2008 aveo transmission flush
 

I have a 2008 aveo with 116000 miles on it. its never had a transmission flush and was thinking about getting one now. Ive heard its a bad thing to do know this late and that i should just leave the old fluid in there. Any opinions on weither i should get a transmission flush or not?

Hard downshift after transmission fluid change
 

I don’t know anything about transmissions but wanted to share my story in case it helps anyone else. I bought a used 2012 Chevy Sonic in 2017 (68K miles). I drove it for four years with no problems. In 2021, at 104K miles, I took it to a repair shop (not a dealer) to have some routine maintenance done, one of which was to change the transmission fluid. After that, the car started hard downshifting from third gear. It was intermittent but seemed worse when it was cold out. Since it was intermittent, I didn’t really worry too much about it, but it got worse, so at 108K miles (also in 2021) I took it to a dealer to have them look at it.

They drained and replaced the fluid 2X stating that the previous fluid was very dark and smelled burned. They even stated that they could not tell if it had been previously serviced it was so bad. However, changing the fluid did not fix the problem of the hard downshifting and stated that there really wasn’t much they could do other than a transmission fix. At that point, I figured I’d just keep driving it until it no longer runs if I was going to have to pay for a transmission overhaul.

I drove it until October 2025 (126K miles) with the problem getting progressively worse to where it was not intermittent and never went away. If you got up over 30 mph at any point, then at around 27 mph, “Bam!” hard downshift. I started to read up on the problem and decided to try a few things before deciding on a transmission overhaul. First thing, I added Seafoam Trans Tune. The problem was immediately better, still there but better. I thought perhaps because it was so immediate maybe the fluid was just low, so I added a quart of Valvoline Max Life. No real immediate change but the problem did seem to be getting better. On the third day, I added Lucas Trans Fix and the problem got even better. It took about 5 minutes of driving around now and the problem went away. However, until the car warmed up, “Bam!” hard downshift.

I drove with these additives for 100 miles. Then I added Lube Gard Transmission Flush, idled the engine for 20 minutes, twice shifting through all the gears 3X each time. I drained the fluid which was very dark red and thick. It did not smell burnt but it did not smell like pure gear oil. It had a very off-putting chemical smell.

I then added Valvoline 100% Dexron VI transmission fluid (blue label) not Valvoline MaxLife (red label). It was striking just how thin this fluid was, like water, splashing and not acting like oil at all. This seemed much thinner than the Valvoline MaxLife I had added previously. Not all Dexron VI fluids have the same viscosity even though they are all called Dexron VI equivalents. I’ve heard that different fluids will thin or shear over time at different rates, but drained transmission fluid should be as thin or thinner than what was there to start with. In my case, the fluid drained from my car was thicker than what I was now putting in (Valvoline Dexron VI blue label). After adding the Valvoline blue label Dexron VI, my car runs like new. No hard downshifting and absolutely no problems.

I am not sure whether the first shop even serviced the transmission but immediately afterward there was a hard downshift. I think it helped that the dealership changed out the old burnt fluid, but the problem remained. I did call the local Chevy dealership to see what fluid they put in the car in 2021. They said they used AC Delco Dexron VI, of which there are two versions, one for older transmissions and one for newer (neither of these is 100% synthetic). They did not know which they used but they did say they have had some difficulty finding out which fluids work best in particular transmissions.

It is somewhat confusing that both AC Delco and Valvoline have two versions of Dexron VI and that all four versions seem to be quite different in viscosity. Long story short, I’m not sure what fixed the problem, Seafoam Trans Tune, Lucas Trans Fix, or Valvoline (blue label) Dexron VI but I am grateful to all three because for about $90, I was able to fix a problem that I had been told would cost thousands.