Originally Posted by DejaVue
Originally Posted by supton
Sonnax's story about new fluid getting just as dirty as old is interesting. Sounds like, on a really neglected transmission, multiple flushes might be needed to truly clean it up. [Of course, just how clean does it need to be? different question altogether.]
It kind of looks like DoubleWasp and some others are arguing with each other about different things.
Or perhaps just partially irrelevant things.
Or just imo a main point is being lost in the process.
DoubleWasp says D+F is no better (and probably worse?) than a full flush.
He quoted Greg from Sonnax as evidence.
Greg from Sonnax said on a 100,000 mile vehicle with the original trans fluid, replacing that fluid with 90% new fluid caused particulates to increase soon after.
Greg from Sonnax recommends on a neglected vehicle to change the trans fluid by any method multiple times.
Originally Posted by DoubleWasp
D&F is only better in people's beliefs. Sonnax studied this extensively and found that even with a D&F, contaminant levels rise hilariously above even pre-change levels shortly after doing a D&F.
Doing a D&F over a flush is good for the spirit, but nothing else.
Cliff's Notes: D+F is not doing anything differently
If it exists, I missed seeing direct evidence showing whether a D+F or full flush increases more particulates than the other.
I would expect particulates to rise on a neglected transmission soon after doing a D+F.
Logic tells me replacing 30% of the fluid will free up fewer particulates and at a slower rate than replacing 100% of the fluid would.
But that's the partially irrelevant part.
Because if the person is trying to catch up on maintenance for their neglected transmission, then both D+F and a full flush should be repeated again fairly soon anyway, since both methods are increasing particulates.
In general, I'd say what Greg from Sonnax said actually favors the typical D+F method since people who do D+F on a neglected transmission tend to do it multiple times in a short timeframe. (Arguably too short if done again in less than 100 miles.) Most people who do a full flush apparently tend do it once and call it good since they replaced all of the old fluid with new fluid.
So, if done multiple times, D+F is probably better than a full flush done once. A full flush done multiple times is probably better than D+F done multiple times. The usual argument can be made that D+F causes less particulates shock to the transmission since 30% new fluid logically doesn't clean as fast as 100% new fluid. But, in either case, the main thing is that particulates freed up by a D+F or by a full flush need to be drained out again anyway in the not too distant future on a neglected transmission. I just thought that point was being lost.
Only point ever was that D+F is not in any way "safer".
I think some people took that to mean that I was lighting my Tiki torches and painting "Heck No D+F!!!!" protest signs in goat's blood.
I do D+F myself. Some of my stuff is just a pain to do anything more than that. I'm just not sitting here telling myself I've taken a "safer" route.
No doubt a neglected trans needs multiple flushes regardless. When possible, I'd rather dump all fluid and dump all contamination than take a soft approach. I'd even prefer a filter to keep the bulk of those contaminants from ever staying in circulation. But that's just me.
