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Old 06-05-2023, 06:44 AM   #1
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Transmission Oil Flush or Drain?

I purchased a used, gas powered 2014 Bay Star 2903 with a F550 engine. I would like to have the transmission oil changed. What method (flush vs oil change) is best practice for the F550 transmission?
Thanks in advance!
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Old 06-05-2023, 06:53 AM   #2
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I never liked the ďflushĒ method which pumps in fresh fluid to exchange the old. To me it stirs up sentiment in the pan and you are never truly replacing all of the old guild. Manually draining from the pan and draining the torque converter via the drain plug and removing the cooler lines at the tranny and blowing them out is the best. If a torque converter does not have a plug I remove the return line from the cooler at the transmission and start the vehicle to pump out the converter.
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Old 06-05-2023, 06:57 AM   #3
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I agree, I would not flush.


If old fluid looks OK just drain, remove pan and clean, replace filter and fill.


If it does look bad, drain fill. Remove return line from transmission cooler and put it a quart glass jar inside a bucket/oil drain pan. Start engine and run until 2 quarts or so are in the bucket. Refill transmission. Repeat until new/clean fluid comes out (easy to see in the glass jar). This replaces almost all the fluid, but does not "knock loose" any contaminants.
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Old 06-05-2023, 07:01 AM   #4
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Originally Posted by Boylegang View Post
I purchased a used, gas powered 2014 Bay Star 2903 with a F550 engine. I would like to have the transmission oil changed. What method (flush vs oil change) is best practice for the F550 transmission?
Thanks in advance!
I would do both. First would be to remove the oil pan and clean any debris out of the pan, replace the filter(s) and add clean fluid back to capacity then flush the entire system. Flushing first can loosen anything in the pan and force it through the entire transmission and oil cooler which may leave some behind.
Just dropping the pan isn't going to flush the torque converter which is where a large amount of fluid will be trapped
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Old 06-05-2023, 08:35 AM   #5
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Synthetic fluid doesn't "wear out". The additives in it do and of course it might get dirty. Personally I would drop the pan and do a traditional inspection and fluid/filter change. Some places will just flush and be done. Kind of blows my mind. I want to at least have look in the pan. A transmission shop's first move when diagnosing an ailing unit is to have a look at the fluid and what's in the pan. I want to do the same to confirm my transmission is healthy.
Generally speaking, if you service your healthy transmission on some sort of regular basis, just replacing the fluid lost when pulling the pan replenishes the additives just fine. Or so has been found to work pretty well for the last 70 years or so. Flushing is a newish thing. Some shops even refer to a transmission flush procedure as a wallet flush.
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Old 06-05-2023, 10:43 AM   #6
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Originally Posted by LETMGROW View Post
I would do both. First would be to remove the oil pan and clean any debris out of the pan, replace the filter(s) and add clean fluid back to capacity then flush the entire system. Flushing first can loosen anything in the pan and force it through the entire transmission and oil cooler which may leave some behind.
Just dropping the pan isn't going to flush the torque converter which is where a large amount of fluid will be trapped
X2 on the above method. When flushing first came about they were pumping fluid into the trans and stirring things up. I believe the way they do it now is to allow the transmission to do the pumping by hooking to the in and out lines and letting the transmission push it out while drawing in new fluid.
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Old 06-05-2023, 10:54 AM   #7
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I pull a Cedar Creek fifth wheel and the manual says change the transmission fluid at a certain number of miles. I have a 2021 GMC 3500 diesel and the transmission doesnít have a dipstick so you can check the fluid level. I let the dealership change the fluid and they didnít put enough fluid in the transmission and the transmission overheated going home. I think when the transmission overheated it hurt the fluid, after about 3 more trips to the dealership they finally changed the transmission fluid and I havenít had any issues since. Anyway that dealership will not see my truck again, I have since found a better dealership closer to my home.
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