In my 2005 Chevrolet Malibu, the RPM drops down to around idle whenever I let off the accelerator pedal. It feels as if it were a manual shift car and somebody pushed the clutch pedal in every time I let off the accelerator, but to my understanding, that should not happen with an automatic transmission. Normally, if I let off the accelerator, the RPM slowly comes back down as the car slows down. Now, even if I'm going 70 mph, if I let off the accelerator, it goes to idle RPM, and if I just slightly push on the accelerator the RPM will jump straight back to ~3000 as if somebody just dumped the clutch.
What is causing these RPM drops?
Potentially related background:
My 2005 Chevrolet Malibu recently threw an engine problem code P0753, which indicates an issue with shift solenoid A. This checks out because it does make a "clunk" sound when changing gears. I intend to replace the shift solenoid. I know some cars have a sort of "limp mode" that gets activated when the engine light comes on. Is it possible that this RPM drop is a sort of "limp mode" that activates when there's a transmission problem code? Will it just go away once I replace the shift solenoid?
I also did some welding in the vicinity of the transmission, so I'm also concerned I might have fried the TCU in the process.
Also of note is that transmission fluid seems to keep disappearing into this transmission. I got it checked out by a shop and they said there's no leaks. But I still have to top up the transmission fluid about every month, and I have no clue where it goes. I guess there's just a black hole in there. Not sure if that's related at all.
I searched and came across this post, but my problem is different in that I have not noticed an abrupt loss of speed. The speed remains the same, it is only the RPM that drops. I also found this post, but my issue is different in that I experience an RPM drop after letting off the accelerator, not while I am still accelerating.
I did find one useful piece of information through my online research, and that is that there are lockup solenoids that control the torque converter. Since the torque converter would seem to be the piece that is malfunctioning in my case, I wonder if one of the lockup solenoids could be bad. What do you guys think?