I have a 2000 SE, bought the car cheap, the transmission was bad (surprise!) Got a tranny for a boneyard with 68000 miles, changed all fluids, installed in car. Now, when starting right off, tranny shifts fine. Shortly after before warm, it feels like a delay, hesitant shift. it does not slip at all, even if i get on it inbetween gears. After I hit highway speeds, the transmission then shifts harshly or bangs gear while shifting. Any ideas or just a bad tranny?

Also, small coolant type smell inside vehicle after car warms up ,not specifically with heat on. Check for signs of heater core leaking and none found. Thanks!

2000 Bonnie 148k

Might want to change the filter on the trans, might be getting clogged.

Check the engine for any signs of coolant seeping out. Check the coolant elbows by the water pump, the O rings some times wear out and leak.

Sounds exactly like mine. Lazy shifts with shudder, then bangs each shift when hot. I'm 4 days into the shift kit fix, but so far so good.

What is involved in the shift kit install?

It'* abotu a 2 hour job if you have the tools and some general knowledge. https://www.gmforum.com/t278802/

Scan for codes. If you have a P1811, you need a new pressure control solinoid.

SGP, I have the exact same issues with my tranny, and it is not throwing codes.

Try hooking a pressure gauge to the line pressure port to see if the pressure is within spec. I've seen a bad solinoid make a massive fluctuation in a pressure gauge.
A P1811 is just a dead giveaway of a bad pressure control solinoid, which is a very common problem.

I used the above reference for the install. It was nice to quickly remove the 4 bolts, and not guess which were which. Thanks!

BTW, I wasn't getting any codes on mine. 400 miles and not one hard shift. I'm liking this car, again.

Do you know the gear ratios to be the same? There were two ratios available in that year. Wrong ratio could upset the shifting etc. Especially when it sees something other than 1:1 in 3rd gear.