, standard, pre-gapped, 4 grounded etc.,) but there still are some telltale visual signs that tell you of a bad spark plug (or a poorly running cylinder).
Can you tell if a Spark Plug is bad simply by looking at it? Not always, a visual inspection can give you an idea of any obvious problems and the running condition of your engine, however, it should not be the end all test. If your spark plugs were hard to access and remove then you should always replace them with the OEM Spark Plug anyways (even if they pass a visual inspection). However, if the spark plug is covered in junk, dirty, the electrode damage, the gap completely wrong, or the electrode worn down, these are all visual signs that your spark plug(s) are bad.
Pro tip: The main thing to remember with spark plugs is to always use the OEM plugs, never go by what the parts store recommends or what is on sale! Call your specific car dealership with your VIN number for the best chance of getting the right new spark plug (I say that because it seems with auto parts even dealerships constantly make mistakes).
Keep reading to see a ton of photos showing good spark plugs vs bad plugs. Hopefully, these will help you learn some of the more common telltale signs of bad spark plugs.
Dirty Looking Spark Plugs?
A used spark plug will never look like a new one put side by side. The used one could still be perfectly functional, but the nature of being inside an internal combustion engine means it will get slightly dirty and darker. However, if it is coated in the dirt or junk then it may be a sign that is not creating a good strong spark and combusting the fuel completely. Or there is another issue causing the spark plug to become fouled.
If you spot any spark plugs in your engine that look like these photos then I recommend replacing them immediately. Pay attention after replacement to make sure your engine isn’t showing any other signs of misfires, performance problems etc.
Here are some dirty looking spark plugs:
Spark Plugs Good Vs. Bad Gap Changes:
After lots of use, the electrode part of the spark plug will naturally start to get smaller and smaller. This depends on the type of plug how fast it happens, but you can visually see it if the spark plug is particularly old. Here are some photos of what it looks like when a spark plug gaps change as compared to new properly gapped plugged.
When the gap is off on a spark plug either it won’t produce a spark, or the spark will be weak and not fully ignite the gasoline-air mixture. Either way, it will lead to performance changes for your vehicle. Always remember to double check your spark plug gap if they aren’t pre-gapped or non-gappable plugs. On many newer vehicles, spark plugs come gapped already and should not be touched. Once again consult your FSM (Factory Service Manual) for the proper spark plug and gapping procedure.
Spark Plugs Good Vs. Bad More Photos:
Here are some more photos to help you get better at seeing a “bad” spark plug. You should think of spark plugs as giving you clues to how your engine is running. If you have a six cylinder engine and one of the spark plugs looks really dirty and fouled, then you should pay close attention to that cylinder(s). Are they having misfires later? Are you getting check engine codes?
That is why it is very important to always keep track of what spark plug came from what cylinder. You can mark them with a paint pen is the easiest way or lay them out in good order.
Can bad spark plugs cause misfires? Yes, bad spark plugs can cause engine misfires. The spark plug is responsible for providing the spark that ignites the fuel-air mixture inside the cylinder of the engine. If the spark plug is bad or failing, it can cause the cylinder to not ignite and a myriad of different problems. The most important part before replacing any part on your car or truck is to make sure your diagnosis is correct, don’t just throw parts at it! Spark plugs are typically the first thing people think of when they hear misfire, but when a professional mechanic hears misfire he only knows that further diagnosis must be done.
Can bad spark plugs cause low compression? No, a bad spark plug will not cause a cylinder to have low compression. Compression is how much pressure the piston inside the cylinder creates when it pushes up into the sealed cylinder inside the engine. The spark plug has nothing to do with compression, other than it being tight enough that air isn’t leaking out past the spark plug threads (would have to be extremely loose for this and about to pop out).
Here is a scenario where a bad spark plug could lead to an engine losing compression over time. If the bad spark plug never ignites the fuel being shot in the cylinder, that fuel will drip down past the piston rings into your engine oil. Over time this may cause the oil to go bad with fuel and stop lubricating the engine. Potentially causing the piston rings to wear faster and low compression to start affecting the cylinders and bearings of the engine.
What does a bad spark plug feel misfire feel like? A bad spark plug will cause what is known in the automotive world as a misfire. That means that the cylinder is not properly igniting the air-fuel ratio. Misfires can happen sporadically or a cylinder may be misfiring consistently. The spark plug is only one of the many components that must be functioning properly for a cylinder to not misfire. Therefore, the better question to ask is what does a misfire feel like? Well, depending on the vehicle and the severity of the misfire it may feel like a bucking, stalling, or hiccuping feeling. It will also typically cause the check engine light to come on in modern vehicles.
Can a bad spark plug still spark? Yes, a bad spark plug may still spark. What makes it bad then? The spark may not be “healthy and strong,” meaning that the fuel will not ignite completely or fully. This leads to poor engine performance and potentially a check engine code being thrown on a modern engine. A spark plug may also simply be gapped incorrectly, and by adjusting the gap you may see the poor performance go away completely. It is also true that the spark plug may be receiving a weak electrical impulse from the coil or distributor, thereby causing it to have a “weak” spark. The general rule of thumb is a bright blue spark is healthy, and a more orange looking spark is weaker. This however is a very rudimentary rule of thumb and is typically not used by professional mechanics to diagnose cars or ignition systems.
Can a bad spark plug cause engine knock? Yes, a bad spark plug can cause engine knock. So can a lot of other components on a car, but engine knock aka pinging or detonation happens when something is wrong with the ignition of the fuel-air mixture inside the cylinder. When a car is experiencing engine knocking you typically hear a “pinging” noise coming from the engine. Engine knocking can cause serious internal engine damage that is very costly to fix, so make sure to take your car in for service immediately if you are experiencing engine knocking. Engine knocking can be caused by using the improper spark plug for your vehicle. That is why I always recommend using OEM spark plugs.
What makes spark plugs wear out faster than it should? If you find your spark plugs are coming out looking dirty after a relatively short period of time then you have some other problem. You need to figure out whether your engine is sending each cylinder the correct amount of fuel, and if the spark plugs are receiving the proper electrical signals. Check the timing of your vehicle as well. As always, if you’re running non-OEM spark plugs then you are setting yourself up for failure. Remove the aftermarket spark plugs and replace with the correctly gapped OEM spark plugs asap and retest.
How do I know when I need to change my spark plugs? All car manufacturers will have pre-determined intervals for changing spark plugs. You can always err on the side of caution and replace the spark plugs before the mileage or date indicated by the manufacturer. However, you should never go past the recommended change interval. In modern cars, it can be upwards of 80,000-120,000 miles before any service to spark plugs is needed. Compare that with older cars that may need new spark plugs every 20,000 miles or less. With longer change intervals it is more important to make sure you’re changing your spark plugs when indicated otherwise you will have performance and gas mileage issues.
Is it easy to change spark plugs? Yes and no, spark plugs can be very easy to change on small engines for example. But on a Ford 5.4l Triton engine, you may be looking at spending $500-600 from a shop in labor for a spark plug change. Whether or not a spark plug change is easy or not all depends on the engine configuration and labor times. As a general rule of thumb the more cylinders your engine has the more spark plugs need to be changed, therefore, the more money it will cost.
What are the benefits of changing spark plugs? Better gas mileage, acceleration, and performance to name a few of the benefits of changing spark plugs. Spark plugs are an essential part of any engine running at peak performance. If you never change them, they will wear out, and they will work less efficiently than new spark plugs would. Your engine will not produce as much horsepower. You will also see your gas mileage decrease and acceleration slow. By replacing your spark plus at the proper service integrals for your car you will avoid all the problems people have when they run spark plugs for too long.
Can you clean a dirty/fouled spark plug? Yes, you can. However, with spark plugs being very cheap typically and harder and harder to replace on modern cars you should only do this if you’re working on a lawn mower or small engine. It is mostly an old-school mechanic thing to clean spark plugs and I have seen some use sand-blasting machines to do it. I would recommend not trying to clean spark plugs at home unless you’re just spraying it with brake cleaner and wiping it clean. Otherwise, you risk dirtying the plug more or damaging the electrode/gap by using a wire brush or other cleaner on the spark plug.
Should I put anti-seize on spark plugs before installing? This depends on who you ask. You can read my entire post examining this question here, but what I believe from my research to be true is if you understand anti-seize and use it correctly you should apply it to spark plugs every time they are removed from an engine and installed. If you are a beginner with anti-seize remember these KEY concepts: anti-seize reduces friction on the threads of the spark plug making over-tightening the plug easier, you must only use a TINY amount of anti-seize and apply it only to the middle of the spark plug threads (do not get any near the electrode or the base), lastly, use only high-quality anti-seize (I prefer Nickel based anti-seize).
Spark Plug Chart for Reference: Here is a spark plug chart showing all the different ways to “read” spark plugs. This is more of an old-school mechanic test. However, it is nice to know the most important tell-tale signs to look for on spark plugs. Spark plug super clean? Head gasket leak causing coolant to steam clean the plug. Spark plug wet and smell like gas? No spark or the plug is broken (cylinder is getting fuel but the plug isn’t igniting it). Spark plug missing the ground strap part, it broke off completely? The piston likely made contact with the spark plug which indicates a timing problem and severe engine problems. Further diagnosis is needed. Spark plug threads rusty and the plug was hard to remove? Make sure to clean the threads out by either blowing compressed air into the cylinder or using a thread chaser. Also, be sure to use a tiny bit of anti-seize on the spark plugs before reinstalling so as to avoid a stuck plug in the future. Here is the chart and the source.
For a supreme guide, not formatted very well, but a TON of great information about advanced spark plug reading for tuned engines click here. Should I replace my spark plug wires every time I change my spark plugs? Yes, if you want to spend the extra money and be absolutely certain your car is maintained to the highest standard. However, unless there is a problem with your spark plug wires they can typically last more than one spark plug rotation.
Spark plug wires can go bad independently of spark plugs and cause all the same issues like a bad spark plug including misfires and performance problems. Search diagnosing bad spark plug wires to see how to go about testing them and finding problems with your wires.
One of the most common sparks plug wire problems is they will have a rip or cut in the insulation and the spark will ground out through the wire before reaching the spark plug (causing an intermittent or constant misfire). You can sometimes find these problems by grounding a test light to the negative post on your battery and moving it around your spark plug wires. If you ever see a spark jump to your test light then you know you have a bad spark plug wire.
Good spark plugs vs bad spark plugs sometimes have some very clear visual telltale signs. However, a spark plug can come bad from the factory and by just looking visually at a spark plug you can’t know if it is working properly or not. Spark plugs typically are one of the cheapest, yet most important part of any gasoline engine. Therefore, it is always best to just replace them if you suspect they are bad. However, if a mechanic is telling you that you need to replace your spark plugs with no reasoning behind it (and your car isn’t due for a change) then go to a different mechanic. Keep up on your car’s maintenance including oil changes and spark plug changes and you will get the best gas mileage possible and performance.
Hope you enjoyed the post and learned something! It’s my style! Share your ways and opinion in comments… How do you check Good Spark Plugs Vs Bad One?
Here are some of my favorite tools for diagnosing spark or ignition problems on anything from a small engine to a car or truck. Also, having a good set of spark plug sockets is absolutely ESSENTIAL to changing spark plugs. You will get nowhere with standard sockets unless it is simplest of simple small engines with easy to access plugs. For most modern cars, you’ll be lucky if a standard socket even fits down the spark plug tube hole.Inline Spark Tester Best Anti-Seize Favorite Spark Plug Socket Set Best Spark Plug Gapper Best Spark Plug Wire Pliers
Click here to read my list of the top ten tool inventions that have made car repair easier in 2019. Do you own any of these?