I took apart the couplings for the water and fuel filter units. I
reinstalled everything using Teflon tape on all threads. My problem went
away.


There can be air leaks in other places than threaded joints.


Wayne.B wrote:
Unfortunately teflon tape has a reputation for degrading over time
when in contact with diesel fuel. Professional mechanics use a
special blue paste to seal fuel fittings.


IIRC there is tape rated for use with various petrochemicals
& solvents... a bigger problem is that little shreds of tape
get in the injection system. Not good.

Fresh Breezes- Doug King