Diesel fuel is drawn up from the fuel tank by a lift pump, filtered once or preferably twice, and supplied to the high-pressure fuel injector pump. Any excess fuel then returns to the tank. Because most of the fuel drawn up returns back to the tank, the rate of circulation is much greater than the rate of fuel consumption and the filters must be sized for this higher rate of flow.
filterThe primary filter, closest to the tank, Fuel usually traps all particles down to about 30 microns in size and can also separate out any water that might be present in the fuel. It generally has a transparent sediment bowl with a valve at the bottom for draining the trapped sediment and water. The smaller secondary filter, nearest the engine, traps remaining particles down to about 5 microns.
Clogged
filterWhen a diesel engine stops running while underway, among the first things to suspect is a clogged fuel filter. Installing a pressure gauge in the fuel line after the filter can help to diagnose this problem before it leads to a shutdown. Fuel filters are most likely to clog when the tank is near empty and after a storm or offshore passage has stirred up sediment from the bottom of the tank. Allowing the fuel tank to empty also increases condensation of water into the fuel. to run rough or sputter occasionally if it allows air to be sucked into the fuel line. Check the input and output fitting threads (which should have Teflon pipe joint compound, not tape), any hose clamps, and also the rubber gasket on the filter itself.
Bleeding
the fuel
lineA diesel engine needs only three things to operate: a proper mix of atomized fuel and air, compression, and timing. When it fails to start, most likely it is not getting fuel. Changing the fuel filters, starting with the primary (nearest the tank), is usually the best first step. If the engine still fails to start, air introduced while changing the filters is a likely culprit.
Some engines are self-bleeding and will dispel air in the fuel line eventually as the engine is cranked. Others must be bled manually by loosening each fuel line connection, in sequence starting at the tank, and operating the fuel pump manually or cranking the engine until fuel seeps out before retightening. An automotive remote starter switch is helpful, along with ample paper towels to stuff under the bleed points.
When cranking the engine for 15 seconds or more, close the seacock for the raw water inlet and wait a few minutes before trying again. If you crank the engine with the cooling water inlet open, seawater will be drawn into the cooling system while there is no exhaust pressure to dispel it. This can cause water to enter the cylinders. If you crank for more than 15 seconds every few minutes, the starter will overheat and seacock.
Water
in the
fuelEven a small amount of water will contaminate a tank of diesel fuel, leading to blooms of bacteria and algae that clog the primary fuel filter with dark, ropy organic debris. A loose or leaky fuel filter cap is by far the most likely cause, followed by a vent that is not angled down to shed rain or that lacks a riser loop to drain water. Condensation is the least likely culprit. There is not enough air in the tank to contain more than a few droplets of water. The inside of the tank is not like a car windshield exposed to a constant flow of moist air.
After eliminating the source of the water, remove it from fuel in the tank by running the engine, draining water that gets trapped in the filter, and changing the filter if it clogs. The engine will return clean, dry fuel to the tank as it runs. A commercial fuel filtering service is also a good option, if available. It will remove the organics but not necessarily the water. Keeping the tank nearly full will reduce condensation; however, condensation is a minor source of water in fuel and it is good to run the tank down on occasion and not accumulate old fuel. Of the many diesel fuel additives available, most are worthless. A few do work and those are not the cheap ones.
Test for
bad fuelIf you suspect that the engine will not start or runs poorly, perhaps making smoke, because fuel in the tank is contaminated, disconnect the fuel line and the fuel return line from the tank and submerge them both into a container of fresh fuel to see if that makes any difference.
for bad fuel Fuel additives A diesel engine that is running rough or blowing smoke might benefit from a fuel additive to clean the injectors and cylinder walls. Opinions vary. A glance at the manufacturer's safety data sheet for any of the popular fuel additives will show that they contain mainly a highly refined oil along with perhaps 10% naphtha, a solvent. You can make your own, for a small fraction of the cost, by mixing 2- stroke oil with naphtha from a home supply store. A little of this should do no harm and might even help, but if it does help, be ready to change the fuel filter where any dissolved gunk will accumulate. Any fuel spill that leaves a sheen on the water ought to be reported to the USCG National Response Center at 800-424-8022. If you spilled it, expect to pay a fine. Fuel spills.