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.

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. MDH Motors offers moderated guest posts! Click Here To Submit Your Article For Review Please feel free to Ask a Mechanic if you have not received a response for your comment within 72 hours, thank you.

370 Comments

  • Jeff Schmidt says:

    I just took my Mercury mountaineer to a ford dealership for a transmission flush. The tech stated that the fluid was black and dirty. Why would a tech change it if it’s not in my best interest, and what if any liability do they have by doing so? They want to put in a new tranny, but if they dislodged a bunch of build up, won’t the new tranny fail as well?

    • Jeff Schmidt says:

      Also 2 days later tranny is shot, brand new fluid was already black again according to tech.

    • Martin Hand Martin Hand says:

      When installing a new transmission the transmission cooler should be replaced or flushed, this is the only part that can contaminate fluid for the new transmission. The shop has no liability in your transmission failing after a flush but they should talk to you about the possible risks before performing the repair. If you we’re not informed then that’s just bad customer service but they are still not liable.

  • Chester Stephens says:

    I had no trouble with power in my car, I had some brake work done and they told me I need a transmission flush. Car has 100,000 mile on it. When I first got the car back it ran just as usual. A few days later I was running errands all around town and noticed I lost power. Got home checked trans fluid it is 3/4 inch past the hot line. Yes car was hot. After checking everything I could that is the only thing I could see wrong. I let it sit for 2 days, went out and drove around a short time and no problems till it gets hot.

    • Chester Stephens says:

      After driving the car again ,when it lost power I would shift to neutral and then back to drive it would run fine for a short while. Had to do this to get it back home. I still think either wrong fluid or too much is causing this issue. Any suggestions would be nice.

      • Martin Hand Martin Hand says:

        Too much fluid can cause fluid leak from vent valve and if level is too low then transmission will not engage. If no dipstick fluid level checked from bottom side. There will be a check plug you remove with engine running in park at operating temperature. Fluid should slowly drip if level correct.

    • Martin Hand Martin Hand says:

      Are you sure the problem is transmission related? Being overfilled will not cause transmission to fail. I purposely overfill certain transmissions (1qt at the most) to help prevent planetary gear failure. Worse case the overfill will cause fluid to leak from case vent valve.

      • Chester Stephens says:

        Thank you. I Wil check more on transmission. My thing is it ran like dream car before. But I do know things do happen very fast and sometimes you can’t see or hear it before it starts. Thank you very much.

      • Anthony says:

        It was because u probably mixed good and bad oil

    • Andy says:

      You can get a little hand pump for about 10 bucks at an auto parts store that you should be able to stick a tube down the dipstick tube and pump the excess out. Despite what is said, overfilling can cause problems on some transmissions. (I don’t have time to explain at the moment, but one can easily search for reasons why.)

      • Martin Hand Martin Hand says:

        Overfilling transmission .5 to 1 quart will not cause problems. I have never seen overfilling an AT transmission cause any problems except blowing fluid from the vent tube. There are claims of all these problems that can occur but from my experience nothing ever happens except fluid leaking or a blown seal at worse. I’m curious as to what failures you have experienced from transmissions being overfilled?

  • andrei says:

    I have a 2001 ford focus. I am having shifting problems, when I put the car in reverse, the car shakes really bad. I checked the trans fluid, it was not black, no metal flakes, color was red, but dark red. I don’t know if the car had a flush and fluid change. I had the car for 4 years now, and I never flushed or changed the trans fluid. Should I try and flush it, change fluid? I also read online and watched youtube vids of ppl putting lucas transmission slipping fix, and it seemed to work for many. Thank you

    • Martin Hand Martin Hand says:

      Possible to fix the reverse only if that’s the only problem. a lot of times you’ll find more problems during the repair so a complete rebuild is the outcome anyways. I do not recommend a flush but you can try the Lucas treatment, can’t hurt at this point. I do recommend to remove pan, inspect for excessive metal or clutch material and change filter of applicable. This won’t fix your problem but will give you an idea of the overall condition of the transmission. Good luck

  • Hi,

    I have a 2008 Dodge Ram 3500 Laramie Quad Cab 4×4. I pull a 30 ft 5th wheel trailer with the truck. About 9000 lbs.

    I changed the transmission oil at about 90,000 mile and it now has 135000 miles. I was told by a transmission shop I should change the oil and have a transmission flush? Your thoughts.

  • Bill says:

    I Have A 72 Chevelle with a turbo 350. It is leaking from what looks like the front passenger side of the pan. I changed gasket and filter, still a slow leak from the same location. Pan also gets hot and stays hot for a while after being shut down. It doesn’t seem to be acting up but am worried about leak and the heat. Pan is pretty hot to the touch. Any suggestions?

    • Martin Hand Martin Hand says:

      Install a transmission temperature sensor and gauge to monitor transmission temperature. The trans pan is probably still leaking because it’s bent. Either flatten the pan and ping the bolt holes or replace the transmission pan and don’t use a cork gasket.

  • Aaron Mones says:

    I have a Nissan altima 2006 2.5s I had a problem with rpm going up and down also hessitation when accelerating I clean up the throttle valve but still having the same problem should I flush and replace the transmission fluid?

    • Martin Hand Martin Hand says:

      No, transmission flush is for preventative maintenance only. It’s not going to fix a problem with transmission if it’s failing, if anything it could make it worse. Make sure the problems is even transmission related also. If anything remove the pan so you can get an idea of the condition of the transmission.

  • Tommy N says:

    Hi I have a 2000 Honda Accord 3.0 I recently have my transmission rebuilt about 3 years ago but lately I notice that every time I start the car and put it into drive the car will jerk forward pretty hard can you tell me what really causes to that. Thanks

    • Martin Hand Martin Hand says:

      It’s most likely related to excessive high engine idle on start up. Could be normal but i wouldn’t know unless I seen the vehicle myself. Also check the engine mounts. Honda uses vacuum controlled engine mounts that can cause harsh forward or reverse engagement when not working properly. I wouldn’t think there’s a problem with the transmission unless your having other problems such as harsh up-shifts. This could indicate transmission has high line pressure.

  • Ted says:

    Hi I have a 2007 Honda CRV LX with 115,000 on it. Have had transmission flush done every 35,000 miles, no problem with transmission. I need to change transmission fluid again. American Honda does not recommend flushing. My Honda dealer always did the flush but not any more. My transmission is good. Should I just do a drain and fill or have flush done. Drain and fill does not replace all the fluid. Now my Dealer said flush can damage transmission. Some Honda dealers still do a flush. I read some where that you can drain and flll the tranny 3 times to get same effect as a flush You need to drive the car 10 minutes between each drain and fill. I am not sure what to do. I never went to Honda for a flush my mechanic did it. I also use another mechanic and he says do not flush just drain and fill once. Help!!!!!

    • Martin Hand Martin Hand says:

      Either way is fine. In your situation sounds to me like you change it often so the fluid is probably not that bad. Just a drain and fill every 15K will keep up on it just fine.

  • Victor says:

    I recently purchased a 97 Lincoln Mark VIII LSC, with 98,000 in Tampa, FL. I took it to a Lincoln dealer that recommended a list of services including transmission change. I took it to aamco, they said the fluid was not too dark but has an odor they did not want to do the service after I let them know it was recently bought hadn’t been serviced by the last owner and mostly sat in a garage and driven occasionally. The car doesn’t really have any shifting issues but I want to preserve it in good condition. Every mechanic has recommended not to change it as precaution but I know doing this means the transmission will eventually fail down the road. Should I risk it? How much would a new transmission/installation cost?

    • Martin Hand Martin Hand says:

      I would recommend to remove the pan and change the filter. This way you can have an idea of the condition of the transmission by looking for aluminum, steel or friction material in the pan, magnet or filter.

  • Bob says:

    I have a 2010 Mazda 3 hatchback, 114K. After an hour of driving only when it shifts from 2nd to 3rd gear, it’ll rev up 750-1000 rpms to shift. But it’s random! I can’t predict it & I can’t duplicate it. It’ll rev like this couple of times and then it’ll go away. It’ll show back up anywhere from 10-30mins later, then go away again, and repeat. Any ideas what it could be?

    • Martin Hand Martin Hand says:

      Intermittent problems are usually electrical but that doesn’t mean there’s not a valve in the valve body hanging up. I’m not exactly sure how those Mazda AT transmissions work I have never rebuilt one yet. I would start by having it checked for diagnostic trouble codes.

  • Laura says:

    I have a 2003 Honda Accord EX V6 with over 180k miles. My transmission was rebuilt two years ago and now I’m having problems again. It’s jerking very badly when changing gears. Would a flush help or should I just get a new car? Thank you :)

    • Martin Hand Martin Hand says:

      A flush is not going to help and it doesn’t necessarily mean you need a new car either. I would have it looked at by a good technician that you trust. It could be a failed shift solenoid, sensor or something else simple you just never know. The transmission was rebuilt so it should not be worn out if done rite. How many miles on the rebuild? Make sure it’s even transmission related…

  • Ken says:

    Best article I’ve read in recent times, simple, thorough and concise, thank you.
    I have a 09 Toyota Rav4 with 2AR-FE, with the U241E A/T Japan built. It has 98K miles but it has a grayish red color, no burnt odor, fluid level is OK, both hot and cold. Shifts fine around town but when driving on freeways in 4th “gear” in O/D, around 45-50 MPH, 1200-1500 RPM, it feels as though there’s a slight slip and a driveline shudder, barely noticeable. No trouble codes stored or MIL at this time. In your opinion, should I go ahead and drop the pan, replace filter, and fluid? Also, I’ve noticed that some after market filter has a foam/sponge like filter element verses OEM mesh type, which is much more costly. Should I go with the OEM or should it matter? Any insight and your expertise is greatly appreciated.

    • Martin Hand Martin Hand says:

      From what your explaining sounds to me like a torque converter lock up shudder. Try adding some anti-shudder friction modifier, Trans Doctor makes a good additive and if you look online you’ll find other brands. I also recommend Lube Guard. If the OEM filter is mesh type just clean it with solvent or brake cleaner and blow it dry with compressed air. Hopefully this helps, good luck.

  • c says:

    1999 Suburban had rebuilt transmission installed about 800 miles ago. My inclination is to have it flushed at 1,000 miles to remove any shavings, debris, etc which may have been created during the rebuilt transmission’s break-in process.

    What do you advise, transmission flush or a service?

    Thanks

    • Martin Hand Martin Hand says:

      No, I would not recommend changing the fluid. When the transmission was rebuilt it would have been cleaned out and the transmission cooler should have been flushed of all metal shavings and clutch material.

  • Brian McLain says:

    I have a 2009 Honda Accord with 145k miles. Honda has recommended I do a transmission service, which I believe for them is just a drain and fill. A local shop has recommended I do a full flush. I bought the car used 90k miles ago and have never had the transmission done. I do not have any shifting issues or other anything to lead me to believe that the transmission is having problems, but I realize the car desperately needs the preventative maintenance. Since it has obviously been a long time since the transmission was serviced, do you think that a full flush is the better option? I believe Honda recommends to not do a full flush, as a standard.

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