Diesel returns to tank.

Could be filter housing fault or a o ring not sealing properly,

Fuel filter and cap replaced a couple of days ago. Fuel started to bubble out round filter cap. It did it again this morning when I had to use primer bulb to start car. After that there was no sign of any leakage under car and filter housing/ cap clean and dry. At the end of my tether now and my mechanic is pulling out the remainder of his hair! Thank you so much for helping.

" Fuel started to bubble out round filter cap .It did it again this morning "

Then you have a leak and it is not sealed , drawing air .