Transmission Flush, When and when not to flush transmission

How a Transmission Flush Works

A transmission flush procedure is generally performed using equipment that either runs off the transmission pump or has its own pump built into the machine. The machines are connected to the transmission cooler lines, and while the old fluid is pumped out the new fluid is delivered at the same quantity and time. Sometimes there is a cleaning solvent that is forced back into the transmission removing deposits of old transmission fluid from parts and components or the technician performing the fush may run a solvent through the vehicles transmission before performing the flush. The cooling lines, cooler, converter and other parts are all thoroughly cleaned. One of the benefits of this procedure is that all of the transmission fluid is removed and replaced. This is different from a fluid change in which only some of the fluid is replaced. A simple fluid change cannot remove all of the built up deposits that accumulate over time as is done during a flush. Also, some of the fluid is usually stored in the torque converter potentially contaminating the new fluid that is put in. What is the point of that?

Transmission Flush Good or Bad?

I constantly hear consumer, customers and technicians debate as to whether transmission flush procedures can be harmful to a vehicle and is a transmission flush good or bad for your automobile. While many mechanics and experts agree that having a clean transmission will extend the life of a transmission, it is thought by some that the flushing procedure may not be the best way to achieve a clean transmission. One common thought is that the process, which forces liquid back into the transmission in the opposite way of the normal fluid flow, could potentially damage components or block tight passageways. When fluid is forcefully pushed back into the transmission, chunks of debris can be dislodged and possibly block narrow channels or one-way valves. When new fluid is put back in, these blockages can inhibit the normal flow of fluid through the transmission causing lubrication issues. Nonetheless, many car manufacturers and dealerships contend that these procedures are not harmful at all and help revitalize auto transmissions. MDH Motors does not use a reverse flush machine and over the thirteen years of being in the automotive repair industry I still yet to have seen a machine that flushes the fluid in reverse. Most machines just pump the new fluid in through the transmsiion cooler lines while containing the old fluid in a separate tank. The best transmission flush operation procedure is to:

  1. Perform the transmission flush
  2. Remove the pan and change the transmission filter
  3. Doing the flush in this order while help prevent the possibility of contaminates going through your transmission and potentially causing a problem by, let’s say hanging up a valve in the valve body.

    Checking your transmission fluid

    In the past every vehicle had a second dipstick, other that the engine oil dipstick, which was used for checking the level of the ATF. For vehicles that are still equipped with such, checking the ATF is very easy. Most cars require that the engine be running with the transmission in park. Some require that the transmission be in neutral. Honda trucks and cars with automatics require that the engine be off. If you are not sure what your vehicle requires, you can sometimes find directions on the dipstick itself. If that doesn’t work then consult the owner’s manual or Contact Me and I will be happy to give you some factory procedures.

    Many new cars do not have dipsticks. On these vehicles the fluid must be checked by climbing underneath the car and removing some kind of plug from the side of the transmission in order to see the fluid level. Some of these newer cars will still have the dipstick tube but no dipstick in it. On top of the tube you will find a plug that says in order to check the fluid level you have to take to take the car to the dealership service department. Once there, the technicians can check it with a special tool that looks just like a dipstick. This seems silly and it probably is. The reason for no dipstick is that the car builders want you to believe that you don’t need to check or maintain the fluid. Many of them actually say that the fluid they use is good for the life of the vehicle. This is not exactly true but with modern synthetic fluids, the fluid is at least good for the warranty period and that’s good enough for them. Some cars have a sensor in the transmission that will monitor fluid, and the level can be checked via the information computer located in the instrument cluster. The level is between the marks, then that is satisfactory, and no more fluid is required. If fluid needs to be added then usually it must be poured down the dipstick tube. These dipstick tubes that double as a filler tube are usually wide enough to put the end of a funnel into them. If the dipstick tube is too narrow to fit a normal sized funnel into the end of it, then there is likely a filler plug somewhere else.

    Most cars also require that the engine and transmission be warmed up in order to get the most accurate reading. The reason for this is that ATF expands quite a bit as it warms up. One might believe the fluid level to be low when in reality the fluid is just cold. Many manufacturers put separate marks on the dipstick that are used if the fluid is cold, but what if the fluid is somewhere between cold and warm? This is why it’s just best to check it with the fluid warmed up. The goal when adding or checking fluid is to make sure that the fluid level is between the two are found on the dipstick. If the level is below the lower mark then some fluid must be added, but if besides looking at the level the condition of the fluid can also be examined. If the fluid contains very tiny black particles that rub off on your oil rag or paper towel this is normal, but can it can indicate that the fluid needs to be serviced. These small black particles are bits of clutch pack material that are suspended in the fluid. This is a sign of normal wear and tear but if the particles become excessive, or if the particles are metallic looking; this could indicate some major problems. The last thing you can do that can help determine fluid condition is give it a sniff. Worn out fluid will have a definite burnt smell to it and fluid from a transmission that has completely failed smells downright disgusting.

    Is a Transmission Flush Necessary?

    Ultimately it is wise to consult the manufacturer owner’s manual and follow the guidelines outlined within. If the manufacturer recommends a transmission flush, it is probably advisable to have the service performed. However, it is true that not all manufacturers recommend this service at frequent intervals. It is not uncommon for flushes to be performed only every 100,000 miles. There are two main type of transmission flush machines and I will explain them below.

    Pump inlet flush machine:

    The first type of transmission flush machine I will explain attaches to the pump intake after the pan and filter is removed. This machine only supplies fresh new fluid to the pump intake and as the fluid passes through the transmission it dumps out to a collection tray and never goes back through for a second pass. All of the old fluid and crud is GONE and replace with fresh new fluid. After the service a new filter is installed, the pan replaced and then it is topped off with new fluid to the proper level on the dipstick. This process takes a total of 20 quarts of fluid to flush out 15 quarts of old fluid, replaces the fluid, and gives the mechanic the opportunity to look in the pan for anything unusual that would indicate a pending failure. Everyone should have this type of service done every 30,000 miles, but definitely before your truck goes out of warranty. By looking in the pan you may get an indication that you are about to have transmission trouble that might show up right after you get out of warranty.I will tell you that this type a flush does take more effort and makes more of a mess, costs a little more, but I think it is worth it. The extra charge will be for 1 hour labor and additional parts and fluid.

    Cooler line flush machine:

    The second type of flush machine connects into the transmission cooler lines. BG makes this kind and here is how it works. This type of transmission flush machine is more common in quick lube places where low level lube techs can operate the machine without any problems. They don’t have to operate any electronics or remove the transmission pan. They simply hook up the cooler line and start the vehicle. The other type of flush machines require you to maintain pump pressure and know the correct transmission cooler line flow. The line going from the transmission to the transmission cooler is disconnected and connected to the machine line in. The line out from the machine carrying new fluid is connected to the line going to the cooler. There is a chamber on the machine that has a diaphragm in it. The top part of the chamber above the diaphragm is filled with new fluid. The engine is started which turns the torque converter and the input shaft on the transmission. The input shaft turns the transmission pump and it makes hydraulic pressure. This causes fluid to flow through the cooler line. As fluid leaves the cooler line it enters the chamber on the flush machine. As the old fluid side of the diaphragm fills it pushes the diaphragm up and forces new fresh fluid into the transmission. After a while the old fluid is collected in the machine and it is replaced by new fluid. Now the transmission has been flushed. Really pretty simple. As you can see the machine cause no pressure and all fluid transfer is done by the transmissions own pump.

    Here is what I do not like about this type of machine. When the fluid leaves the transmission pump it passes to two different pressure regulators. One regulator supplies fluid at one pressure to the transmission itself that operates the pistons and controls gear shifting. The other is supplies the torque converter and the transmission cooler. So you can see that all the fluid leaving the pump does not go to the cooler. A bunch of it is cycled through the transmission and dumped back to the pan without going through the cooler. This type of flush machine does not remove all the old fluid, but it continuously dilutes it down with new fluid. It never really removes all of the old fluid, but is far superior to just drain and fill.

    The other thing I do not like about this type of flush is that they sell the supposed benefit that they do not have to drop the pan and change the filter like that is a benefit. Dropping the pan is very important. Looking in the pan is a fantastic diagnostic tool that can tell you if something is going wrong in your transmission. Now let’s say some crud is flushed out of the transmission with this flush method. Where does it go? It can go into the pan, and then sucked up into the filter that may clog the filter causing the pump to starve for fluid and a pressure loss. On the engine the filter is after the pump and if the filter gets clogged there is a bypass valve that opens and oil bypass the clogged filter so the engine is still supplied with oil. Unlike the engine oil pump and filter the filter is on the intake side of the pump. If it gets clogged, that is it, it is clogged and stuff does not get lubricated and the clutches do not get enough clamping pressure and they slip and burn up. In just a fraction of a second you just bought a new transmission if the filter clogs.

    When not to flush your transmission

    Before draining or flushing you should pull the dip stick and look at the fluid. If it is dark, burnt smelling, and you see little flakes or speck in it, DO NOT FLUSH IT. The fluid and transmission possibly has hard part damage, but the transmission just has not figured out it should die yet. If you flush a transmission in this condition it could fail right away. Real strange, but that is what seems to happen. If your transmission is in this condition just drive it while you save for a replacement transmission. There is no way of telling when it will fail. It might be today, next week, or next year, but it is doomed.

    The goal here is to flush the transmission BEFORE the fluid gets contaminated. What you are taking out should look like what you are putting in. Do not wait for a color or smell change. Most manufacturers calls for transmission fluid change at 30,000 mile intervals. The industry standard is two years or 24,000 miles whichever comes first. It is your vehicle, you decide what is best for you.

This post was written by: Martin Hand


If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance Thank You

Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Daniel says:

    hi I have an 08 impala that my wife drives weekly to work about 30miles a day. it has 150k miles but has never had the transmission flushed, the vehicle does not slip but does have a burning smell. do you recommend I flush it?

  • charlie says:

    My 2008 Suzuki SX4 Sedan Sport with 72000 miles recently developed a chirping noise. Almost like the sound of bearings going out. It shifts fine, no slips, no warning light on etc. After a quick google search I’ve come up with a bad front pump or clogged filter. I added a can of Seafoam Trans Tune and the chirping noise went away dramatically. It is still there but definitely a noticeable difference in how often. This leads me to believe the issue is probably a clogged filter. Would a flush help unclog the filter? Any advice would be helpful.

  • David Umana says:

    I have a 2014 mustang GT, it was pre-owned when I bought it at 20,000 miles, I’ve put 10k miles on it so far and I recently went for a maintenance and an oil change and. I was told it needs a transmission flush because it has dirt and debree, should I? Or should I not? Pls let me know, the car is currently at 30k miles.

    • Martin Hand Martin Hand says:

      I would check your factory service recommendations for that specific vehicle. Most newer vehicles with automatic transmissions use synthetic automatic transmission fluids which do not require service or not until 60,000 miles or even higher.

    • Martin Hand Martin Hand says:

      I would check your factory service recommendations for that specific vehicle. Most newer vehicles with automatic transmissions use synthetic automatic transmission fluids which do not require service at all or not until 60,000 miles or even higher.

  • a lot of good information here. my question was if I should change my transmission fluid and filter in a 2003 Camry – 4clyinder with 126, 000 miles . SO I will keep driving it and forget it. got confused on all the answers .

  • patrick M. says:

    Hello Martin,
    I have an ’06 Mazdaspeed 6 with 140k miles. My 5th gear hasn’t worked since I’ve had the car (about 2 years now), it slips out of gear as soon as I try to let off the clutch. i was planning to flush the fluid and filter until I read the last part of your article saying that the flush might alert the transmission to its impending doom. I saw in some of your responses that it is wise to check the pan and the filter to get an idea of the health of the transmission. Can I remove the pan and change the filter without changing the fluid, thereby keeping the transmission in its current operative state but at least giving me a clue on how it’s holding up?

    • Martin Hand Martin Hand says:

      Changing the fluid in a manual transmission will not change how it operates. You have a bad 5th gear synchro, bent shift fork or some type hard part damage. Transmission will need to be removed and repaired.

  • J.R. Snow says:

    This article was very helpful! the most helpful article after hours of internet research. I have a 2000 Toyota Tacoma that had the transmission flushed (don’t know what type of flush, but it wasn’t just a drain and fill) at about 75,000 miles. It now has 192,000 miles. Shifts fine, drives fine, but I recently looked at the fluid and it is brown, the color of engine oil when it is almost ready to be changed. is this too far gone for a flush? Should I just take it to a transmission shop (a Pittmans) and let them look at the pan/filter and decide?

    • Martin Hand Martin Hand says:

      If the transmission shifts good then maybe just drive it or try removing the pan and changing the filter. If you take it to the transmission shop I would only assume that they will try to sell you a transmission rebuild.

      • J.R. Snow says:

        Ok, so too far gone for a flush? If a flush won’t help then I’ll change the filter and fluid myself, but if a flush will help then I want to do it, I really want to keep the car.

        Thank you so much for your advice!!

        • Martin Hand Martin Hand says:

          When you remove pan and change filter check for excessive metal or friction material in the pan. This would indicate if the transmission needs repair and if has hard part banshee or just burnt clutches. Most problems start from the valve body.

  • Gerard J. Mottau says:

    My 2003 Dodge caravan with 90000 miles went into ‘drive home’ mode on the highway. I found the control module case was defective and corroded. I found one in the local junkyard that looked ok and put it in my Caravan. It’s working, but has a harsh downshift when I slow down to stop at a light. It came from a 2003 Chrysler Town and Country which is basically the same vehicle. Should i be concerned, and change the control module again, or just drive the vehicle? They get 35.00 for the module with no warranty.

  • Linda says:

    I have a 2011 Chevy Malibu. It started slipping last week. I was told that there was no debri in the pan, but was also told that the transmission need a rebuild. I have 73K miles on the car. I am at a loss as to what I need to do. Will changing the fluid and filter help? I am not sure if it has ever been flushed. I have had it for 23 months and only put 13K miles on it in two years. Please help.

  • Mike Laker says:

    I have a 2002 Honda Civic with 150,000 miles I purchased used with “little” documentation. Recently in the cold weather in Northern VA when the car is shifting from 2nd to 3rd at ~28 mph, the engine revs a bit before the shift completes. Kind of like applying the clutch in a manual without releasing the gas pedal. This only happens when the car is cold, once it warms up it seems fine. Any thoughts or suggestions and what to do or check? Fluid level is good and fluid appears read and fairly clear.

    • Martin Hand Martin Hand says:

      Most likely a valve body control issue or even worn out clutches but either way you would have to completely disassemble the transmission and do a rebuild to correct the problem. I would drive it until the problem progressively gets worse then think about repairing it but if nothing changes just keep driving it

  • Bill Murphy says:

    Have a 2008 Outlander V6 Mitsubishi. Transmission has not been serviced in many yrs. the car now has 110,000 miles. Should I drain and refill or flush at a Mitsub dealer.Is there a right answer going forward. Will a flush create possible problems?

  • Cristian says:

    Hello! It is good to have somebody experienced giving advice on maintenance and repair.
    I have a 2011 Rav4 automatic with no dipstick. I wander if I can change the ATF by just let the cold fluid drain out and replenish with the exact same amount. I can repeat this after some time until the fluid comes out clear. I want to avoid the dealer and the high prices. I saw the Toyota tools needed for ATF change and there is no way I will purchase those.
    Thank you!

    • Martin Hand Martin Hand says:

      The transmission requires a special fill tube for filling. You fill it from the bottom plug. There should be two plugs on the pan. One for draining and the other for filling, the fill tube rises inside the pan to the correct level height. With the vehicle at operating temperature, running in park, the fluid level is correct when fluid slowly drips out of the check plug.

      • Cristian says:

        Actualy, on the model I own there is un uper plug on the transmission and the filling is quite ordinary. There are no problems there. The problem I have is with the temperature of the ATF. I cannot read that so I intend to do it at cold temperature.

  • H.Otani says:

    Our 2004 Toyota Corolla’s (145,000 miles) check engine light turned on about 3 weeks ago and we took it to our mechanic right away. When he hooked it up, it had indicated that it was due to our transmission. Since then, he has turned off the light to see if/how long it would take to come back on again. The light has not turned back on and it still seems to be driving normally. He’s recommended that we first do a drain and fill or a flush to see if that would take care of the issue, and if not, we are prepped to put in a new transmission. In this scenario, should we stick to just a drain and fill or do a whole flush?

    • Martin Hand Martin Hand says:

      It really depends on what transmission diagnostic codes are being set. I recommend to remove pan, inspect and change filter. This will give you an idea of the transmission condition and what type of failure.

  • Sheila Johnson says:

    I had a 2008 Ford Escape. I say had, because the place where I took my car to be serviced suggested that I get a transmission flush before 100,000 miles on it, so I did. There was nothing in the driving that gave me any warning that the transmission might be going. I had put about 500 or so miles on it (had to go to Upper Michigan with it and back) and almost half way home the transmission gave out. I called the shop that did the flush and he told me on the phone, they have a machine they use so the filter does not have to be changed. Is that normal? The tech that did the flush said the fluid was only orange-brown a little. Do you think the transmission failed because they didn’t take out the filter? Any comment will be greatly appreciated? Thanks.

    • Martin Hand Martin Hand says:

      As long as the correct fluid was used then it’s likely coincidental. If the filter is plugged up then there is most likely some other problem causing the filter to plug up in the first place.

  • Bill Cobb says:

    My 2016 Ford F-350 runs the transmission temp around 205 degrees. Not pulling anything. Is that normal?

  • Mario Johnson says:

    I believe I’m past the option of a trans flush. The liquid is dark however I don’t see any debris. I’m starting to look for a transmission repair shop in the metro Detroit area. I have a 2007 Chevy Monte Carlo with 149000 miles. When I drive it hovers at 20mph for an extended period of time before shifting. I realize this is a indication that I need to repair/replace the trans. Any recommendation on a cost friendly repair shop. Preferably one that has optional financing. Thanks for the help.

    • Martin Hand Martin Hand says:

      I wouldn’t have any repair shop recommendations since your out of state. I’m not sure about repair costs either. Probably close to $2000.00 or more for a complete rebuild and all the proper valve body update or modifications.

  • Donna says:

    I have a 2006 Pontiac Vibe and never had my transmission flushed or cleaned. What would be the best recommendation for my car it has over 170,000

    • Martin Hand Martin Hand says:

      I recommend to start by removing the pan and changing the filter. Then you can inspect for excessive clutch material or metal shavings in the pan and filter to get an idea of the condition of your transmission.

  • PJ Brien says:

    Question I have a 2008 Camry V6 Automatic 6 speed and a AAMCO tech told me to not to have a ATF flush done and should only get the drain and re-fill done? I am curious what I should get at this point of up to 150000 miles on it now.

    • Martin Hand Martin Hand says:

      Either service should be fine unless you have pre-existing problems with the transmission. The service should be done on good transmissions as a preventative maintenance only.

  • Amarjot S. Vohra says:

    So my family just got a brand new transmission put in on our 2011 Camry XLE v6, before the old trans had given problems with gears slipping and rough and sluggish shifting. Toyota did a trans flush, which I knew was going to ruin it, but it helped for a while, then one day, boom, free Rev everywhere in Drive. So what I am asking is, is a trans flush after break in necessary? Or after about 30-50k miles? The car is driven hard at times (such as the drag strip) and mostly use tiptronic mode, rarely ever have it in auto, and would a trans cooler help?

    • Amarjot S. Vohra says:

      My real question, which I forgot was, is a flush required for all old and new auto transmissions or just the ones in older vehicles.

      • Martin Hand Martin Hand says:

        It’s not required but is good for preventative maintenance. Newer vehicles commonly use synthetic ATF and may not be recommended as frequently if at all. Always check with your factory service recommendations first.

    • Martin Hand Martin Hand says:

      Maybe not a transmission cooler but if you could monitor transmission temperature that would be very beneficial to the life of the transmission. Excessive heat is the number one failure of automatic transmissions. This way you can drive it easy if it ever gets hot. Anything over 220-250°F can be damaging. As far as future services I would recommend a fluid exchange every 30K.

  • grigore says:

    Hello, on my toyota avensis auto gearbox, i changed the oil filter and emptied the cooling exchange radiator with air pressure via one pipe line going into radiator. Then we filled up via dipstick.
    The gear oil level is not increasing to hot spot when engine is hot; I wonder the oil did not filled up exchange radiator due to existing air inside it ? Level is on low cold spot allways.
    Thank you,

    • Martin Hand Martin Hand says:

      I’m not sure what your asking here. If ATF does not come to hot on dipstick then add more fluid. If you drained coolant you will need to run vehicle with radiator cap removed and possibly a funnel to prevent spilling, this will bleed air from cooling system.

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