When my ford 150 xlt 2003 truck is running it gave off a fishy smell why?

Hello, it's really hard to say why an abnormal smell occurs. A few things I can think of causing a smell would be: -Oil leak from a component burning on hot engine or exhaust components -Sulfur smell from gassing battery -Moldy smell from the ventilation system inside the car None of these sound like "fishy smell" -- so it's super hard to know without experiencing it first hand. Perhaps you could take your vehicle locally and see if someone could get a "nose" on it and give you an opinion? https://repairpal.com/repair-shops/ford-repair-and-service-in-atlanta-georgia

Old antifreeze has a similar smell - suspect engine coolant leak.