Minutes after driving this car off the GM dealer lot the transmission had a delayed shift that became a hard shift when it finally picked up! But it only happened 3 -4 times a week, you could never tell when it would occur.

Dealer said they found "nothing". I know now that's not true. Eventually the transmission computer stores a fault code that does NOT turn on the engine trouble lite! In my case the computer stored a " P1811 " code, meaning "low ratio, delayed shift". When a shift is made, but delayed, the computer ups the oil pressure thinking the transmission is slipping, so when it does engage it's with a thump or a big bang. This easily starts to break parts in the transmission.

The next dealer I went to did a warranty service replacing lots of parts but not the parts that cause the problem! The shift problem got worse after just one week! I gambled that the original dealer never changed the tranny oil filter, the filter is small and is designed to plug up! The dealer that did the warranty work never replaced the filter either! So on the " DL " I replaced it myself. The tranny shifts improved by 80%, but that' still not good enough.

When the warranty work was done, gears and clutch packs in the tranny were replaced, these are damaged parts but not the "root-cause". Sonnax.com and other manufacturers of parts for the rebuild industry found that the GM 4T65E transmission in this GM car and many others, has weak parts in the Valve Body that causes bad shifts, namely shift valves made from soft aluminum that gall in the valve bores and stick during a shift causing a delay. The second Dealer did not replace any valve body parts since new GM parts are the root cause - they are 'design-defective'.

The tranny oil filter cannot remove wear metals that are shed from gears and bearings so these circulate thru the valve body freely scoring the shift valves and binding the valve bores making shifts worse over time. GM has a single cheap ceramic ring magnet in the oil pan to pick up "some" but not all the wear metals in the oil.

My DIY fix so far for the last 3 years is:

1 - Install 6 neodynium bar magnets (2" x 3/16" sq) inside the oil pan near the GM magnet. 2 - Install 4 neodynium bar magnets on the oil filter next to the filter inlet. 3 - Install a Trans-Go shift kit parts in the 1st and 2nd shift accummulators 4 - Add one full can of SeaFoam "Trans Tune" to the tranny oil 5 - Disconnect the car battery for 1 hour to clear the TCM codes and shift pressure table.

It's worth noting the wear metal bits are super fine, not "shavings" like most folks think. Typical size is .1 micron to 15 microns, this means they can go anywhere in the transmission. GM knows their tranny filter and even after-market filter cannot stop this process. The filter at best picks up only non-metallic wear bits from the clutch surface material.

The steps above seems like a lot of work, but the cost of a proper rebuild with Sonnax parts would be $2,000 or more.

I also learned a lot from a GM dealer fraud website called: WilsonNiblett-Ripoff.com

What can you do during your next car purchase?

1 - Google " (car model) problems" - and look at forums that discuss issues

2 - Ask the dealer what the model number of the tranny is, not just a "4 speed" but " 4T65E " or " 6T70 " for newer GM 6 speed transmissions, etc..

3 - Google the tranny model and see what pops up, ie " 4T65E problems "

4 - Then you can make a more informed purchase!