Automatic Tranny flush, HD gmc 6 liter.

[SuperDave]

Ok, So my 99 HD, 100k miles, is delaying (5-10seconds or so) shifting into overdrive on the highway and sometimes even shifting into 4th. I thought I might get a flush done and put in some synthetic tranny oil.
Any of you guys had anything good come from flushing? One guy told me to just drain it and add two qts to run through and then fill it back up, said flushing was a waste of money.

Let me know what you guys think.

Thanks,

 

[Mahoney Pursley Ranch]

Flushing is not a waste of money as it removes ALL the fluid from the trans and torque converter. That being said I would not advise it at 100,000 miles nor would I advise going synthetic at that many miles. In any event with the miles on your vehicle the fluid may be PART of your problem but that alone is NOT going to cure it.

 

[Stocker Steve]

Flushing is a rip off at the garage. Draining and replacing the tranny filter is part of good maintenance. Git'r'done.

If it still shifts slow with the new fluid (and I expect it will) - - tranny mechanic in a bottle does work sometimes. Most of them swell the O rings and increases the fluid viscosity. I had a Chev that needed some (tranny mechanic in a bottle) at 180K, and that tranny is still going at 229K.

 

[dieselbeef]

flushin at this point is probly futile and may cause more troubles than it solves. have someone ya trust look after it quick before its too late and change the fluid and filter. probly the converter. although it could be somethin simple like a seleniod stickin.

 

[novaman]

I would never flush a tranny. Know a few people that did and had the tranny go out shortly after. A mechanic friend of mine told me that by flushing you stir up any dirt or gunk from the oil pan and run it through the tranny. Obviously that isn't a good thing. I would just drop the pan and keep all the crap from being run through.

 

[Mahoney Pursley Ranch]

I would never flush a tranny. Know a few people that did and had the tranny go out shortly after. A mechanic friend of mine told me that by flushing you stir up any dirt or gunk from the oil pan and run it through the tranny. Obviously that isn't a good thing. I would just drop the pan and keep all the crap from being run through.

The proper way to flush a trans is to drop the pan,clean the pan and replace the filter/screen first then do the flush to remove any remaing fluid from the entire system including trans cooler(if vehicle has one) and the torque converter.This is the procedure that was used at the last dealership I worked for. Done this way there is no chance of stirring up any gunk from the rest of the system. Again I am not suggesting this is a cure for trans problems but for good preventive maintenance. Going with synthetic trans fluid or engine oil on a high mileage vehicle that has not used synthetic fluid almost from new causes leaks as synthetic fluids will find wear passages that dino oils will not.

 

[Earl Thigpen]

Flushing the transmission IS a good idea. Here's why.

1) The transmission pan contains only approximately 20% of the fluid contained in the whole transmission. The remaining 80% is captured in the torque convertor, oil cooler, lines, valve body etc. Flushing exchanges ALL the fluid, not just what is contained in the pan. When you either drain the pan (if there is a drain plug) or drop the pan you still can't get to the remainder of the fluid.

2) Particles of the friction discs that are eaten away over the years gets trapped in the valve body passages and more importantly, they get trapped in the vanes of the torque convertor. Since you can't drain or exchange the fluid in the convertor you never clean the trapped particles out of it.

3) Something happens to the transmission fluid over time. Not a chemist so I don't know what goes on there but it gets really nasty. It's carrying parts and pieces of the clutches and bands that have worn so maybe that's why the stuff gets nasty. Anyway, this goes back to #1 - you can't get to the 80% that really needs to be changed.

Hope this helps. Little long winded - sorry.