I have heard really mixed opinions about the necessity of engine flushes. My quick research suggested that manufactures do not recommend doing them. In addition, I have heard of extreme cases of blown engines after engine flushes.

My mechanic performed an engine flush on my old car and so far, it seems to be ok.

  1. Is it safe?
  2. Is it necessary?
  3. What are the pros and cons of doing it?
  • Welcome to mechanics.SE! We're glad you're here!
    – anonymous2
    Commented Oct 28, 2016 at 13:03
  • Why do you think that you may need to flush your engine in the first place? Commented Oct 28, 2016 at 13:11
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    They are always a good idea if you are running a service operation, because they make you more profit. And if you are unscrupulous and running a service operation, most customers won't even know whether or not you did the flush, so long as you put it on the invoice...
    – alephzero
    Commented Oct 28, 2016 at 16:28
  • After a leak (e.g. head gasket leak) that allows coolant to mix with the oil. You want to clean that out thoroughly. Otherwise, probably never. Commented Oct 29, 2016 at 2:47
  • Ok so reading into this would I take the advise that it is better to change the oil 2/3 times and running it in between is better than chemical stuff you buy off a shelf.
    – Jason
    Commented Jun 30, 2017 at 13:54

6 Answers 6


An engine flush is basically the process in which a mechanic puts chemicals in the engine oil to break down sludge or carbon deposits from old oil. The difficulty with it is that it can break down sludge that had formed over rubber seals and is actually serving as a secondary engine seal.

Furthermore, if your engine has had regular engine oil changes at the specified times, a flush should be totally unnecessary.

If you do have serious sludge build-ups, it could be the best route to take; but you're taking it at a risk. If the sludge is actually protecting the rubber gaskets, you may have a bigger job on your hands replacing them.

  • 1
    Great explanation. I have also heard of undissolved sludge (in very bad cases) getting lodged in oil passages and pickups causing more damage by lack of flow.
    – CharlieRB
    Commented Oct 28, 2016 at 15:34
  • One can prevent clogged passages by changing the oil with a high quality synthetic oil like Mobil 1, substituting 40% of the oil with Marvel Mystery Oil (MMO). Run the engine warm at high idle for 15 minutes. Change the oil again, substituting 20% with MMO and change the oil after 200 miles of highway driving. Use 1 quart of Berryman's B12 Chemtool in the crankcase for five minutes before each oil change.
    – Carguy
    Commented Mar 6, 2017 at 8:19
  • Certain engine designs may be higher risk such as those which share engine and gearbox oil like the old A series or those with a turbocharger which uses the engine oil for the turbo. Commented Mar 6, 2017 at 11:48
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    Do not use marvel mystery oil, it is a mineral oil and it will reduce the protection and lubrication properties of your synthetic oil. See: mechanics.stackexchange.com/q/20942/14704 and see: en.wikipedia.org/wiki/Marvel_Mystery_Oil#Composition Also you don't need to change oil twice. You should simply use semi or fully syntethic oil and change with intervals recommended by manufacturer. Actually, you shouldn't need to use any additives to keep your engine in good condition. Today, all the oils and gasoline already have necessary chemicals to do that job. Commented Mar 6, 2017 at 12:59

I'll do an engine flush as soon as I see it recommended in the factory service manual. My opinion is to let sleeping dogs (and oil particulates) lie.

It sounds like a good idea on the surface, but as explained in the other answer; once an engine is old enough that a mechanic thinks an engine flush would be a good idea, then it's too late. And if the engine flush is not thought of as a good idea.. it's too early.

The exact timing of when to do an engine flush depends on when the mechanics next Boat Payment is due.


I believe a flush is a good thing, just not with chemicals.

I'm disabled so cannot afford a garage to do the easy stuff, but I also get satisfaction from doing things myself. I always do a flush (not with any chemicals or additives) I simply perform a double oil/filter change, I make sure I do it after a run so that the oil is hot, just drain it via the sump and take off the oil filter, when it has all drained out put on a new filter and fill with new oil, a week later I do it all again.

Is it necessary, NO, but I like the fact that my oil never goes black from soot from the EGR system and the cost of doing it twice myself (even with fully synthetic oil) is still less than a quarter a garage would charge me to do it just the once (at least here in the UK) and I know it has been done, unlike some of the horror stories you hear about from the 'specialist' garages.

If I drove a petrol car rather than a diesel van I probably wouldn't.

  • 6
    Don't you think you'd get the same (or better) improvement by doing oil changes twice as often than doing 2 oil changes each time?
    – Johnny
    Commented Oct 28, 2016 at 22:06
  • Johnny, your suggestion of changing the oil/filter every 6 months is something I had not considered before, if I'm honest I'm not sure what I do gives any Improvement other than making sure 99.9% of the old oil is flushed out, maybe a 6 month change is the way to go.
    – David
    Commented Oct 28, 2016 at 22:40
  • 6months? isn't that ridiculously short period? I thought diesel require even less frequent changes (at least according to vw) Also the color of the engine oil does not define if it is doing its job or not. auto.howstuffworks.com/5-engine-oil-myths1.htm In my VW, I have long life service intervals and it can do 30000km(diesels can actually last longer to 50000km) or 24months. See the answer from VAG mechanic here: mechanics.stackexchange.com/questions/6856/… Also this: volkspage.net/technik/ssp/ssp/SSP_224.pdf Commented Mar 6, 2017 at 21:33

My understanding of the engine flush process appears to differ from everyone else that has commented or answered so below is my take on it.

An engine flush (engine oil or transmission oil) is the process in which ALL of the oil is drained from the system and re-filled; this does not necessarily mean that additives are added to break down sludge and inadvertently cause issues with seal.

For instance my car has 12 quarts of transmission fluid. If I simply drain it from the bottom I only get 3 quarts so I only have to fill 3 quarts.

If an engine flush is performed then they need to put back in the exact same amount of oil which they took out. So if they truly flushed 12 quarts via an oil vacuum then they need to put back 12 as well. In this lies the problem.

When filling the transmission with oil they might not reach all of the places where they took it from. There are a LOT of nooks and crannies which might not get lubricated unless you knew to specifically target those areas.

If a nook or cranny is left unfilled then YES, you can absolutely experience a blown transmission.

This same principle applies to engines, brake lines, and cooling systems.

  • 2
    Well for engines there is really no where for the oil to hide it just goes down the sump, of course after an oil change there will still be some old oil but a very small amount
    – method
    Commented Oct 28, 2016 at 20:21
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    Is this conjecture or would you have references to cover what you're saying? Commented Oct 28, 2016 at 20:57
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    - Its not the chemicals that affect the seals, the seals has gone hard/cracked before. The engine flush is removing the sludge that seals the seal. - Replacing the entire oil (actually more like 99% of the oil) is not an engine flush, it is a plain and simple oil change.
    – Martin
    Commented Oct 28, 2016 at 21:07
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    The OP is asking about engine flushing here, not replacing tranny fluid. And an engine flush isn't the same as an engine oil change
    – Zaid
    Commented Oct 28, 2016 at 21:30
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    @Johnny that's true for some transmission boxes, especially automatic ones but not for engines. Automatic transmissions in general should not be drained empty if you dont happen to have special equipment and knowledge on that particular transmission unit, those wont be happy with air pockets inside and requires specialized air bleeding equipment. That said, some units have air bleed valves, some are designed to bleed air out through dipstick pipe when unit is tilted to angle and some wont need anything special. engines are simpler than manual transmissions which are simpler than automatic Commented Oct 29, 2016 at 2:51

Do NOT ever let them talk you into doing an engine flush. At one of my top offs I was told I would need to get my system flushed cause there was a lot of sludge and such. Well I had them do it at my next oil change. 1 week and a day later I'm driving down highway and my vehicle starts to shake, get to gas station. My engine was knocking something bad. Had to end up getting picked up and my vehicle towed back. Which my dad (vehicle guru) and guy who towed it (my good friends friend) have both confirmed that it my engine is shot. Had I not gotten the flush done, I would still have my vehicle

  • 1
    I agree that engine flush is unnecessary in modern cars with syntethnic oils. But why exactly engine flush caused this in your car? Do you have an explanation? Commented Mar 6, 2017 at 21:36

I just did an engine flush with 20 percent Marvel Mystery Oil in my crankcase with fresh Mobil 1. Sure, the low pressure oil light began to flicker after 200 miles, but it disappeared after I changed the oil again. The car is is 23 years old and has 60k miles, all stop-n-go, and the oil was changed on time (so there was no excessive sludge visible on the oil filler neck).

Now, the tachometer no longer fluctuates at idle. I have seen MMO flush correct fluctuating tach on several cars over 30 years. It is most likely because lower piston rings tend to stick on old cars, even if the crankcase oil is changed regularly.

The main risk with a flush is particles dislodge and clog oil passages, especial the oil pump pick-up screen. The best way to prevent any such problems is to change the oil in intervals of 200, 500, 1000 miles when 20% Marvel Mystery Oil is added. It is not necessary to change it so frequently if used before the car reaches 50k miles.

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