Best Penetrating Oil For Stuck Spark Plugs

To remove a stuck spark plug, you'll need penetrating oil to help loosen it. Remember, use penetrating oil, not lubricating oil. Lubricating oil is lethal for spark plugs.
By Kelvin Wamalwa Written by Kelvin Wamalwa
Updated on July 16, 2022

Spark plugs provide the electric spark that ignites the air-fuel mixture to start your engine. As such they’re extremely important components of your engine and need to be inspected and changed if need be during routine maintenance or else your engine won’t start.

Spark plugs do get stuck sometimes. There are numerous causes. Corrosion of the spark plugs is a major one. The others are rust, slime, dust, and grease. Faulty ignition coils can also cause a spark plug to fuse to the engine due to excessive heat. To remove a stuck spark plug, you’ll need penetrating oil to help loosen it. Remember, use penetrating oil, not lubricating oil. Lubricating oil is lethal for spark plugs.

While the terms are sometimes used interchangeably, penetrating oil is very easy to differentiate from lubricating oil. It has low viscosity (it’s thin) and usually but not always comes in a spray can. 

What factors to look for when choosing a penetrating oil for stuck spark plugs

There are several things to consider before buying a penetrating oil. The most important of these is penetrating power. Many penetrating oils use a strong solvent mixed with a light lubricant. 

In fact, you can make your own penetrating oil at home using vegetable oil and acetone. The mixture should be 90% vegetable oil and 10% acetone. Be sure to mix them in a metal container because acetone eats through most plastics. 

This kind of oil-solvent mixture is the most common. The strength of the solvent varies from brand to brand so keep an eye on that. Generally, the stronger the solvent and the higher its concentration, the faster the penetrating oil works.

High-tech penetrating oils use refrigerants instead of solvents. The refrigerant absorbs heat and lowers the surface temperature to well below freezing. The target spark plug then contracts, and the lubricant lubricates the hole, making it easier for you to pull it out. 

Refrigerants are more expensive than solvent-based penetrating oils. Their biggest selling point is that they won’t eat away the non-metallic comments of your engine like rubber belts should they spill.

What can you spray down a spark plug hole?

You can’t just spray anything in a spark plug hole. Any solids are a no-no. So is oil. It may interfere with the functioning of your spark plugs and cause an engine to misfire. 

It’s best to clean your spark plug holes with compressed air, either from an air gun or a can. Scrub the dirt and grime then blow it away with an air gun.

In the case of caked grime, you can use a penetrating oil or carb cleaner but you have to be careful that you don’t use too much. Once the oil has dissolved the gunk, blow it away with compressed air. Wipe away whatever liquid is left with a clean rag. You can easily do this by attaching the rag to a screwdriver. You can also spray your penetrating oil onto the rag and use it to clean inside the spark plug hole.

Once the hole is clean and dry, spray WD-40 to keep away moisture and replace the spark plug. Clean the holes one at a time as well so you don’t introduce more dirt to other holes by leaving them open while you’re working on another.

Can you use WD-40 on stuck spark plugs?

You can but you shouldn’t. WD-40 works by displacing moisture and providing light lubrication, an act that prevents corrosion. WD-40 is an acronym for Water Displacement, 40th formula.

On the other hand, penetrating oils generally work by dissolving gunk, grime, and rust, then “washing” them away, making stuck spark plugs easier to remove.

While WD-40 can lubricate rusted nuts or spark plugs and make them easier to remove, it’s not the best choice for a stuck spark plug. It is not entirely useless if it’s the only thing you have but it’s not ideal. 

You would be better off with a dedicated penetrating oil like Kroil, PB B’laster, or Liquid Wrench.

Most effective penetrating oils to use on stuck spark plugs

The debate over which is the best penetrating oil is a tough one. Everyone has a different opinion colored by both experience and feelings of brand loyalty.

Nevertheless, we did our best to list some of the best penetrating oil brands in the market. In no particular order, we give you:

  • Kroil
  • Free All
  • PB B’laster
  • Knock’er Loose Plus
  • Liquid Wrench

What brands should you avoid when choosing a penetrating oil for stuck spark plugs?

This is an even tougher one. To avoid getting my teeth knocked out by anyone angry that I denigrated his favorite penetrating oil brand, I went to Amazon and checked the worst-rated penetrating oils. All were obscure brands like Promech and Anti-Seize Technology. Others had no customer ratings or reviews at all. All the well-known brands had pretty good ratings (4.2 stars and above).

This is not surprising. At its core, penetrating oil is a very simple product. That’s why it is so cheap. It is a solvent in light oil. You don’t need a Ph.D. in Chemistry to make it. Drexel University students found that a mixture of 10% acetone and 90% vegetable oil made a better penetrating oil than some commercial brands. Anybody can mix acetone with vegetable oil. Perhaps that’s what the penetrating oil makers are selling us. It could be an infinite money glitch for them.

As environmental regulations keep tightening, companies will be forced to create designer solvents or refrigerants. It’s only then that differences will start emerging. At the moment, the biggest difference in penetrating oil brands is how fast they act, a factor driven mostly by solvent type and concentration.