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.

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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.


  • Rocky says:

    Hi I have a 2012 nissan altima 4dr and I’m not sure what the problem may be.. it seems to jump a little at around 60 km/h and feels funny sometimes changing gears.. I got the car at 40K and now it has 140k.. have not gotten a flush or changed the tyranny oil was wondering if it’s time to do that

    • Martin Hand says:

      At this point your taking a risk because that’s a lot of miles without service. There is something going on I would have the pan removed and inspect for metal or friction material.

  • DaveC says:

    I just found your site and I have a question for you.

    I have a 2007 Saturn Vue with 106k miles on it. It has the v6 Honda engine I am told. I have never had anything done to the transmission. It is, and has been, running fine.

    I just purchased a used car and it was suggested that I get the transmission flushed on it which got me thinking about the Vue and how I had never had anything done transmission-wise to it.

    I have read on the web, or been told by local mechanics:
    1. I’m hosed – sell it quick
    2. Go and get it flushed
    3. It’s a sealed system – it will be fine
    4. Leave it alone and if you start to notice anything transmission related (slipping, etc) sell it fast.

    I am very interested in your thoughts on my situation. Your post here leads me to believe you know your stuff.

    Thanks for your time.

    – DaveC

  • Courtney says:

    I have a 1998 Mercury Sable and just this weekend, the service engine light popped on. I brought it to have the codes read and it said that my transmission is slipping. I have not done a transmission service since I have owned the car. (3-4 years unfortunately) Would you recommend getting is flushed and a new filter put in or is the transmission completely gone?
    Thank you!!

  • Spencer says:

    I have a Mitsubishi ralliart with the dual clutch sst transmission. Manufacture recommends it changed every 30,000 miles. But I smelled the fluid and it smells fine as far as I know it had not had the fluid changed since new 75,000 miles ago. I am concerned because it is a $10,000 transmission. Should I get it replaced even though it smells fine?

    • Martin Hand says:

      If the manufacturer recommends a service every 30K then I would definitely do it, there is a reason for the manufacturer recommending this frequent of a service. Re-check with Mitsubishi on the service intervals also, I would think it would be every 60K.

  • joseph colon says:

    not sure to do a flush in my ford van * windstar 1998 at 109000 miles. had 3 oil pan changes in last 4 yrs.,
    including filders. thinking abut a flush. somewhat not sure. I am concern about dirt left in transmission that may
    cause it not to untion properly. Please provide a suggestion and or advise.

    • Martin Hand says:

      Transmission flush or service is preventative maintenance only. As long as your servicing it for that reason you should be okay. If your servicing it in attempt to improve shifting or performance problems then I don’t recommend it.

  • Khal says:

    I have a Honda Accord 2002 V6 3.2L. While I was driving my car suddenly it shown me a Check engine and TCS sign and My D4 started flashing I put my car on side and shut off the engine. I started again after a few minutes the D4 disappear but the check engine is still. Second thing the transmission is slipping when I put in 1st gear. The mechanic said that you have dirty transmission fluid but not burning. I am really in great trouble. I don’t know what to do. Should I suppose to flush the old transmission fluid and filter with a new one. or should I stay with the same fluid because I hear a dirt transmission fluid changes means you collapse your transmission. please advice me.
    Best Regards


    • Martin Hand says:

      That vehicle does not have a filter that can be serviced unless you remove and completely dis-assemble the transmission and from there you should probably have it rebuilt. I have seen these filters plug up many times causing transmission failure.

  • Awesome, awesome, and awesome!!! Very informative article. I was not aware of the different methods/ machines used to flush a transmission. The clear explanation describing the differences makes it obvious to me which one I would choose. This is something I need to look into after the holidays. It’ll be a gift to my truck!!!

  • kurstin says:

    My car manual said to get transmission flushed at 100,000 mi. Unfortunately, I missed that chance. When I was going to go have it flushed, someone mentioned that having a transmission flushed after the fact could cause the transmission to give out, and everything I looked up on discussion boards seemed to support that, as well. My car is a 2006 Saturn Ion – I’m not sure if it’s just that type of car that has issues, or if it’s in general. Now, my car has almost 200,000 miles, and I would like to get a transmission flush, but I’m worried about frying the transmission. Is it recommended to get the junk out of the transmission as long as the car is running well as you described, or should I be concerned that it will have issues if I have waited too long?

  • Chris Mann says:

    1966 Oldsmobile Toronado. FWD with GM TH 425 version of TH400. Cooler in radiator failed causing mix of coolant and ATF. I think I caught it in time. Any experice with coolant in the ATF and the success rate of flushing under this condition?
    Years ago I had a Jaguar with a Borg Warner that was driven into salt water. I flushed it and it worked OK for a while. It seemed the band material would pile up in the pan and clog the filter. I would clean it out again and it would work again…for a while. I recall drilling a hole in the torque comvetert for a drain and then sealing it back up. Will the flush eliminate the need for that procedure? Can I do the cooler line flush myself or do you recommend the other professional type? I’m not too keen on a reverse flush…doesn’t should healthy.

    • Martin Hand says:

      You can just remove the cooler line, start vehicle and pump it out that way. Just make sure to keep filling it through fill tube. Do this until fluid comes out clean at the cooler line.

  • Kristin says:

    My daughter recently bought a 2001 Cadillac Seville and had trouble getting it to reverse (she said it seemed as if the car only wanted to go forward). She got it to her mechanic for an oil change & he recommended transmission fluid for $160. I’m guessing for that price he’s planning on a flush. The car won’t start so she feels as though it’s her only option to go thru with it. Do you know if this make & model need transmission flushing every so often?

  • Roman says:

    I flushed my transmission oil on a 2010 Mazda 3 at the dealer. First flush was around 30,000 and the next flush was around 80,000 miles. There is no guideline on when to flush the transmission oil in the owners’s manual.
    I changed it at 30,000 because the oil was still relatively new and I read that its good to do a flush around 30,000. Then after that to do one every 50,000 to 60,000 miles. There is no filter on this
    vehicle I was told so they didn’t drop or
    clean the pan. Your thoughts.

    • Martin Hand says:

      Does the transmission even have a pan? If it doesn’t have a filter it has a screen that can be cleaned. Sounds to me like you have taken very good care of the transmission, I wouldn’t worry about removing the pan in your case.

  • Adriana M says:

    What a great read!!! I flushed my transmission about 5 months ago, that day..later in the evening it completely gave out and would not start it just keeps turning. I knew it was the flush that did it because it was kicking before but it still ran. After I did the flush that was it everything you said in the article to not do, is what I did due to being misinformed by a mechanic. So many codes came up when I had the diagnostic test that they wanted to charge me an additional $500 to the $200 I had to pay already for the test. I got the results from the test and had my car towed to my place..its been sitting ever since. When the car gave out that evening the dashboard started going crazy all the gauges were on 0 or E nothing was working not even the radio..its like it messed with a sensor in the computer. At this point I’m not sure what to do any advice? I pay the car off in a few months at that :/

    • Adriana M says:

      I have all the codes that came from the test but the shop that did the test charged me $200 to do it and since almost a dozen codes came up he said it would be another 500 to figure out exactly why the car wouldn’t start.

    • Martin Hand says:

      I would find someone else for a second opinion first then decide what to do from there. If the transmission has failed maybe find a used one for it or weigh the options of a rebuild.

    • chris says:

      I have a 99 Chevy K1500 truck. Sometimes at start up, the gauges do not work, and the shift indicator does not light. It will go into reverse, but not Drive. The blogs said check the ignition switch and sure enough if I jiggle the switch and try it a few times everything comes back. Bad connections can really mess up a computerized car, and its computerized transmission. Failing alternators, Failing batteries are bad news in new cars.

      • chris says:

        You get a whole slew of codes with bad connections too. I would say check get your connections checked. Mechanics will say “rebuild” before they look for electrical issues.

  • Jaime Cruz-Loya says:

    I have a 00 Pontiac Grand Am GT, automatic. It’s got 161,000 miles. I had a transmission fluid flush done 50k miles ago. I’ve always been told to get it done every 50k miles. Ever since that last flush in temperatures below 60 degrees farenheit, unless I let the car warm up for 10 minutes or so it will not shift when it should it will rev up higher than normal but it doesnt shift hard. After the warm up period its perfect, shifts flawlessly. My question is what might be causing this? Would performing the flush hurt it, help it? Any help is greatly appreciated. By the way it doesn’t have a dipstick.

  • Linda Ash says:

    I have 2004 Buick Century. It is jerr king at times while I am driving. I stop and put in drive and it stops. I was told to get a transmission flush by one mechanic. After calling around was told not to unless I go to a transmission place incase it won’t work anymore after the flush. What should I do?

    • Martin Hand says:

      I don’t recommend a flush, you need to find out why it’s jerking. Possibly take it to transmission shop for an inspection, go to more than one if you don’t trust them or there advice.

  • N Howard says:

    I own two 1977 Jaguars that I dispatched to the USA, in 2012, for storage in my son-in-laws extensive garage.
    One was an XJ 5.3 L. I had a gearbox change out made by a major Jaguar dealership, in the UK, but right from the date of the change out there was a leak from the gearbox. It was an insignificant leak but was enough to be a nuisance. The two cars, once in my son-in-laws garage were to be “exercised” on a regular basis. However, the person who was to carry out the exercising thought it wise to add an additive to the oil which was designed to stop oil leaks. It was later reported to my son-in-law that the gearbox was faulty and when selecting a specific position another position would be delivered by the transmission. This news was passed back to me together with the news of the addition of the additive.
    Can flushing with a solvent remove the contaminant from the elastomer and bring the swollen elastomer parts back to original size?

  • tim says:

    I just bought a 2001 honda accord ex.It has 173,00 miles on it and is an automatic.The tranny fluid is brown,not pink.I am afraid it will hurt to get a fluid change but what if the tranny leaks? What can I do? I do not even have 100 dollars to save and cannot afford a new tranny.I may have to sell the car and I just bought it 4 and a half monthd ago.

    • Martin Hand says:

      If the transmission is working properly then you need to have it serviced. If it’s not working like it should then I would just leave it alone until you can afford the repair.

  • Shannon Walsh says:

    I have a 2003 Hyundai Santa Fe with 120,000 miles on it. I bought it with 68,000 miles on it. I have not changed the transmission fluid at all since I bought it. I recently took it for inspection, and my mechanic said that the fluid was dark brown, and he recommended a flush. I’ve always heard that a car with over 100,000 miles shouldn’t have the fluid flushed if you don’t know the last time it was done. My car drives fine. Very, very occasionally it will buck when I am stopped at a red light. Would you consider that a problem with the transmission? Would you give it a flush if it were your car?

    • Martin Hand says:

      I would remove the pan first and inspect for excessive contaminates. You can tell a lot by whats in the pan. Could be brass, steel, aluminum or friction material. Change the filter and test drive. If it’s okay then consider the complete fluid exchange.

  • […] can have an adverse effect. any or either way u look at it the old dirty fluid needs to be removed.…-transmission/ lexusman is online now   Quote Quick […]

  • Bob Schilling says:

    Thank you for this information. Now I’m educated on the correct flush. Do you know if Honda uses the “pump inlet flush machine”? Just purchased ’09 CRV with 94000 miles. Want to maintain it correctly. If not Honda, do you have any recommendations? I live in New Wilmington, Pa. 16142. Thanks again for your help. Bob S.

    • Martin Hand says:

      All flushes that are done today use a standard pump machine that runs off the transmission cooler lines. I believe the pump inlet procedure is pretty much obsolete presently.

  • travis says:

    Hello. I recently bought a 94 Toyota 4 runner with 214000 miles. It’s an automatic and when starting from a dead stop it is gutless, like it’s trying to go in third gear. I can manually shift down to second or low and it takes off like it should. The overdrive off light also blinks. Transmission oil appears light brown with a musty, oily smell. Do you recommend flushing it?

    • Martin Hand says:

      Definitely not, sounds to me as if the transmission is in a fail safe mode and needs repaired. Take it to a repair shop in your area and find out what codes are stored in the TCM.

      • travis says:

        Thank you so much for your help! I did not know the transmission could go into a fail safe mode! I appreciate it. I will get the codes from the tcm. I’ve done research on problems with older 4 runners and sounds like my shifting problem may be caused by 3 solenoids located on bottom of tranny under oil pan. Hope this is the case, if so it shouldl be an easy fix.

    • Anonymous says:

      Low atf?

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