Cody,

I finally had a chance to read things a little further. Without more information, I'm not sure what is driving you to replace both the engine and the transmission. It's very possible the engine is fine.

First question.... Can you have the car scanned for any engine and transmission codes being thrown? I'm looking for the code numbers, not their definitions. As long as the engine issues are not internal, they should be easy to resolve.

Now, the trans giving you issues. Unfortunately, the 4t65e in these cars has a bad rep for problems. When we bought my wife's '05 Impala with a 3400 and 150K on it, we new the trans was shot. I don't do trans work, I got a salvage trans with about 70K on it and had a shop put it in, out the door my total cost was $1200.

The sad part is, my guess is the fluid in the trans was never changed. At the point your trans begins having problems, changing the fluid becomes a gamble of fixing the prob or taking the prob from bad to worse. If you decide the change the fluid, you MUST do a pan drop and filter change (but again, keep in mind, you are running the risk I just mentioned).
I change trans fluid and filter every 100K miles or sooner.

I know many members have noted intermittent issues with these transmissions feeling like they "slip" into 1st gear from a dead stop (both my Monte and wife's Impala do it, but these days the Monte next to never does it, I think it's due to some mods and changes made to the PCM for the transmission).
The wife's Impala has an issue of intermittently banging into gears as it shifts. I've read that those issues can sometimes be adjusted by changing trans settings in the PCM (something I am researching more). But, even with tuning adjustments, it's not a fix, it's a bandaid on a bigger problem.

I hope this helps a little and hopefully if you can get us more info, hopefully we can steer you into the right direction.

 

I am also having problem with my 2000 Chevy monte carlo ss using way too much gas. I drove from AR to SC. Usually I spend 100.00 on gas that's two tank fulls. I filled up 3 and 1/2 times. that is like 864 miles. It cost 20.00 bucks to drive from Atlanta Ga to Anderson SC..can anyone give me some advice please?