E31 850 Transmission Filter Change and Flush

    • Official Post

    I've just got around to changing the fluid, filter and flushing the trans on my 1991 850i. It's been about 60,000 Km since it was done previously but the results as far as improvement in shift-smoothness have made it very worthwhile and made me think I should post the process for other's enjoyment.


    It is quite a simple job for someone who has reasonable mechanical aptitude and a reasonable selection of tools. Takes 2-3 hours. Most important is a clean place to do the job, auto transmissions have little tolerance for dirt and, as changing the filter means removing the oil pan, you don't want any dirt to be blowing around while it's opened up. Also make sure you have lots of rags around to wipe your hands continuously, it's VERY important to keep any dirt, dust or grit out of the trans.


    Before you start, you will need a new pan gasket and a filter and O ring set, these are all available after-market for around US$20ish. Plus you'll need about 10 liters of Dexron 3 ATF. It's also worthwhile to give the underside of your car in the trans area a high pressure wash, as it will make the job a lot cleaner, especially if it's greasy under there.


    1. Take the car for a run so the trans fluid is warm and well stirred up, you want any contaminants to be well mixed in the fluid, so they drain out readily.


    2. Raise the front of the car up on stands or ramps (apply common sense safety precautions when working under your car), so you can crawl under and get to the trans area. Make sure your work area is clean and there is no dirt or dust likely to be blowing around.


    3. Loosen (because it will be on real tight) the huge bolt holding the dipstick tube to the front of the oil pan. Also loosen the trans cooler return pipe where it connects to the bottom of the trans, just forward of the oil pan (19mm spanner) The reason for doing these 2 steps now is that they are the most likely to trip you up. If you cannot budge either of these then you're best to enlist help.


    4. Remove the drain plug from the centre of the transmission pan, there will be 4-5 liters of fluid that will come out, so be prepared, it may also be scalding hot. Once it's drained completely, reinstall the drain plug.


    5. Remove the huge nut and then dipstick tube from the front of the oil pan. No need to remove the upper dipstick tube mounting, as there's enough movement to swing the tube away from the pan with this in place. There will be a little fluid that comes out when you do this, so have your drainage container underneath.


    6. Place a good thick mat of newspaper on the floor under the oil pan. This will catch and contain any drips and spills that will occur in subsequent steps.

    6. Loosen the 6 x 10mm bolts holding the oil pan to the trans body by turn at a time, so as not to put uneven stress on the pan. To make this job less messy, I recommend loosening them all a few more turns then giving the pan a good whack with your hand to un-stick the gasket and pan from the body of the trans. This will allow a "controlled descent" of the pan, which will still contain some residual fluid. When the pan is loose from the trans body, next remove the 4 bolts holding it in the corners. Finally remove the final 2 bolts along each side, while holding the pan up with your other hand. Lower the pan, keeping it level but with rear end slightly lower, so the residual fluid doesn't come out the hole where the dipstick attaches.


    7. Place your drainage container under the trans to catch the continual drips that will be falling. Resist the urge to wipe the internals of the transmission to clear these away. The reason for this is 2 fold - firstly any lint or fluff on your shop rag will end up inside the trans (not good) and 2ndly it will continue to drip no matter how much you wipe!


    8. Remove the filter from the bottom of the trans. This is held in place by 3 x Size 27 Torx head screws. When it comes away, there will be a fair bit of fluid inside it, so be ready with your drainage container. There is also an O ring which seals the filter outlet with the valve body in the trans. Make sure it comes away with the filter, else remove it from the valve body. Your new filter kit will come with a replacement O ring. You will notice that the old filter is covered in a very fine black film, this is normal and does not mean your trans is on it's way out.


    9. Remove the plastic wrapping and the plastic protector ring from the new filter. Install the new O ring on the new filter, the lip on the filter, around which the O ring locates is sharp, so take care not to damage the O ring or your fingers.


    10. Locate the new filter complete with O ring on the bottom of the trans, ensuring it is correctly seated and the O ring is in place and not pinched. Secure it with the 3 x Torx head screws and tighten them snugly. Always be careful not to over tighten screws and bolts.


    11. Now have a look in the oil pan. When you drain the remaining fluid from it, check if there's any sediment in the bottom. Apart from the thin black film, it should be fairly clean. If there are metal bits, lumps or sludge in significant quantity you could be in trouble. Note there are 2 magnets in the pan to collect metals from the fluid. It's normal to find these covered in a black sludge, which will feel smooth like grease. If it's gritty or there are bits of metal in there, again you could be in trouble. Thoroughly clean the pan internally and externally and clean the magnets. Make sure everything is spotless. Ensure the magnets are put back in their circles inside the pan. Check the sealing edge of the pan to ensure it's free of any damages. If you have had fluid leaks from the oil pan previously, check that the pan is not bent or warped. Best way to do this is to place the pan on a sheet of glass (even if it's still in a window!) It should sit flat with no significant gaps anywhere around the edge.


    12. Back under the car, clean the lower face of the trans against where the oil pan seals using some lint free rag and solvent (petrol works well) Ensure this face is spotless and free of any damage, this will allow the pan to seal leak free.


    13. Install the new pan gasket on the pan. Do NOT use any gasket sealant! The gasket doesn't stay put very well, so take care not to dislodge it when you reinstall the pan on the bottom of the trans. As before, hold the pan in place with one hand while you reinstall the 2 bolts and pan retaining fittings in the middle of each side of the oil pan. Leave these loose at this stage while installing the final 4 bolts in the corners. Once all 6 bolts are loosely installed, carefully check that the gasket is correctly positioned around the entire circumference of the pan. If it doesn't look right, remove the pan and reinstall the gasket correctly. Once you are happy that the gasket and pan are correctly installed, progressively tighten, in a zigzag pattern, the 6 securing bolts. Finally, only tighten these enough to bring the pan retaining blocks into contact with the trans body. These are designed to put exactly the right amount of pressure on the pan and gasket to ensure it seals - do NOT over tighten.


    14. Reinstall the dipstick tube on the front of the oil pan. Wipe the outside of the pan so it's clean and dry, this will make checking for leaks easier when you refill it in the next step.


    15. Refill the trans through the dipstick tube; note this will need a long funnel or a funnel with a piece of tubing on the end of it. Fill it slowly (this bit can be tedious) as the dipstick tube takes the fluid very slowly and has a tendency to overflow at the slightest provocation. What I did was to get a piece of plastic tubing that fitted tightly inside the dipstick tube and also fitted tightly around the outside of my funnel, therefore there was no chance of overflowing around the dipstick tube. Ensure the funnel and tubing is spotlessly clean before using them. At this stage you need to put 5 liters of fluid in the trans. While and after filling, check for any leaks around the pan and dipstick attachment to the pan. If there is any seepage at all, fix it before proceeding. This may mean draining out your new fluid and removing the pan again.


    16. Now we're going to prepare for the flushing of the torque converter. A bit of transmission theory here... when you drain the oil pan as you have just done, you only remove about 4 of the total of 8.1 liters of fluid from the transmission. The remaining 4 liters is in the torque converter. Every time the transmission shifts gears, a very small amount of friction material wears off the clutches. This friction material stays in suspension in the fluid. The fluid is pumped around the trans, through the torque converter and then through the trans cooler in the radiator before being returned back to the oil pan. Flushing the converter ensures that as much of the old fluid, contaminated with friction material, is replaced with new clean fluid.


    17. Assuming your oil pan is leak free; loosen the securing bolt that holds the trans oil cooler pipes along the side of the engine. This bolt is right below the starter motor. This will allow some freedom to remove the cooler return pipe.


    18. Next, remove the cooler return pipe from the bottom of the trans, just in front of the oil pan (19mm spanner). Have your drainage container there, as fluid will leak out. Note, there is an O ring on this pipe where it fits into the transmission housing. Be careful with this O-ring, because you're going to re use it - unless you have another handy (12mm O ring).


    19. Fluid will be dribbling out of this tube continuously and you need to attach a length of drain tube to it, so you can direct the outlet of this tube into a container. I used a piece of garden hose, as it seemed to fit perfectly and secured it with a hose clamp. You will also need to plug the transmission body from where this pipe was removed. I used a synthetic cork from a wine bottle, which I was able to screw into the threads in the pipefitting. If you don't plug this hole, your new fluid will come out in step 23, as I found out the hard way!


    20. Now you need to measure the quantity of fluid that you have drained out of the transmission in total thus far. This will help guide how much is left in the torque converter and needs to be flushed out. I had drained about 4.5 liters from mine by this time. Meaning there was 3-4 liters (trans and converter hold 8.1 liters in total) remaining to be flushed.


    21. Place your drain tube into a suitable container that will allow you to measure how much you flush out - I used an old 5-liter oil container, with graduations on the side. 21. Add some more fluid to the trans via the dipstick tube, you will need at least 6 liters in the trans when you start the flushing process, yes it will be overfilled but don't worry at this stage.


    22. Make sure you have a drain pan under the trans, in case any of your plumbing springs a leak. You also need to have an assistant at this time to help you.


    23. Now start the engine and run at idle in Park. When you do this the transmission will pump the remaining old fluid out of the torque converter and into the container you allocated at step 21. It pumps it out by replacing it with the new fluid you have just put in the oil pan. Watch the container and switch the engine off once it contains the quantity determined in step 20. In my case I drained a further 4 liters. You could get keen and flush a bit more through but you need to be mindful that you only have around 6 liters in the trans to work with, so you may need to add more new fluid during the process, or do it in stages (not ideal in stages as the old and new are more likely to mix in the converter). Also keep an eye on your plumbing during the flush and switch off the engine if you get any significant leaks in your drain hose or plug you have installed in the bottom of the trans body.


    24. When you have completed your flush, remove your drain hose and reinstall the cooler return line to the transmission body, don't forget to have the O ring on the end of this tube before you install it. Ensure this tube is correctly located in the transmission body before you start screwing in the 19mm securing nut. It should be possible to do this nut all the way up with only fingers, if you have correctly seated and aligned this pipe in the transmission housing, before finally tightening with a spanner in step 25.


    25. Reinstall the cooler line clamp below the starter motor before you finally tighten the 19mm nut securing the cooler return line into the body of the transmission.


    26. Work out in total how much fluid you have drained from the transmission, this will allow you to work out how much to put back in at this stage. In my example, I had drained a total of 9 liters, so I put back 8 liters at this stage. I did this because this would be sufficient to check the level using the dipstick and add exactly the right amount to fill the trans to the correct level. It’s easier than overfilling and draining out the excess.


    27. Run the engine again after reconnecting up the cooler return line and roughly topping up the fluid to check for leaks. Also move the gear selector into each position to confirm the car engages gear correctly (keep the brakes applied hard if the back wheels are on the ground!). Fix any leaks or seepage from the connections before lowering the car to the ground. If the car doesn't engage gear (i.e.: you don't feel the characteristic "bump" when you go into D or R). Make sure you have sufficient fluid in the trans (i.e.: recheck how much you have drained in total vs. how much you have poured back in). Do not rev the engine during this test, or if it fails to engage gear.


    28. Lower the car to the ground and check the fluid level as normal. Add more fluid until correct. Test-drive the car - I did and gear changes were now VERY smooth, especially at light throttle openings. After a test drive, re-check the fluid level again. Note, it is almost as bad for the trans to be over filled, as it is to not be full enough, so spend some time here and get it spot on.


    29. During the next week or so of use, check daily under the car that there is no evidence of leaks from the transmission. It should be dry.


    Hopefully this is a useful guide to those keen DIYer's to do this job. Happy to answer any questions or fill in gaps in the above.


    Good luck, Richard.
    (RAC69 on http://www.clube31.com)

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