I am hardly an expert but if I remember the science correctly your PC2 is closer to the truth.
- Gas might not ignite at all, whatever you are running on (diesel? kerosene?) is fuel which is not enough, to ignite you also need oxidizer... air will do but as PC1 only just opened the cap there might simply not be enough (especially since match will be burning out oxygen as it falls and might actually be extinguished before it touches the fuel)
- Assuming fuel ignites, it is fairly likely to go off before exploding - same issue there might not be enough oxidizer to sustain the fire.
- If the fuel does catch... Explosion is by definition a rapid increase in volume. Burning fuel will quickly create high volume of gases... but with cap off, the time to explosion will be determined by the tag of war between gases generated by burning fuel and gases venting though the opening.
- Several other factors will be significant as well... How full is the tank? How long has the engine been running? The more tank is filled with fumes rather then liquid fuel the more likely the explosion will be.
In the end you are GM, hard science and realism are subjects to your discretion.
Personally in this situation I would not bother with roll at all, I would consider explosion a 100% given, however will give several minutes before it happen (on one hand giving players a chance to get to safety, but on the other hand messing with their timing).
If I can get players to believe the plan failed and there will be no BOOM... so I can follow up with KA-BOOM as soon as they relax... So much the better. ;)
This message was last edited by the user at 16:40, Fri 11 May 2018.