Originally Posted by aaalbedo

Sorry to ghost y'all for a month, especially after all that excellent information and advice. This is a fourth vehicle for me and I'm waffling a lot; hate to see another Estoril 4.6is parted out, but choking on the $9K quote to install a rebuilt (and also doubting that the shop can actually find one). There were two silver 4.6is's posted for sale here in Denver in the last month for $2,000 and $1,200 but I missed them both.
Anyway, here is more information.

Transmission was serviced (flush and filter) by PO at 140K miles, now at 170K. I bought it at 160K and asked a local indie if I should service again, and they said no, they have never seen a 4.6 transmission last to 160.

At about 165 I noticed symptoms; after braking it seemed to slip out of gear as there was a very mild lurch as I opened the throttle; most noticeable after a sharp right turn. It also seemed to fight me when braking; it was still trying to pull as I came to a stop. Took it to a different, less specialty shop (tech claims to be a 540 fanatic). They said it was overfilled (which is a red flag since that's nearly impossible) and the fluid smells burnt, needs rebuild and estimated $6K.

About a month later I was on the highway at ~70 mph and the trifecta lit, but it did not seem to go into failsafe, shifting normally for the rest of the 4 miles home. Only short drives for the next couple of months with no change, and eventually the trifecta went away. Then I drove it ~20 miles to work, and trifecta returned at the very last stoplight. Still shifted normally for the drive home with trifecta on.

I bought a reader based on forum advice (I've used OBD fusion heavily for my 330, and have INPA but no german). But the reader results seem pretty vague; four drivetrain issues and here is the entirety of that report:

1.EGS: Symptom, Gear Monitoring
2.EGS: Speed Signal N-Ou (Output)
3.EGS: Converter Lockup Clutch, Excessive Slip
4.EGS: Batt. + Supply, Term. 87

So I think I need to fire up INPA and have a better look; hope to do that this weekend. Also note that there are 3 safety and 17 body issues reported by the reader (including an apparently dead left rear wheel speed sensor), and I may have some electrical issues as well.

Truck is super clean inside, has some fairly major paint issues on the front end. Hoping to keep it alive! Thanks for all the input so far. Stay tuned.

Larry