You got it on the first try, it's a propellant intended to give extra thrust power (aka Penetrating).
Scimitars are mostly slashing weapons though (that is, after all, why they're curved

). The steam might be a better fit for some of the non-Toughness Save drains as well. Could be useful to keep both alternate SFXs in mind when describing specific uses of your scimitar is all I'm saying.
But yes, the idea is that with time and access to high tech goodies, Deezy will unlock additional functions and goodies within this machine. It is, after all, a relic from a time when technology was much higher than now. And there are questions too, about it that are unanswered. What was it doing, all by itself in the middle of nowhere, with no pilot, and no real damage (beyond the damage done by time and exposure)? Was it abandoned? Why? Was it hidden? Why?!
I'm suggesting you define the mystery a little more, mostly to give Relique something to lock on to in the imagination departement. This supposes you want Beast-upgrade to shape the direction of your roleplaying experience, of course.
[sblock=Decision tree to AWESOME]Granted that there are unknown systems within the Beast, two basic questions come to mind: (1) Do those systems share the same nature as the systems that have been repaired? If yes, then the Beast is a single-build machine with no real mystery attached to it construction-wise: i.e. no involved roleplay necessary, just need extra PPs. If no however, then (2) How do (some of) the non-fonctional systems differ from the working ones? To which I can see a few non-exclusive answers (2a) Technological levels: the Beast is a multi-build machine with an ancient (more advanced?) core over which successive generation of builders have added, modified and repaired according to their own understanding, tech-level and purpose. Somewhat similar to 1 in game-implementation unless some of those systems answer to 2b, c or d. Speaking of, (2b) Mysterious origins: (some of) those systems are NOT standard historical human tech (alien, pre-human martian civilisation, time-travel and/or parralel universe origins for example). Repairing those systems require delving into the mysteries of those civilizations (and checking with Relique that they exist in his world in the first place

). They might have (2c1) Uncommon power sources: (some of) those systems are not powered by diesel or aether at all, but by more esoteric means, be it magic, psionics, blood sacrifice, etc. This one is mostly to help involve one or more PCs in the research stages, say she goes to see James Featherston-Smythe for systems based on sorcery, or Brian notices that activation of his thermo powers causes certain pieces to twist and liquidly align on him, etc. Or (2c2) Seperate functionality: the separate system are meant to trigger a series of power completely different in nature from its "armored weapon's platform" function. Time-travel, Dimension travel, Space travel (great tie-in with The Puppeteer's long term goals!), Desolidification, Teleport, Doom's Day themes come to mind. Tied to the decision taken in 2c, of course, but it's not so much a question of defining the exact powers as it is establishing a general theme, I think.
Hope this post was more helpful than the last.

[/sblock]Nice picture, Jkason! I can't draw, which is why I like Heromachine. Speaking of, I'll try to patch something together helmet-wise during the week-end, Blackrat. Let you know how it turns out!