1. Fuel delivery problems

2. Engine performance problems
Poor engine performance: If air gets into the system causing a less than ideal air-fuel mixture, the engine may run rough, have reduced power, or have difficulty starting.

3. Air locks

4. Damaged fuel pumps
Running the fuel pump with air in the system can cause it to overheat and malfunction, as the fuel acts as a coolant. Long-term exposure to air can damage the fuel pump and lead to expensive repairs.
5. Reduced Fuel Efficiency

6. Emissions Issues
A poor air-fuel mixture can lead to increased emissions, which can result in emissions test failures and damage to the environment.
7. Possibility of Contamination