Background:
The vehicle in question is a stock 2009 Chevrolet Impala with the 3.5L engine and 4T65E transaxle. It has around 125,000 miles on it. There is no check engine light, and I haven’t been able to pull any trouble codes with a basic scan tool. The transmission fluid was changed by the dealer about a year and a half ago at 115,000 miles. They did not change the transmission filter or pull the pan. The fluid looked to be in okay condition before it was changed. The transmission fluid, as it should, looks new and doesn’t seem to be frothy or otherwise full of bubbles. It looks to me like it might be slightly overfilled. The problem has been occurring for at least a year; I’m not sure if it coincides with the fluid change.
Complaint:
Occasionally, when I come to a complete stop, then go to take off again, the engine will rev up to 2000ish rpm but the car moves forward with very little power as if the transmission is slipping, but then it will lurch forward, sometimes quite harshly depending on how much gas you’re giving it, then resume normal operation. It’s as if it was taking off from a stop in a high gear then downshifting suddenly to first, but I don’t know if that’s what really is going on. This happens maybe once or twice per hour of driving. Sometimes more, sometimes less. It seems like it tends to happen when the engine/transmission are hotter, but that’s not the case 100% of the time.
Aside from this issue, the engine runs well and the transmission upshifts/downshifts fine and the torque converter seems to lock up and hold fine. When it’s not having trouble, it will launch off the line with plenty of power if you want it to; the transmission does not feel weak.
I’ve read a bit about some problematic EPC solenoids in these transmissions, but I don’t have any way to do further diagnostics on this transmission.
Question:
With that all in mind, does this issue sound familiar? Ultimately, I’ll likely end up bringing it to a transmission shop, but I’d like to be as informed as possible before doing so. My experience with other vehicles is that any transmission shop just wants to do a full $3000 rebuild, but I refuse to believe that is necessary here. It seems like an electrical/sensor/computer issue or a pressure control problem. Unfortunately, it seems that most of the relevant sensors and solenoids are not in the pan and are difficult to access. The intermittent nature of the issue will prove challenging for most transmission shops as well.