Disclaimer:
These are my suggestions for things to do. Attempt these modifications at your own risk. I accept no responsibility for damage to your figure.
His backpack is rather annoying. It keeps popping off. You try and push that grey lock into place and all you can achieve is this:

when what you need is this:

(notice the lack of gaping hole under the grey piece)
The problem is twofold but easy to fix if you don't mind slicing and dremeling.
First Cause:

The rear of the car (where the thrusters are) is pushing down on the pegs that the forearms should clip onto in car mode, between the arms and feet in car mode.
Solution:

Using a modeling scalpel trim away part of the angled edge. Take care not to damage the tab that slots into the arm hole (to the right of the green circle, above).
If this hasn't improved the staying power of the backpack then continue:
Second Cause:

The slot (1) is at the wrong angle for the pegs (2).
Solution:

Using a dremel enlarge the channel in the outside corners on the edge nearest the head, furthest from the tail fins. The adjustments here need to be small because cutting out too much will reduce the friction needed to hold the grey piece in place. The problem with making this adjustment is that dremels are round but the pegs are rectangular. If only I had a square dremel! You may think it would be easier to trim away the pegs instead but they're already small enough. Trimming them would only weaken them.
While not a miracle cure it has certainly improved the backpack on mine a lot and it doesn't pop off anywhere near as much.
I hope these instructions are clear enough for you to follow. If you are interested I can post another mod tomorrow that will improve Hot Rod's shoulder articulation so he can do things like this:

but only if you ask nicely!