When he mentioned his concern about our engine, he replied that any air bubble will have gone back into the tank.
This friend sounds like one of those that is often referred to on this site .............. the ones you want to take with a grain of salt. Be careful.
It's highly unlikely that any air will have
"gone back into the tank". Air bubbles will always hang at high spots. If you follow your fuel supply hose from your primary filters (the one you changed) back to your fuel tank, you will see the the fuel take-off is on the top of the tank. Inside the tank is a vertical tube which goes down near the bottom of the tank. There is no way for air bubbles to float down through the fuel and into the tank. Scratch that theory.
In addition, if you look at your fuel line leading from your primary filters back to the fuel tank it's most likely laying flat on the bottom of the hull. Additionally, the fuel line going to the engine is likely lying flat as well. Bubbles are not completely removed until the fuel really starts to move.
And lastly, once errant bubbles get past your fuel lift pump, the only way is to go forward, towards the injector pump (as shown on the previous diagram).
If you're worried about a hidden bubble appearing when you least expect it (on the stormy high seas) and not comfortable bleeding the two fuel supply lines at the injectors (back to the diagram), try this:
1. Make the boat fast to the dock and slowly load the engine to a
safe level (be careful doing this). This will make the fuel move faster than being in neutral at any RPM.
2. If still no problems, check out how to bleed the engine (check the archives here for some good ideas), but only for a Yanmar 2GM 20F.
3. Head out on a dead calm day and run the engine full throttle to test if there are any bubbles still lurking in the lines.
4. If the engine does shut down, you'll know how to bleed at the injectors.
5. If still no hiccups, you're headed for the open seas
.
PS. I'd give anything for a first mate like yourself who takes an interest in engines the way you do.