Well...
All I know is, back some time ago, (back in the early '80s ) a buddy and I were out cruzin' in his '67 Galaxie 500 convertable...hot summer night...and it over-heats...

It's night-time, and he goes to check the coolant level...takes off the radiator cap...can't see anything, so he whips out his cigarette lighter...

Whooosh![X(]
A jet of white-blue flame shoots straight past his face right to the under-side of the hood!

My guess would be yes, the fumes are flammable...