Engine Oil Sludge – Symptoms (& How to Remove it)

Engine oil sludge is unwanted in any engine, but unfortunately, it happens, especially if the engine is not well maintained. Here's what it is, what causes it, and how to prevent it

Car Engine Sludge

Engine sludge is not something anyone wants to deal with. This tarry accumulation shows up in the crankcase and can lead to a lot of trouble. Because your car engine is meant to run with clean oil, you don’t want to deal with sludge. 

We look at what engine oil sludge is, how to remove it and steps to prevent it so your car motor can receive the lubrication and cooling it needs. 

What is Engine Oil Sludge?

Engine Sludge

Engine oil sludge is the black tarry substance that shows up in the motor. It occurs as a result of chemical reactions.  It is old motor oil that degrades naturally when it is exposed to high temperatures and oxygen.

The higher the temperatures go, the quicker the degradation goes. For every 18-degrees the temperature rises, you can expect oxidation rates to double. 

The by-products of these chemical reactions include organic acids combined with polymeric products. Together, these products will continue to react until they create sludge.

At first, the sludge might just be a thin film, but eventually, it starts to bake onto the surfaces to create a horrible mess.

Engine Sludge Symptoms

The most common symptom of engine oil sludge is low oil pressure light on your dashboard or a noisy engine. Not only is engine sludge gross in nature, but it can also cause some disastrous effects.

As the sludge becomes thick, the oil can block passages, including the oil-pump pickup screen. This blockage leads to oil starvation. These problems happen over time, not suddenly.

In addition, if the engine has variable valve timing, engine sludge can impact the needed mechanisms. The sludge will always decrease engine performance, reduce efficiency and lead to costly repairs. 

How to Remove Engine Sludge

  • Use Engine Additive 
  • Drain the Oil
  • Install Oil Filter
  • Add Fresh Motor Oil

1. Use Engine Additive 

Various companies make an engine additive meant to treat sludge. Park your vehicle on a level area and remove the engine oil cap. This is one we recommend:

Liqui Moly (5200 Oil Sludge Remover - 300 Ml
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Liqui Moly (5200 Oil Sludge Remover - 300 ml
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Pour a can of your selected treatment into the engine. Make sure you follow the instructions on the can.

Put the engine oil cap back on and run your car for 10 to 15 minutes. You don’t want to drive the car with this treatment in the engine; you just want it to warm up completely.

With this engine treatment, you can help liquefy the sludge inside the motor. It will mix better with the dirty oil so you can flush it. 

2. Drain the Oil

Once the engine has been warmed, you are ready to change the oil. Remove the drain plug from the oil pan and let it run into your drain pan. 

Some people swear that using an air compressor helps to remove the sludge from inside the engine. However, you want to do research before you choose this option as it could introduce new contaminants into the engine.

3. Install Oil Filter

With an oil filter wrench, you can take the filter off. More oil will drain from the system when you do this. 

With your new filter, put some clean oil on the gasket to create a solid seal. Using your hand, turn the filter clockwise to tighten it. 

4. Add Fresh Motor Oil

Once you have drained the oil from the system and put your new filter on, it’s time to reinstall the drain plug. Check the grade of oil you are using to ensure it is compatible with your system. You can find this information in the service manual. If you use the wrong motor oil, it can harm the engine and encourage more sludge buildup.

Pour in new motor oil. Check the level with the dipstick and add more if needed. Just don’t overfill the system with oil. It should fall between the ADD and FULL lines.

Screw the oil filler cap back on tightly. Take your car for a test drive and see if it runs better.

RELATED: 10 Best Synthetic Motor Oils

How To Prevent Engine Sludge

Engine Head Sludge

With a little due diligence, you can prevent engine sludge from occurring in the first place. Here are a few steps to prevent engine oil sludge:

  • Regular Oil Changes
  • Drive with Care
  • Visit Your Mechanic

1. Regular Oil Changes

The best way to prevent engine sludge is to keep up with the oil changes. When you remove the dirty, contaminated oil, you ensure that sludge can’t build up.

If you use conventional oil, you might need to change it every 3,000 miles. However, synthetic oils can go up to 10,000 miles without a change. Check your service manual for more guidance.

RELATED: 6 Best Motor Oils for High Mileage Engines

2. Drive with Care

The way you drive can have a profound impact on the integrity of the oil. For example, if you do a lot of stop-and-go driving, you are subjecting the engine to a lot of wear. 

Additionally, if you use your pickup truck to haul and tow often, the engine is going to need more care. Consider changing the oil more frequently to avoid sludge.

3. Visit Your Mechanic

The best way to prevent engine sludge is to visit the mechanic often. If you do your own work, it’s important to keep an eye on the system and perform regular inspections. If you catch problems early, you can avoid expensive repairs later.

For example, if the engine starts knocking or is smoking, these are warning signs. You should also be aware of poor fuel economy and a decrease in performance. 

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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.