When I had those symptoms it was the fuel pump, even though you can hear it running it does not mean its pumping pressure. Make sure to replace with genuine only.
Its the fear that it could unscrew and loose all the oil and the engine that causes it. Just a little bit more to make shure, I had to use a 3ft bar to get it off so I will do the same to put it back.
To do that job everything has to be striped off the bulkhead inside and out before starting. If you paid a professional to do it take it back and tell them to sort it.