Is an Engine Flush Good or Bad?

To flush or not to flush.

It’s a question whose answer is obvious in the bathroom, but vigorously debated in the garage.

Let’s get right to the point. Is it safe to use an engine flush in an engine with high miles?

Spend a few minutes perusing online forums and you’ll find a range of answers to this question, often involving a 1980s Trans-Am, Camaro or other car that someone thrashed on for years, parked in a pasture for a decade and now wants to revive with an engine flush.

Here’s what we’ll cover:

What is an engine flush?

An engine flush is an aftermarket chemical additive designed to clean accumulated deposits, sludge and other gunk from your engine. You pour it into your engine’s oil-filler port and idle the engine for about 10-15 minutes. It mixes with the oil and circulates through the engine, helping dissolve sludge and clean deposits. Then, you drain the oil (along with much of the gunk, in theory), change the oil filter, add fresh oil and return to the business of driving.

How deposits and sludge form

If it did its job, your engine’s performance will return to the heady days of its youth, when it delivered maximum power and efficiency. Over time, however, harmful deposits and sludge may have accumulated, causing power and performance loss.

Oil pickup screen
The tiny openings in the oil pickup tube screen can easily plug with sludge, starving the engine of oil.

Deposits and sludge can form for several reasons, including frequent short trips that don’t allow the oil to fully warm up and evaporate moisture, the ingestion of air-borne dirt, fuel dilution and high heat breaking down the oil. As it settles, sludge can clog narrow oil passages or the screen on the oil pickup tube, restricting oil flow to vital parts, especially the upper valve train. Deposits can cause the rings to stick, reducing engine compression and horsepower.

Is an engine flush necessary?

A good engine flush can help loosen deposits and dissolve sludge, returning your engine to like-new condition. However, in old engines with high miles, sludge may be the only barrier keeping oil from seeping through worn or cracked seals. Removing the sludge exposes the seals for what they really are – junk. Soon, your engine begins leaking oil, and you’re mind instantly associates the engine flush product with an oil leak.

In reality, the seals were already bad; the flush simply revealed their true condition.

If you suspect your vehicle falls into this camp, leave well enough alone and skip the engine flush. It’s probably not worth trying to revive an engine in such poor condition without first fixing the bad seals or other defects. In effect, you’re choosing your problem: either sludge and deposits robbing performance or, if you clean the engine, the seals showing their true condition.

Part of a good maintenance regime

But that’s not to say an engine flush is never a good idea. In fact, it’s often the first step in helping restore a neglected vehicle to top-notch performance. And, often when you buy a used vehicle, that’s what you’re getting – a vehicle whose owner found antiquing on Saturday afternoon more enjoyable than changing oil or dropping the transmission pan. Consequently, your “pre-owned” ride, while not complete junk, may boast a sketchy maintenance record.

In these cases, a potent, detergent-based flush can help prepare the engine for new oil, loosening sticky valves or rings and helping remove harmful sludge. While not a required step when switching to AMSOIL synthetic motor oil, we do recommend flushing your engine if you want to give your vehicle a fresh start.

Some people also claim an engine flush will free large chunks and cause an avalanche of debris to clog passages inside your engine. Don’t worry. AMSOIL Engine and Transmission Flush cleans at the molecular level, ensuring the deposits are dissolved and properly exit the engine with the oil when it is drained. This helps the new oil get off to a “clean start” and perform to its engineered capacity.


Testimonial time

For the record, I’ve used AMSOIL Engine and Transmission Flush on three different pre-owned vehicles in my time, and it’s worked great. One of them, a 1999 Honda CR-V, has 206,000 miles on it and still runs like a top without burning hardly any oil. Another, an Oldsmobile Intrigue, ran great until a computer problem forced me to trade it off…for the CR-V. The third I sold to my fellow blogger, Ed Newman, for $500 and a telescope. Last I heard, it still ran well, but rust was getting to it.

He may have parked it in a pasture behind his house.

In sum, flush your engine if you want to give your vehicle a new lease on life. AMSOIL Engine and Transmission Flush, as the name indicates, also works great for cleaning automatic transmissions. But if you have any reservations about disturbing sludge or deposits that may be holding your old, high-mileage engine together, consider skipping it. It’s up to you.

How to do an engine flush

If you are looking to do an engine flush this weekend, here’s a quick video that will walk you through the process, courtesy of MyJeepStory.

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  1. I have a 2005 Lincoln Town car and she is mint. I only use synthetic oil and change the oil every 9,000 to 10,000 miles. Is it necessary to do a engine flush with synthetic oil compared to regular oil. I have had my mechanic flush my engine twice in 6 years and she runs like a top, absolutely no problems. I bought the car in 2012 and flushed the engine because I did not know the maintenance schedule of the other owner and then only using synthetic oil I flushed it again 5 years later…….can I keep this way of maintenance or is a flush every 5 years with synthetic oil over kill? Thanks for any help you can give me…..again the car runs great!!!!

  2. Yes, engine flush is good as it is important for engine oil. Generally, an engine flush washes the gunk out of your car’s engine. In an engine flush, a technician takes out some of the oil and adds a flush additive. In your blog you give the clear idea that whether engine flush is good or bad. It is better that you can ask for the expert.

    1. And I heard people said after flush will have some engine flush left inside engine and that not good for engine, that why manufacturer not recommended the enigne flush?

    2. Hi Erik,

      It’s true that during an oil change some of the old oil remains in the engine, but the amount is minimal. AMSOIL designed its Engine and Transmission Flush with that in mind, and we have conducted significant testing to ensure it is both effective and safe for the recommended applications. You can use it with confidence.



    3. Hi Erik,

      Add AMSOIL Engine & Transmission Flush to the oil already in your engine. There’s no need to change the oil first.



  3. Hi there. I recently had my oil changed in my 2016 Kia Sorento. The dealership neglected to tighten the oil filter and over the next three days unbeknownst to me the oil drained out. When I saw my oil pressure light come on I wasn’t sure exactly what to do. I drove for maybe another 30 seconds and then pulled over. After discovering the problem and refilling with 4.5L of 5W30, I carried on as normal. My daughter said that I should get my engine flushed as there could now be small pieces of metal floating in the new oil as a result of the loss of oil. I have already told the dealership what happened and they accept responsibility but said nothing about an engine flush. Should this be pursued?

    1. Hi Brenda,

      Since the dealership made the mistake and accepted responsibility, I’d let them handle it. They should do whatever is required to get your vehicle back up and running properly. If your car acts up, just be sure to take it back immediately.

      Thanks for reading.

  4. I have 150,000 miles on 2007 Toyota sienna LTD. Dealer told me that the engine is sludged. Would cost $5000 to clean.

    I tried Seafoam. And I’m getting oil changed as we speak.

    Will it be safe to use your product now? Do you think it will make a difference?

    Can I add it after the oil is changed and drive with it inside until my next oil change?

    1. Hi Chelley,

      We recommend having the engine cleaned before using our product. While it will help to clean out the engine, the concern is still with the potential for existing debris in the engine plugging the oil pick up tube. Was the dealership able to identify what was contaminating the oil causing the sludge build up?

  5. Hello, I have a 2006 Sierra 2500hd with the duramax engine. The truck has 430k on it. Recently I dropped the lower oil pan and to my surprise tgere was no caked on sludge in it. My question is do you think I might benefit in using the engine flush? Or do you think with that many miles I may do more harm than good? I change the oil every 10k miles.

    1. Hi Tad,

      It definitely wouldn’t do any harm, as it is a detergent based cleaner, not solvent-based. It probably wouldn’t hurt with that many miles on it, in case there’s any buildup in the top end of the engine, but it sounds like this one is very clean.

      Thanks for reading.

  6. I inherited a 1983 Mercedes diesel , 206000 miles, that smokes a fair amount. The oil was usually changed every 3000 to 5000 miles. The engine needs some work, but it sounds that the engine flush might be a good thing to do for this one. Does the engine flush work on all engines or is there a special flush for diesels?

    1. Hi Greg,

      AMSOIL Engine & Transmission Flush is recommended for gas and diesel engines, so it’ll work fine in your Mercedes.


  7. Would this adversely affect the clutches in my Yamaha xv1900 Raider? I have 109,000 miles on my bike and have used 20/50 Amsoil since my second oil change which was around 6,000 miles. I put new clutch plates in there at around 80,000 just cause I thought it was due. Everything in there was spotless. I have started to notice a little faint ticking lately and was wondering if this product or one such as Marvels Mystery might help the problem.

    1. Hi Ron,

      AMSOIL Engine and Transmission Flush is not recommended for use in wet-clutch-equipped motorcycles and other powersports equipment. We do not recommend oil additives, such as Marvel Mystery Oil.

  8. I have a “97” Civic with 180k miles on it, I’ve had it for 2yrs now, I add about a half a quart of trans fluid in the engine n drive it for a 100 miles n then change my oil afterwards, I use Valvoline 20\50 (people who know use Valvoline ) because of of the high milage, it used to use a quart of oil a month until I switched to 20\50, I haven’t had to add oil since! I’ve been doing this for many years n had have great results from it, the trans oil is a high detergent n cleans the internal engine parts gradually, rather than all at once possibly damaging the engine, my uncle uncle use to use diesel fuel to flush out his engine, for some reason I didn’t like the idea or the smell it created! I’ve had many cars n have done this to all of them resulting in very high mileage, its very inexpensive n a lot easier to use! Wat do u think of this?

    1. Hi Danny,

      Switching to a higher viscosity can sometimes reduce oil consumption since the thicker oil does a better job sealing worn rings or valve guides. This could certainly be the case with your Civic after switching to 20W-50. Also, thicker oils (and synthetics) typically have reduced volatility, meaning they’re more resilient to evaporation at high temperatures.

      As for adding transmission fluid to the oil, I’m not going to tell you what to do – it sounds like you have plenty of experience working on cars.

      But I will tell you why I wouldn’t do it.

      1) The detergency, or cleaning power, of ATF is much less than motor oil. ATF doesn’t see the combustion byproducts or contaminants like motor oils do, so they are designed with much less detergency. So, in most cases, adding ATF to motor oil actually reduces cleaning power. Instead, ATFs have elevated levels of friction modifiers and other additives that help with protecting gears and clutches.
      2) ATF can disrupt the engine oil formulation and reduce its effectiveness. A good motor oil is a fine balance of base oils and additives that are designed to work together to fight wear, reduce friction, prevent deposits and provide cleaning power. Adding a foreign substance to the oil – tranny fluid, diesel fuel, an aftermarket oil additive – will throw off oil chemistry and possibly negatively affect wear protection, oil life or more. You may see a benefit in one area, but at what expense?
      3) ATF can alter the viscosity of the oil, reducing wear protection. This likely isn’t an issue in your case since you’re running a higher-viscosity oil than what the engine manufacture recommends.

      A good motor oil contains enough detergents to prevent sludge in clean engines and slowly dissolve accumulated sludge in older engines. The cleaning power of motor oil is designed to work gradually over subsequent oil changes and not necessarily all at once. Plus, our Engine & Transmission Flush is specifically designed to clean sludge, whereas ATF isn’t designed to be used in an engine.

      My two cents. Thanks for reading.

  9. My 02 sierra has 256,000 miles on it! It’s starting to loose oil pressure once it gets hot and under a heavy load causing lifters to tap. I’m thinking the strainer is picking up gunk. If I run a flush thru it should I go back with an engine restore product ?

    1. Hi Jody,

      Engine restore product? We don’t recommend using aftermarket oil additives. Check out this post to find out why.

      If you suspect “gunk” has accumulated in your engine, AMSOIL Engine & Transmission Fluid may help you, so you can feel safe giving it a try.


  10. Aside from the 2 Toyota dealer provided oil changes I have used Amsoil. Oil change interval 10,000 miles. My next oil change will be at 60,000. Would using the Amsoil flush be recommended on my 2010 Prius? And, if so, could an oil analysis be run at the same time?

    1. Hi Michael,

      Sure, you can use AMSOIL Engine and Transmission Flush on your Prius. And you can certainly perform oil analysis. To get the most bang for your buck, however, I suggest you perform the oil analysis first before using Engine and Transmission Flush and changing your oil. The report may come back saying your oil is suitable for continued use. No sense changing it if it’s still performing well.

      Thanks for reading,


  11. Hi,
    I purchased a 2002 Sierra with a 5.3 liter a few months ago and now that the temps are colder it appears to have the cold engine knock for about the first 30 seconds. One GM statement says that this is from build up above the piston seal. Do you think a flush will help?

    1. Hi Sierra Dan,

      I reached out to our Technical Services Department, and here’s what they said:

      As temperatures drop, the oil thickens. This means it takes a little longer for it to circulate on initial startup, and build up pressure and completely circulate through the engine. It has nothing to do with piston sealing. Another occasional cause is if the oil filter is mount so the oil can flow out when the engine is shut off, and the anti-drain back valve is not working properly, it can mean it takes longer to refill the filter and then circulate through the engine. Changing the filter will correct this problem.

      I hope that helps. If you need more info, contact them at [email protected] or 715-399-TECH.


    2. Cold knock on those GM Truck engines is caused by carbon build-up on the piston above the rings. An oil additive is less likely to reach that area than a fuel additive like AMSOIL PI. I would try running a couple tanks of fuel with PI in it and see if that helps. A mild cold knock on these engines is usually harmless too.

  12. Does Amsoil recommend using the flush occasionally even on strictly Amsoil Signature run engines? I only go 10,000 between changes but its severe service…lots of short trips in winter. Thanks!

    1. Hi John,

      No, we don’t. It’s strictly up to you. If you’ve used Signature Series exclusively, there’s likely not much that needs cleaning. However, you’re safe to use it between oil changes if you want to go that extra mile.


  13. Hi AMSOIL, my car has about 16,600 miles and I am the original owner. I have my oil and filter changed every 7,500 miles per maintenance schedule. When should I start using engine flush and how often should I use? Thanks.

    1. Hi,

      Per our product recommendations, flushing your engine isn’t required when using AMSOIL products. But, if you want the peace of mind that comes from doing a little house cleaning every so often, you can use AMSOIL Engine and Transmission Flush as often as you feel comfortable with it.


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