Air in the fuel being supplied to the engine can cause a number of problems including hard starting and poor performance. Air can enter the fuel system at several points such as:

+ Cracked pickup tube in the fuel tank (low fuel)
+ Loose suction side fuel supply lines
+ Damaged primary fuel filter seals
+ Leaking frame mounted primary fuel water separator housing and fittings
+ Leaking injector sleeve to injector seat
+ Damaged engine-mounted fuel pump seals

TEST RESULTS AFTER TEST LINES INSTALLED

Start the engine and run at idle a minimum of 5 minutes to fill the lines with fuel and purge out all the air introduced when connecting the lines. Monitor the test line for aeration several feet away from the engine. Do not check at cylinder head outlet. Fuel exiting the cylinder head will show disturbance, this is normal and should not be considered air in the fuel return.
Note: Some very small air bubbles in the line are acceptable.