so try this first? will this make the problem worse?

 

Worse? You already planned to have your transmission replaced!
Don't jump to the worst case scenario, just clean it first and see what happens,(add a can to the transmission and drive a few hundred miles) if shifting improves, flush it!
After many years in the auto repair industry, 9/10 clean&flush corrects transmission performance issues.
Most automatic transmission shifting problems are caused by sticky varnish residues that form in old fluid. Sea Foam’s Trans Tune works fast to reliquefy varnish deposits from valve bodies, solenoids and shift actuators. Before you spend $$$$ on a new transmission, try adding a can of Trans Tune to your fluid filler – super easy! Also works great as a fluid cleaner and conditioner, or as a pre-flush cleaner.

 

Sea Form is good, however its not a cure all. And if it is slipping now, the transmission will not last a few hundred mile test drive.

 

it will not repair burnt clutch packs. or jammed valve ball bearings in the valve body plate.

 

It will not repair a transmission leak either.
An honest mechanic will diagnose(check fluid level/condition/drop pan/fluid exchange, etc)..A transmission shop mechanic will do a symptom diagnosis to sell transmission replacement.
It's easy to type: "REPLACE THE TRANSMISSION", on a forum, since it cost nothing to type that.
Engine has an issue, "REPLACE THE ENGINE".
Check engine light is on, "REPLACE THE PCM".
I can type that too since it costs me nothing, however if it was my ride or my customer's ride, I would DIAGNOSE first.

 

First I would NEVER, NEVER, never.............. did I say NEVER..... take an out of warranty vehicle to a dealership. In Websters when you look up the definition of "stupid" it says, person who takes an out of warranty vehicle to a dealership for repair.

Try a clean, drain filter replacement per hummerz' suggestion first.

If it still persists, then find a reputable local transmission shop to take it to for a rebuild. Add in any aftermarket HD performance parts to the rebuild (like Monster sun shell etc...). You can build a 4L60E trans to take 1000+ HP in track cars, you can easily build a bullet proof H2 trans as well.

I have never replaced any trans in 40+ years. My 50 year old Olds 442 has its OEM TH400 trans. They only need replacement if damaged, like collision, trail damage, fire...

 

Thanks for the tip. Gonna add a bottle drive it around for a bit then get it flushed at the dealership.

 

Post results first, after you add a bottle and put some miles on it, and shifting performance improves.

 

New here...but been down road of rebuilding transmissions plenty of times. 1. NEVER...let me say that again just to be clear - NEVER do a transmission flush on a high mileage transmission...NEVER...ok last one for good measure. 2. Correct course of action, drop the pan, change the filter. Replace the pan. Add (5) quarts of Dexron VI. Drive. If it still slipping, then you talk about a rebuild or new transmission. If it gets to that conversation, you will need to ask yourself this...1. How long do you intend to keep the vehicle? I am a keeper...so with that said, a rebuild would be out of the question...unless the re-builder is replacing all the weak points in the 4L165E. There a ton of after market Transmission companys that provide updated bullet proof 4L165Es....expensive yes, but they will with stand superchargers and nitro.

Cheers!

 

I highly doubt that flushing atf would cause imminent transmission failure. If your transmission were to fail after a transmission flush or just a transmission service, the transmission would have failed regardless. There is no scientific, logical, or mechanical justification that I’ve ever been shown, seen, or read that proves otherwise. Changing the fluid either completely or partially will have no negative affect on it, period. I do know, atf flush/exchange with new manufacturer recommended fluid, does correct transmission shifting quality & performance. Over the years, thousands of atf flush/exchange services were performed under my watch and never had a transmission fail.