3.8 2002 Impala with 170K runs great, once it is warmed
 

We are the only owners of a 2002 Impala with a 3.8 engine. This care runs great once it is warmed up. This car burns on oil and we have been very religious about changing the oil and filter every 3k. a few years ago we got a check engine light and the dealer could not fix it. They thru every part available at it which I made them remove since it did not help. the problem is that the car stumbles very bad or just dies during cool weather or hot weather. I have changed the MAF sensor with one from the local parts store only to make thinks worse so i put the original back on after cleaning it with the correct cleaner. This is a good car other than keeping it running until it is warmed up.

It's a 16 year old car, so things are bound to happen due to age and mileage. First thing I'd do would put a can of Chevron Techron or Seafoam in the gas tank and take it out for a good high-speed run. It could be many things, like fowled fuel-injectors, gas-pump going bad (they usually just stop pumping one day, however). Might even be the exhaust system catalytic converter going bad, if it's burning oil, which can clog it up with carbon residue.

Did the dealer give you the OBD II codes, when they tried to repair the vehicle?

oilcanhenry makes a good point, any available codes would help figure this out. I wouldn't want to just throw parts at it either, but when I had my 2001 Impala, I remember having to change the spark plugs and the wire set when it was running poorly, also if I remember correctly I had one intermittent coil pack that made it run really bad.

How is the fuel pressure on the fuel rail? If it is not ok, could be a bad fuel pressure regulator or fuel pump. Those regulators are easy enough to change. When was the fuel filter replaced?

Looking at data with a OBD2 scanner would help too.