Ted Klastorin of Seattle, Washington, asks:
"I have a Universal XP25 diesel engine with approximately 2,200 hours on it. The engine runs well, but is difficult to start after it sits idle for a few days. To get it started, I have to bleed air out of the fuel system for several minutes. Once started, however, it runs smoothly.
I have checked and replaced both fuel filters; the sediment bowl on the Racor filter is clean. I replaced the fuel line from the tank to the engine two years ago, and there is no diesel odor or any other indication of a leak. It seems air is somehow getting into the system, but I am mystified as to how this might be happening."
Nigel Calder replies:
Are you sure the problem is air in the fuel system? The engine may be suffering from poor compression. When it sits for several days, the oil lubricating the cylinder walls drains down into the crankcase, reducing the seal around the piston rings, resulting in blow-by on start up. When cranking the engine to remove air from the fuel system, you also get the oil moving, are warming the cylinder walls, and are probably injecting some diesel into the cylinders, which then dribbles down and helps to reseal the rings.
But let's assume there is an air leak. Could there be pinholes in your fuel lines from chafe or corrosion? If your primary fuel filter has a transparent sediment bowl, loosen the bleed nipple on the secondary filter and operate your fuel lift pump. If you see bubbles in the primary filter bowl, your leak is between the primary filter and the tank, or else in the seal for the primary filter (the most likely place). If there are no bubbles, the leak is probably upstream of the filter.
Another place you might find an air leak is in the return lines from the injectors. These often tee into the secondary fuel filter and from there return to the tank. I have seen air get into fuel systems on older engines through these lines when the engine is shut down (I had this problem once myself). If this is suspected, you can try plugging the connection at the secondary filter and running the return line directly back to the tank.