I have a 2000 impala 3800 engine...... Last week while it was raining I started having the following symptoms:

-very high idle. stayed at 30% or so steady throttle. The engine was racing.
-rough or failed starts cold starts unless gas was applied. After a few seconds the engine would start racing again.
-high RPM shifting.

Did some research and consulted with a mechanic friend and he suggested the TPS sensor. Replaced this $30 and 10 mins but sadly no immediate change. Same condition continued until about 2 days later (dry weather) and then it started running and shifting better than it ever had the whole time i have owned the vehicle.

It rained again today and the condition returned.
-revving but not as high
-cold start problem is gone.
-late or high rpm shifting. To get a shift to happen I have to run up the RPMs and then feather the gas back off and then the shift will happen. Same for all gears.

Any thoughts? I am broke and need to try and fix myself but don't want to throw anymore parts guessing at the problem.

 

Get the car scanned/.