Here, RealTruck.com will explain how and when to test fuel pump performance, the signs of a bad fuel pump, and the cost of replacing it. Signs of a Bad Fuel Pump: What to Look ForSome signs of a bad fuel pump are painfully obvious, while others might lead you to believe something else is the root cause of your problems. Decreased Fuel EfficiencyYour car’s fuel efficiency is directly tied to the fuel pump’s performance. How to Test Fuel Pump PerformanceYou can test your fuel pump with a fuel pressure gauge. Older vehicles have mechanical fuel pumps that fail faster, while newer cars have electrical fuel pumps that are more reliable.