Cleaning Pistons Through Spark Plug Hole: Experts Tips

If you love fixing things at home, you may have tried cleaning pistons through spark plug holes on your petrol generator. In generators, the DIYer’s approach to cleaning pistons is to scrap the pistons with a tiny long screwdriver. But what you do not know is that you can clean your car pistons through plug holes.

If you are sure you have carbon buildup on the piston, you don’t need to visit the mechanic to take off the cylinder head for cleaning purposes. You can clean it at home without any assistance.

In this article, you’ll learn what to spray in spark plug holes when cleaning pistons, the causes of dirty pistons, and other ways of cleaning pistons without removing head. Grab a seat while I uncover all you need to know.

what to spray in spark plug hole

What causes dirty pistons?

Factors like wrong plug gaps, dirty air filters, dirty fuel injectors, a rich air-fuel mixture, and more of stop-and-go driving are the possible causes of dirty pistons.

Rich air-fuel mixture

Every gasoline-powered engine needs a predefined air-fuel ratio for an optimal combustion process. Every car engine has many components and car sensors that come together to help the engine determine a balanced air-fuel mixture at any point in time. If any of these components go bad, the air-fuel ratio will be off-balanced.

Rich air-fuel mixture can happen in two ways. It can occur when too much fuel enters the combustion chamber or when too little air enters the engine. Too much fuel in the combustion chamber will reduce the spark plug tip temperature, preventing it from getting to its self-cleaning temperature. Once this happens, car deposits will accumulate on the pistons and the plugs.

Driving habits

I mentioned earlier that spark plugs have some self-cleaning properties. However, the engine must run under load for some time before reaching the spark plug self-cleaning temperature of about 450 degrees centigrade or 842 degrees Fahrenheit.

Factors and habits like continuous low-speed riding, too much stop-and-go driving, prolonged idling, and frequent traveling in icy areas can limit your spark plugs from getting to their self-cleaning temperature.

In this case, the best way to prevent fouling on the plugs and pistons is by traveling long distances at least once a week, preventing long idling, and doing more aggressive driving.

Too large spark plug gap

Most spark plugs come pre-gapped from the factory. It’ll not always match all engine specifications. Driving with an incorrect spark plug gap will cause poor acceleration, engine misfire, bad gas mileage, and carbon fouling on the spark plugs and the pistons.

If you think your spark plug gap needs adjustment, measure the gap using a spark plug gap tool and make the necessary adjustment.

The spark plug heat range is too cold

Aside from city driving, long idling, and driving in icy areas, spark plugs with too cold heat range may not reach a self-cleaning state.

The spark plug’s heat range tells us at what range the plug can dissipate its firing heat to the head cylinder. Using plugs with low heat range can cause carbon fouling on the plug’s center and side electrodes. The spark plug temperature has to get to a self-cleaning state before it can burn off or prevent carbon deposit.

Electrical issues

If there are electrical issues causing weak sparks, it’ll lead to incomplete combustion. This will invariably cause the plugs not to get to their self-cleaning temperature. If there’s a weak ignition in the combustion, it’ll cause carbon buildup on the spark plugs over time.

cleaning pistons through spark plug hole

How do you Clean pistons through a spark plug hole?

Whether you are looking for how to clean piston rings without removing them or how to clean pistons through plug holes, this is the right guide.

The job may be tedious, but it is worth it if you get the steps right. Here are the simplified steps to follow.

Step 1: Getting ready

The first step is to prepare the necessary tools and equipment and park the vehicle in a secured garage. Get the best piston cleaners from your favorite auto spare parts and a Depstech borescope. I recommend chem-dip carburetor parts cleaner if you want excellent results.

Other cleaning solvents like brake cleaner, WD40, etcetera can still do a good job. So, can I use WD40 to clean the pistons? The answer is yes.

Step 2: Remove the spark plugs

Get the right socket and handle, and remove the ignition coil bolts or nuts. Take off the coil or plug wires and unscrew the plugs one after the other. Do not forget the label the plug wires so you won’t mistake one for another.

Step 3: Check the carbon buildup

With the spark plugs out of the way, inspect the amount of carbon buildup on the pistons using a borescope. I recommend using Depstech products. You can get one for $30 to $50. The purpose of the inspection is to record the before and after carbon buildups so you can tell if the cleaning is effective.

Step 4: Clean the carbon buildup

Turn the engine to the top depth center and clean the no.1 and 4-cylinder pistons by scrubbing them with a long screwdriver. Turn the engine to bring the other pistons to the top and scrub them.

The next step is soaking the pistons to remove the carbons. Pour in piston cleaner spray in the hole and leave it overnight. This will allow the cleaner to do its magic. Don’t worry about the cleaning solvent. You’ll get it out the next morning.

Step 5: Vacuum the solvent and re-install the spark plugs

Vacuum the solvent the next day and check the piston with your borescope to ensure the solvent is cleared. Don’t be afraid of wetness on the pistons. It will dry off with time.

Examine the piston to compare the before and after pictures. Don’t worry if the difference is not up to your expectations. But the cleaning solvent should dissolve at least half of the carbon, and the dissolved buildups will come out when you vacuum. You may have to drive the vehicle for 30 minutes at highway speed before you can have your expectations. Repeat steps 3 and 4 if there’s no difference at all.

Reinstall the spark plugs and test drive the vehicle aggressively. Don’t panic if you have a “smoke show” when you start the car. It’s normal. The smoke will disappear after driving for 30 minutes to 1 hour. The engine may also run rough for a minute or two.

Note: If you fail to vacuum out the seafoam in spark plug hole or any cleaning solvent you used, it’ll cause hydro-locking. The liquid doesn’t compress. So, if you leave sufficient fluid in the cylinder, it will bend or break the piston connecting rods or other parts.

Step 6: Change the engine oil and spark plugs

Drive the vehicle for a few days to have the plugs properly burn off the remaining carbon deposits. Lastly, change the spark plugs and the engine oil and oil filter.

Other ways to Clean Pistons without Removing

Cleaning pistons with heads off is the most effective way of removing carbon buildup. However, that’s not the only way. Aside from the method above, you can clean pistons by adding piston rings cleaner additive to the gas tank. I prefer using seafoam. You may try other cleaner additives if you are not a fan of seafoam.

Add the seafoam on a half tank and drive with it until you run low on gas. At first, the engine may stutter, and you may have large smoke from the tailpipe. Everything will normalize with time.

Another method is adding piston carbon cleaner through the intake valves. This requires spraying carbon cleaner into the intake through the vacuum hose that connects to the valve cover and the intake manifold while the engine is running. You can also pour it through the vacuum hose that goes from the brake booster to the intake manifold.

Frequently Asked Questions – FAQs

Q: What is the best way to clean pistons?

Undoubtedly, the best way of cleaning pistons is to remove them before cleaning. My best practice is soaking it with chem-dip carburetor parts cleaner for a few hours before wiping off the dissolved carbons with a cleaning brush. Scrub off any stubborn carbon deposits with soft sandpaper and rinse again with the parts cleaner.

Q: What is the best cleaner to remove carbon from pistons?

There are several effective piston cleaners in the market. But the best I have come across is the chem-dip carburetor parts cleaner. While not specifically designed for cleaning carbon-fouled pistons, they are quite effective. Chem-dip carburetor parts cleaners are water-based, safe, and effective solvents formulated to remove stubborn carbon, oily, and grease-fouled engine parts.

Q: Can I spray wd40 in spark plug hole?

Yes, you can spray WD40 in spark plug holes. It is engineered to remove moisture and carbon from spark plugs, plug wires, and other engine parts. If you are also contemplating if you can spray brake cleaner in spark plug hole, the answer is yes. Both wd40 and brake cleaner are suitable for der-carbonizing engine parts.

Q: Can pistons be cleaned and reused?

Yes, pistons can be cleaned and reused. Cleaning pistons for reuse using chem-dip carburetor cleaner and vapor blasting are the most effective and easiest means you can deploy. Auto mechanics do not replace carbon-fouled pistons when overhauling engines. They only replace burnt, broken, or damaged pistons.

Q: What is the life of a piston?

Car manufacturers design pistons to last the lifespan of an engine. As a result, engine pistons last between 75,000 miles to 250,000 miles, depending on your maintenance culture. On average, engine pistons last 100,000 miles.

However, this is subjective depending on your maintenance habit, driving habits, and other factors. So, pistons can damage before the engine reaches 100,000 miles.

Final Words

Cleaning pistons through spark plug hole may seem like a daunting and ineffective task. Some motorists and mechanics argue that you won’t get a good result, but that’s not true. However, the best method is cleaning pistons with the head off. But if you get the necessary tools and follow the above guides religiously, you’ll get an excellent result.



While growing up, I knew I had a thing for car repairs though my parents never wanted me to learn mechanics. I always visit a mechanic garage in my small neighborhood after school. As I grew older, at age 16, I got addicted to anything automotive. My parents had to enroll me in that mechanic garage since giving up was never an option for me. As a dedicated mechanic who got into the industry from an early age, I'm graced with an addiction to diagnosing and rectifying automotive problems with ease.

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