hi guys ive got a 1998 p38 & lately it takes a good turn to get it to start whether it be hot or cold in fact that much of a turn it flattens the battery ? i noticed this morning that there was a air bubble in the pipe that goes from the Diesel filter to the engine , is that something that could be causing the problem .

 

When was it last serviced, fuel filter, injector additive maybe fuel distributor timing checked?
May want to look at your glow plugs, 1 or more may be failing and you could also have a battery that has about failed.