RPM drops quickly after letting off accelerator

Question

The problem:

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.

The question:

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.

My research:

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?

Answer 1

The following is not an answer but information related to 'limp home mode', specifically addressing EFI using drive-by-wire systems; throttle plate driven by servo motor commanded by the ecm as directed by electronic pedal. GM uses a pair of feedback sensors in pedal and a pair in the throttle body. If one sensor outputs incorrect signals, the ecm detects this as out of range of normal operation and immediately invokes reduced power mode; the ecm has a built in program to reduce engine power to around 15%, enough to drive off to a safe place to make the emergency call instead of dying out in the middle of nowhere. Reduced power mode ensures against a runaway engine since drive-by-wire systems are 100% electronic - no steel cable connecting pedal to throttle plate. In my opinion, your descriptions doesn't fit reduced power mode, commonly described as limp home mode.

Answer 2

Solution

I got the issue fixed, and it had nothing to do with the transmission. My best guess is that the issue was an over-restrictive exhaust. Blowing up the muffler ended up fixing the issue. Read further for more details on that.

Troubleshooting

I replaced all the transmission solenoids except for the torque converter clutch solenoid (it was too hard to get to), and while that fixed the shifting issue, it did not fix the RPM drop issue.

I also had a new issue creep up on me, which was a bouncy RPM during idle. It would start at about 1000 RPM, and slowly drop to about 400, at which point the engine would start shaking, and then it would shoot the RPM back up to 1000 and repeat the cycle.

As a completely unrelated issue, the engine has always made a quiet ticking sound when running. I've been told it's nothing to worry about, but I figured putting some SeaFoam engine cleaner in the intake can't hurt. So I did that, and the directions said to apply "spirited acceleration" after using the product. So I got on the freeway and hit the gas, only to hear a loud "bang" sound. I looked back and saw my rear bumper hanging off the car. When I pulled over, it turned out my muffler had exploded and had taken my rear bumper cover with it. I've included photos below:

Ever since that muffler exploded, The RPMs have been perfectly smooth. No drop off when letting off the accelerator, and no bouncy RPMs at idle. I zip-tied the bumper back on and it's good as new!

Theory

My theory is that my muffler was overly restrictive, which was causing too much back pressure and therefore causing the RPM problems. When I floored the accelerator, it finally put enough exhaust pressure on the restriction to explode it.

Conclusion

I hope this helps someone else who finds themselves with RPM drops after letting off the accelerator and/or a bouncy idle. If you're reading this, maybe you should just replace or cut out the faulty exhaust component before it blows up like mine did.