It sounds lie you need to just drive a little faster for this test. The best suspect is the transmission's torque converter clutch. The clue is the 40 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're 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 can't 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 35 and 45 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, as I mentioned, 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'll 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.
If my suspicion is correct, the solution is to have the normal maintenance transmission fluid and filter change, but be sure the correct fluid is used.
Thursday, November 2nd, 2017 AT 8:14 PM
