canonically in the comics, some transformers have a preferred mode that they reflexively revert to after death - that may be their vehicle mode or their root mode (aka robot mode) so sometime after death they may sproing back into their preferred mode if they died in their unpreffered one, so even if you got a slick cybertronian ride going with some sort of sparkless sorcery, it could revert back to its robot mode unexpectedly within a relatively short period of time - say… perhaps somewhere between a breem and a vorn? (8.3 minutes to 83 years) 

and - get this - that post-mortem reflex is called rigor morphis (and it can happen with or without a functioning t-cog! spoooooky!!!)