Why does my 2016 Mustang GT Manual jolt/thud when I shift?

Question

I just learned how to drive manual in my 2016 Mustang GT. It has 3300 miles on it. It makes a "thud" noise when I put the stick into low gears (1,2,3) sometimes. This noise makes the car jolt, such that even passengers can feel it (similar to going over a concrete road joint).

To reproduce, I do this (for example):

1. Be driving in 1st at 2500 RPM with the accelerator moderately down
2. Release the accelerator completely and press the clutch to the floor
3. Wait a second for the tach to drop to ~1900 (the Mustang holds it at the right RPM for the next gear up for two seconds)
4. With the clutch still to the floor and the accelerator still completely released, pull the stick down into second gear
5. Ease off the clutch and begin applying gas through the catch point

While doing #4, the car jolts and makes a thud noise--sometimes. It seems to do this at lower RPMs more often than higher ones. I can definitely feel it through the seat; it's not just a noise I hear.

It does not feel the same as the thud/clank I would get when I was first learning and was applying the accelerator unevenly and the car would jolt forward (which would scare me and make me let off the gas) and then would switch from being driven by the engine to driving the engine by inertia (the cross over point had slack in it, and the thud was caused by hitting the end of this slack suddenly). It could be drive train slack, but I'm not going back and forth between engine breaking and accelerating like I used to.

It is possible to reproduce this from a complete standstill: just shift into first from neutral with the clutch hard to the floor. Thud! Jolt! Then, hold the clutch down and row through the gears while stopped: 1st, 2nd, 3rd, 4th, 5th, 6th. All smooth. Back to 1st. Smooth. Drive, come to a stop, neutral, clutch, first: jolt! thud!

Why is this happening?

Answer 1

Check the transmission mount, which is at the rear of the transmission (where the drive line yoke attaches to output shaft). Often times if you've gotten on the accelerator a little bit and do some hard shifting you can easily break the mount (or more likely after several times of doing this it will start splitting then finally give up and break). This is a common failure point due to torque output of the engine. It happens more often on manual shift transmission than on automatics, but can happen on either.

If you jack up the car and put it on some jack stands, then get under the car, you can probably see if this is the issue. It should be fairly self evident. You may even be able to push up on the back of the transmission and see if it's separated. Replacing it is usually an easy task, as it's held on by either three or four nuts/bolts.

Answer 2

I test drove 2 new GTs yesterday and got the same issue. While shifting from lower to higher gears (1 > 2 > 3), definitely heard the thud like sound. It seemed to come right from the gear box. If it is occurring in most new cars, it might be 'natural' or a 'flaw' they overlooked. It might be as flimsy as a plastic that close to the transmission or something more serious -- don't know if I should buy this car though...I like the sound and pull of the engine, but this sound has planted doubt in my mind.

Answer 3

If it is the IRS sub-frame & differential together making this sound, Steeda sells a sub frame brace that just bolts up to the bold and rear sub-frame under the rear of the car. Go on American Muscle or CJpony and you will see there videos on how it works and the install for this. Most are very happy with this add on because is cuts back on wheel hop and rear flex in the corners and tightens us the rear suspension as well. I am putting this on my 2016 v6 mustang in the very near future.

Answer 4

It is a well known problem caused by the softly mounted IRS sub-frame and the differential.