A fuel line leak spraying onto the hot exhaust manifold can ignite from the heat.

I was in a Lanscaping truck that burst into flames under the engine compartment, while we were driving.

We pulled over and jumped out of the truck with little time to spare. The bed of the truck had old wooden sidewalls and was completely loaded with grass clippings.

Within a minute or so the entire truck was engulfed in 20-foot flames.

Edit: And yes, just like in the MythBusters episode, the fuel tank did not explode, it just added fuel to the fire, igniting the load of grass in the truck bed.

The fuel line leaking spraying onto the hot manifold can ignite from the heat.

I was in a Lanscaping truck that burst into flames under the engine compartment, while we were driving.

We pulled over and jumped out of the truck with little time to spare. The bed of the truck had old wooden high walls and was completely loaded with grass clippings.

Within a minute or so the entire truck was engulfed in 20-foot flames.