What Is an Engine Flush and Do You Need One?

What Is an Engine Flush and Do You Need One?
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As your vehicle accumulates miles, your mechanic may recommend an engine flush. Depending on how you’ve maintained your car, more miles may also mean the accumulation of contaminants. While scam products that solve nonexistent problems are out there, engine flushes were designed with the more legitimate contaminants in mind.

But is an engine flush right for your vehicle? And what is an engine flush, anyway? Learn more about engine oil flushes and if your car needs one.

What is an engine flush?

An engine oil flush cleans the engine’s internal parts of excess oil, dirt and sludge built up over time. A small amount of oil is removed from the engine. Next, a nonsolvent flush additive chemical, designed to break up carbon deposits in the engine, is added to the system. Then, the car is taken for a test drive to work the chemical through the engine.

An engine flush also helps remove corrosion inhibitors to help extend the life of motor oils. This can result in reduced oil changes throughout the vehicle’s lifetime. Engine flushes are a popular service offered by automotive repair shops because they’re affordable, usually costing between $100 and $150.

How does sludge form in an engine?

Oil sludge is a gel or solid in your engine oil caused by wear and contaminants. How does sludge form? The longer oil stays in your engine, the more it oxidizes and picks up contamination as it cleans and lubricates the engine. Common causes of contamination and oxidation are:

Oil breakdown

Engine oil is subjected to a variety of harsh conditions throughout the life of an engine, resulting in degradation over time.

Thermal degradation

Thermal degradation results from excessive heat caused by friction between metal parts inside an engine.


Oxidation occurs when oxygen chemically reacts with the engine oil—the primary purpose for using antioxidants.


Corrosion happens when additives designed to minimize wear within the engine meet water vapor or humid air drawn into the crankcase because of its low boiling point (roughly 212 degrees Fahrenheit or 100 degrees Celsius).

Any of these may result in fuel contamination, meaning the oil breaks down into corrosive acidic compounds once inside the combustion chamber. As it further degrades, oil loses viscosity and forms sludge.

Fortunately, you can protect your car from the accumulation of engine sludge by keeping up with factory-recommended maintenance. Ignoring checkups and oil changes puts your vehicle at risk of developing costly problems such as engine sludge.

Do I need an engine flush?

In most cases, you don’t need an engine oil flush as long as your car is in good working order and you follow the recommended maintenance schedule. However, while your engine doesn’t necessarily need an oil flush at that moment, keep track of when your next oil change is coming up. Then, use the time for research.

Vehicles last longer these days thanks to technological advancements. Plenty of cars only need simple oil changes every so often rather than full-blown flushes. However, if you have a car with 150,000 miles or more, chances are your engine needs an oil flush. That’s especially true if you find you frequently need to fill your car with oil.

Always consult a mechanic before deciding to do an engine flush. They may be bad for older cars where sludge is bolstering worn seals and gaskets. In such cases, the flush could dissolve the sludge, leading to oil leaking from the engine. That’s a worse problem than reduced performance due to sludge buildup.

Why is there a debate over engine flushes?

While getting an engine flush is safe for most vehicles, some online mechanics claim it can reduce the life of your engine because of the caustic chemicals used. In addition, they say a flush can loosen deposits and move them around to more vulnerable areas of the engine. But many will counter that neither claim has merit. Adherents contend that no commercially available engine flush is caustic enough to harm an engine. And, provided the chosen flush contains powerful enough detergents, they add, the sludge will be fully dissolved as opposed to broken up and transferred.

It has been said that there are no known harmful side effects from performing an engine oil flush if you use a licensed mechanic. A DIY engine flush should only be done if you have extensive automotive knowledge and can accurately determine that an engine flush is necessary and won’t harm more than it helps.

Engine flush pros and cons

Engine flushes have more going for them than against them. There are, however, some concerns worth considering.

Engine flush pros

Decreased engine wear

Using an engine flush on a high-mileage engine will clean the engine of accumulated sludge, carbon deposits and contaminates that increase engine wear. A flush can also increase performance and fuel economy.

Cleans internal parts

An engine flush will clean the engine components you can’t see that would otherwise require a complete engine teardown.

Improved fuel economy

When sludge and contaminants increase, so does the engine oil viscosity. This thicker oil can increase wear and resistance, requiring the engine to work harder. Eventually, your fuel economy will be affected and your engine will feel sluggish.

Better emissions control

Carbon deposits and contaminants can cause piston rings to stick. Deposits can eventually cause damage to the piston rings and combustion chamber walls. As a result, the oil will make its way past these areas and enter the combustion chamber, causing high oil consumption and burned oil in your exhaust emissions.

Engine flush cons

Other motor issues may appear

On the flipside, removing sludge and unwanted deposits may also be the reason not to get an engine flush. For example, older high-mileage engines with built-up sludge may rely on the sludge to cover cracks and openings in worn-out seals. Once the sludge is gone, fresh oil will flow through those cracks and allow oil leaks in or out of the engine.

How often should you do an engine flush?

Suppose you purchase or own a vehicle with a sketchy or unknown oil change regimen. In that case, an engine flush can help remove sludge and contaminants from years of neglect and prepare the engine for new oil and a new filter.

If you don’t fall into that category and have followed recommended oil changes, you probably won’t need an engine flush as part of your vehicle’s regular maintenance.Even if your mechanic recommends one, you likely won’t need another as long as you take care of the engine.

About Bumper

At Bumper, we are on a mission to bring vehicle history reports and ownership up to speed with modern times. A vehicle is one of the most expensive purchases you'll likely make, and you deserve to have access to the same tools and information the pros use to make the right decisions.

About Steven Mitchell

Steve’s first car had pedals, and he’s been filled with passion for four wheels ever since. Steve lives and breathes the auto industry as a technologist, futurist, and writer while keeping up with the whys, hows, and plans of the automotive industry. His automotive journey began with a fully restored Triumph TR3b that he sadly never appreciated until decades later. Ownership of X-1/9’s, BMW’s, an SVT and a Dodge Ram Hemi have given way to an Audi Sport.

Disclaimer: The above is solely intended for informational purposes and in no way constitutes legal advice or specific recommendations.