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.


  • Haylee says:

    Thanks for this post! I have a 2002 Toyota Camry with 262,000. At an oil change a few months back, a flush was suggested due to black fluid. More recently, I had to get my starter replaced at a more reputable shop, and they said to absolutely NOT flush it, and they wouldn’t even do it because of liability. Do you suggest I just get the fluid changed instead of flushed? I’m nervous about debilitating my car, as I can’t afford a new car or new transmission. The car seems to drive okay, no huge noticeable problems shifting. Thank you for any help in advance!

    • Martin Hand Martin Hand says:

      That’s pretty high mileage so if your going to service the transmission start by removing the pan and changing the filter. A flush could possibly plug up the filter so change filter after the flush if performed.

  • Gina says:

    I’ve just found out that a seal on my transmission is leaking while bringing my 09 Camry in for other maintenance. When my mechanic called to tell me this he said that he’d like to flush the transmission fluid . Is this necessary as he will be changing the seal and putting new fluid back in already ?

    • Martin Hand Martin Hand says:

      There are a lot of different seals on the transmission, not sure what seal your referring to. If it’s leaking it should be replaced and transmission service is good for preventative maintenance.

  • Brosif says:

    I have a 2000 Honda Prelude with an automatic transmission but the transmission has no “filter” or dropable pan, rather a sealed in “strainer” is used and the drain bolt has a magnet. If a flush is performed does the strainer have to be replaced or is it a metal mesh that does not require service? The flush product I was planning to use does not recommend it be used on transmissions without a serviceable filter or drop pan. In order to service the strainer the entire transmission has to be dropped. Transmission is slipping when taking off from a dead-stop

    • Martin Hand Martin Hand says:

      Transmission fluid exchange or flush will not help at this point. You need to remove the transmission and have it repaired. It’s common for these filters to plug up but you might as well rebuild the transmission while you’re in there.

  • LaShonna says:

    I bought my teenage son a 2001
    Volkswagen Jetta GLS 2.0 in 2013 . It had about 110,000 miles on it at that time, now it has 151,000 miles it jerks once every time the gears are changed. I wanted to get the fluid drained and replace and filter changed. I am a single mother and know nothing much about cars but I don’t want it flushed. Should I just leave it alone. Any recommendations would be greatly appreciated?

  • Inder says:

    I have s Suzuki Swift 93, 5 speed manual. Fantastic car. I change transmission oil every
    10000 km. My car has done 180000 km. Should i flush the transmission. The transmission is still slick

  • Alex Nicolson says:

    Thanks for this. I am not a qualified mechanic but have some experience. This gels entirely with my experience.

  • l johnson says:

    2004 Pontiac Vibe, 158,00 miles with AT. Transmission fluid never flushed or changed. Fluid is black but appears to be clean. Drives good & shifts good. Should it be flushed/changed or just leave it alone.

    • Martin Hand Martin Hand says:

      I would exchange the fluid and remove pan to change the filter also. Make sure to use the correct ATF. Valvoline Max Life ATF is compatible I believe.

  • James Erve says:

    Hi thanks for the writeup. I have a 2006 Pontiac Grand Prix G6. Over the last three months intermittently it has started driving “rough” to the point becoming hard to accelerate. After pulling over it refuses to shift into gear (reverse, neutral, drive). After waiting for about 30-45 minutes and restarting the car it seems to work just fine.

    Ever hear of such a thing?

    *Local mechanic after hearing of the symptoms was equally as baffled, especially after I had it towed to him for said problem but after it sat overnight it drive “normally” when he took it for a spin. He is suggesting a trans mission flush before escalating it to a bad trans

    I’ve only had the car since March, but I’m fearing the worst now

    • Martin Hand Martin Hand says:

      I wouldn’t recommend a transmission fluid flush, that won’t help anything, for preventative maintenance only. Sound like electrical problem causing it to go to fail safe mode. Check for codes and repair those first before considering fluid exchange.

  • Christopher says:

    Hello I have a 2009 Chevrolet Cobalt LS with an automatic transmission and 137XXX miles. I’m getting a PO752 code. It’s saying Shift Solenoid A is stuck on. It revs really high when it tries to shift into 3rd gear. Sometimes it just hesitates and shifts hard. Any ideas? Should I replace the solenoid or just invest in rebuilding the transmission since it has so many miles?

  • Amador says:

    I have a 2016 Camero SS. They had a recall in a transmission part. After the recall replacement it vibrated while shifting from 4-8 cylinders. They keep flushing the it 4 or 5 time now. Vibration is smaller. Do flashings wear out any part, damage or age?

  • jon presho says:

    I have a 2008 Yukon Denali 6.2 either 6l80 or 6l90, not sure. I had the trans heat exchanger fail inside the radiator, cross contaminating both fluids. What would be the best course of action to salvage my trans? I should add this began as a small leak with mysterious coolant loss over a few months before total failure, trans cooler failed and in turn radiator tubes failed. Still runs and drives fine. Perhaps a gallon of coolant in the transmission.

  • Nancy says:


    I have a 2009 Mazda 6 at 120k miles. I took my car in for an oil change and transmission fluid flush (which has been recommended for about a yr now). The guy at the shop ended up calling me back saying there was a cooler line leakage (he said it was a pretty significant leak) by my radiator and he recommended getting that replaced in order to do the fluid flush. Being a woman, this was a another language.

    My questions is, would you recommend getting this replaced asap? Or is this something that can be delayed till another month? I walked into the shop to get something simple done and then I come back out with a whole lot more! Please help!

    • Martin Hand Martin Hand says:

      The transmission cooler lines are used to perform the transmission fluid flush so if they are leaking they should be replaced at the same time. If you have questions have the shop show you the leak or request to see your old parts.

  • marcy says:

    I have a Mitsubishi Endeavor with 90K miles. There is a vibration at 25 – 32 mph. Mechanic says the transmission fluid is not burnt and that it is the torque converter. It may be good for another yr.. there isn’t any slipping when in gear. is it worth spending the $ for a flush?

  • Yolanda says:

    I have a 93 Mercedes 300SE with 229k miles! Its a real beaut but its jerking alot trying to get through the gear changes. A transmission flush was suggested, what do you suggest? I haven’t had a diagnostic as of yet, but was advised about the metal shavings and if they are in the fluid, the transmission is about to go.

  • Christopher Hunt says:

    Hello sir,

    I have a 2012 Honda CR-V. Currently when I drive, at exactly 20 mph it makes a quick vibrating noise kind of like a cell phone vibrating on a desk. Between 25-30 mph, the vehicle shakes if I am accelerating slowly or makes the same exact vibrating noise at exactly 25 mph if driving uphill. The fluid is clean but I have never had the fluid changed (has about 75,000 miles, should have done at 60,000 if I remember right). Is it maybe the torque converter or should I get a flush and use a shudderguard first? Thanks.

  • alex says:

    Great write – thanks heaps.
    I had a thought though, provided a reverse flush is NOT carried out, what could still go wrong if a flush is performed on a transmission which has had damaged (as discussed in the last paragraph)?
    Thanks again

  • Victor N says:

    I have a 2003 dodge ram 1500 that has 192000 miles on it no smell or dark color. The truck shifts as it should was wanting to know what you would recommend? What type of flush machine do you recommend?
    Thank you

  • Henriette Clardy says:

    I have a 2017 Mitsubishi Mirage G4 SE, I bought last August and I have 40.000 miles in my car. The service said I should have a transmission flush with filter change. I heard about that the transmission flush is not needed before 100.000 miles.
    Can you please let me know when I should have a transmission flush?
    Thank you.

  • LTD says:

    Hi, I have a 2013 Ford Escape and am having transmission issues. It tries to get into gear but mostly revs in drive and reverse. My mechanic states that the fluid is brown but it doesn’t sound like any metal flakes. Is it still too far gone for a flush and fluid change?

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