Lesson 2/Learning Event 3
If the in-tank pump is operating as it should, but no fuel reaches the injector pump, the trouble can easily be
located. With the in-tank pump running, loosen the fittings on the fuel lines or open the bleed valves, one
at a time, to see if fuel will flow out. Start at the final stage (engine secondary filter) and work toward the
tank. Let's say that fuel leaked when you loosened the fitting on the tank side of the vehicle primary filter.
However, no fuel leaked from the fitting on the engine side of the filter. This indicates the trouble is in the
filter. It's probably stopped up and needs cleaning.
If fuel reaches the injector pump and the linkage is all right, the problem must be in the injector pump or
the injector nozzles and lines. You are authorized to replace the high-pressure lines; however, it is unlikely
that they are all plugged. The next step would be to notify your support unit of the condition.
Another problem you may find in the fuel and air intake systems is an improper fuel-air mixture. A clogged
air cleaner can cause smoky exhaust and low engine power. It can also cause the truck to use too much fuel-
air mixture in the combustion chamber. The mixture is too rich (does not have enough air). Let's see what
else can cause an improper mixture of air and fuel.
On some 2 1/2-ton, multifuel engines, a turbosupercharger helps supply air to the engine cylinders. If the
turbosupercharger is defective, the fuel-air mixture becomes too rich, especially at higher engine speeds. A
defective turbosupercharger will generally show the same symptoms as a clogged air cleaner. This includes
smoky exhaust, low power, and use of too much fuel. You should check the air cleaner first when these
If the air cleaner element is not clogged, check the manifold heater gaskets for leakage. The gaskets could
be blown, or the manifold heater elbow could be loose. Any leak in this area will decrease the
turbosupercharger output and cause the fuel-air mixture to be too rich.
If the manifold heater gaskets are not leaking and the air cleaner is not clogged, the fault is in the
turbosupercharger. You should notify your support maintenance unit because you are not authorized to test,
remove, or replace the turbosupercharger.
High fuel consumption is a problem that you should look into as soon as possible. This problem may not
seem important; however, the driver and other personnel aboard may be riding a very unsafe vehicle!