Shaking low RPM

I dealt with this same problem recently. The check engine was on, but there was no indication of an engine problem such as a misfire. Upon further investigation, I found the torque convert clutch (TCC) was not functioning properly causing the vehicle to shake. Basically, it felt like an engine misfire problem. It only did it around the same speed under the same conditions you mentioned. The customer had to replace the aforementioned item and the problem was solved. If you have a scanner that can read transmission codes, check that first.

The best suspect is the transmission's torque converter clutch. The clue is the forty mph.

The torque converter is the fluid coupling between the engine and the transmission that allows the engine to remain running when the vehicle is standing still, in gear. It starts to become efficient at around 1000 rpm, but that slippage always exists while you are driving. The lock-up clutch eliminates that slippage to achieve better fuel mileage, similar to what a manual transmission's clutch would do.

That lock-up clutch cannot be allowed to engage at lower speeds, and when it does engage, it has to do so gradually to avoid causing a harsh clunk. Most lock-up clutches begin to engage gradually between thirty five and forty five mph. It is cycled on and off very rapidly to modulate its engagement, but there are special additives in the transmission fluid to allow it to slip at first. The most common cause of this shudder is using the wrong transmission fluid. Without the needed additives, the clutch engages too hard, then engine torque causes it to break free, then it locks up again, very rapidly.

To prove if this is the cause, when the shudder occurs, hold the accelerator pedal perfectly steady, then lightly tap the brake pedal with your left foot. That will cause the lock-up clutch to disengage for a few seconds, (in preparation for coming to a stop), then the shudder will occur again when it engages. If you have a tachometer on the dash, if you tap the brake pedal at higher speeds, you will see engine speed increase about 200 rpm. That is due to the lock-up clutch releasing. It also releases at wide-open-throttle, (for more torque), and it will not lock up until the engine is warmed up. That is to insure the transmission fluid is warm enough to flow easily to the cooler.

Yes. If the shudder stops, have a standard fluid and filter change performed, and be sure they use the correct fluid listed for your vehicle.

No need to replace the filter again so soon. To insure you get the right fluid, you might want to buy it from the dealer's parts department.

You will need to remove the transmission to swap out the torque converter. Here is a guide that give you an idea of what you are in for the service and the removal:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

and

https://www.2carpros.com/articles/transmission-replacement-removal-manual

Please let us know what happens so it will help others.

Cheers, Ken

Keep us updated. I am interested to see if is the same problem I dealt with. If so, we will most likely see more of the same and it will be helpful to others.