It is at 160K, and its never had an ATF flush. I checked the fluid today and it is very dark. So my intention is simply to do a drain & fill, and not try to stir up any gunk that might be lodged in the galleries where the ATF flows. I don't know if what you suggested might stir stuff up, but my plan is to do this first, and then see how the trans reacts. Perhaps this is unnecessary caution, but I'm trying to avoid a flush right now. I've heard that flushes on an unmaintained transmission can trigger issues. After searching threads here on that issue, I know some here think this is an old wives' tale, but others warn against it, so I'm going to err on the side of being too careful.
Originally Posted by JMJNet
On that model Sienna (same what I owned before), you can also do a cooler line flush using the return line output radiator flow after the d&f.
Use about 12 qts of ATF and you will be good for a long time.
This procedure is available from Toyota forums like Toyotanation, Siennachat.
In addition, you can splice in a Magnefine filter, or something like it, into the ATF return line.
Then the ATF in there will outlast the van (not litterally).
With an extra quart of ATF, you can also flush the PS fluid, if you have not done so.