Poor fuel economy Poor performance Decreased horsepower   Metallic or ‘rattling’ sounds when accelerating Backfiring or ‘popping’ sounds when the engine is revved. Dull, knocking sound coming from under the hood when accelerating. Loud roaring noise while driving down the road at a steady speed.         Fuel leaks in the engine compartment     Burning smell coming from under the hood.   Loss of power during acceleration.     Increased gas consumption Backfiring or knocking sounds when accelerating Decreased horsepower   Stalling out at high speeds