I have a 60 seiries detroit deisel in a 2000 volvo vnl The motor used to start right off even if it had been sitting for a few weeks. Sudddenly it began taking 10 second of cranking before it will start, even if it has been sitting for less than one hour. I replaced the check valve at the back of the cylinder head a few months ago ,because allthou it would start right away it would sometimes quit after it ran for 15seconds or so. Replacing that valve did't fix that problem but I found if I keep the fuel tanks full when I park it I don't have that quiting problem. The truck has 1.2 million km or 780 thousand miles.

See if this could be part of your problem.

In cold weather this year, many diesel owners have had problems with the new ultralow-sulfur fuel. The problem comes during the refining process used to attain the ultralow-sulfur ratio. That affects the naturally occurring wax in diesel in such a way that it can cause the fuel to turn from liquid to gel more readily in cold temperatures. Gelled fuel clogs the fuel filters and starves the engine, causing it to stop. Diesel fuel additives, such as Amsoil's, and ultraclean kerosene added to the fuel help prevent this problem.

Clogged fuel filter may result to a dead engine because it will fail to start due to loss of fuel. For maintenance, you have to replace your volvo fuel filter periodically to reduce the risks.