I have a very hard time believing that an internal combustion engine would sustain significant damage prior to stalling. An engine could run, albeit very poorly, with extreme backpressure (say, an exhaust blockage but perhaps some leaks elsewhere in the exhaust system). If the exhaust was perfectly sealed, there would be so much backpressure that the mixture would be starved of air and there would simply not be any explosion in the cylinders. I have limited knowledge of diesel engines but would expect a similar result.

Here’s a video where an exhaust pipe is plugged. You can see how quickly the car stalls (at 10:00): https://piped.video/watch?v=jnoW0skAChA

I can’t say that I’ve ever used a single Megalixir in several Final Fantasy games due to them being limited in supply.