An attack against one of my heros is declared. If I use Fuel Injection to block LOS to the hero what happens?

Fuel injection shouldn't trigger until the attack has resolved, at this time LOS is no longer necessary. If it was when it was declared and you blocked LOS then the attack would be voided and the action would still be expended (RRG 15).

Fuel Injection says WHEN an attack resolves and is an interrupt. Shouldn't it happen before the attack?

When an attack resolves is after step 7 of an attack. When an attack resolves is when Blast, Cleave and Conditions are resolved if the target suffered damage from the attack. There are plenty of other abilities with the same timing:

* Mission/Core Rules
- Blast, Cleave, Conditions
* Imperial
- Arc Blasters
* Rebel
- Opportunist
- Suppressive Fire (if Havoc Shot used)
- Sidewinder
- Get Cocky
- Brutal Cleave, Rampage, Staggering Blow
- Snap Kick
- No Escape
- Execute
- Stay Down
- Return Fire (Han Solo)
- Rage
- Figurehead
- Adrenaline Rush
- Quick As A Whip
* Rebel (Other)
- Jeswandi Training
- Fuel Injection
- Peacemaker, Life Debt

(You must be thinking of when an attack is declared .)

Yeah, LOS is only important when the attack is declared, right?

But since it is an interrupt, Fuel Injection is ressolved before Blast?

The target suffers damage on step 7 of the attack, so blocking the line of sight afterwards does nothing.

See Timing. During attacks the order is mission/core rules, attacker abilities, defender abilities. Outside of attacks it's mission/core rules, imperial abilities, rebel abilities.

Fuel Injection is a rebel ability, so happens after Blast, Cleave and Conditions, which are (attacker/)imperial abilities. (Being interrupt does not affect the timing, only that the game resumes after it is performed. Most abilities actually interrupt without using the word interrupt.)