I replaced the fuel filter the other month on my 2003/53 2.2 Di Sport Diesel X-Trail and since then I have not been able to obtain the consistent performance I had previously. Genuine Nissan filter was used.

Whilst the engine always starts first time and idles smoothly, intermittently and when under load, i.e. accelerating up hill or towing, it will loose power in 3rd or 4th gear requiring a premature change down to 2nd in order to maintain progress. If I leave it in 3rd or 4th the power will intermittently surge.

The problem I’m sure is down to fuel starvation caused by air in the fuel system that wasn't there before the filter change.

I have primed the system by removing the fuel filter outlet pipe ensuring no bubbles remain and then reconnect the pipe continuing to prime until the priming pump goes ‘hard’. However if left for 5 seconds it will go soft again and will take a few more pushes. I then start the engine and continue to prime for a few minutes in case any air is stuck beyond the filter.

This process fully restores the performance for a few weeks only for it to slowly revert to its previous state.

I have checked the filter housing/priming pump by installing a clear fuel pipe to the outlet, that loops higher than the filter, and there are no bubbles whilst engine is running.

What I do notice is when the engine is switched off a large bubble appears from the direction of the fuel pump and sits at the top of this loop. It clears when the engine is started but returns when stopped. I’m not sure if this is normal behaviour or whether it has anything to do with my intermittent issue? On disconnecting the clear filter outlet pipe the fuel immediately drains away, I assume back to the tank via the return pipes, again I assume this is normal too? - but the filter remains full as expected so the non-return valve is working ok.

So to diagnose this further my questions are :-

1/ Should the priming pump stay hard until the engine is started?
2/ Does it make any difference if the ignition (engine not started) is on when priming?
3/ Is it normal to have to continue to prime once the engine has started?
4/ Should an air bubble rise from the direction of the fuel pump to the fuel filter outlet when the engine is stopped? Or do I still have an air lock/leak maybe in the fuel pump area?
5/ Should the fuel drain away from beyond the fuel filter if I disconnect the filter outlet pipe or should it hold there waiting for the main fuel pump to suck it in and compress it?

All hose connections are tight and there are no obvious fuel or air leaks in the system that I can see.

I would welcome any comments, observations or advice so I can resolve this issue once and for all, and hopefully help others who are experiencing this frustrating problem. Thanks in advance.