Fuel System Diagnosis And Testing - Ford LSG 875 Service Manual

3-02 FUEL SYSTEM
3-02
DIAGNOSIS AND TESTING
GENERAL INFORMATION
Water and dirt that accumulate in the fuel tank
can cause a restricted fuel line or filter and mal-
function of the fuel pump or carburetor. Conden-
sation, which is the greatest source of water
entering the fuel tank, is formed by moisture in the
air when it strikes the cold interior walls of the fuel
tank.
If the accumulation of dirt and water in the filter
is excessive, the fuel tank should be removed and
flushed, and the line from the fuel pump to the tank
should be blown out.
Air leakage in the fuel, inlet line can cause low
fuel pump pressure and volume.
A restricted fuel tank vent can cause low. fuel
pump pressure and volume and can result in col-
lapsed inlet hoses or a collapsed fuel tank.
Low pressure is the most likely fuel pump trou-
ble that will affect engine performance. Low pres-
sure will cause a lean mixture and fuel starvation
at high speeds.
Dirt accumulation in the fuel and air passages,
improper idle adjustments, and improper fuel level
are the major sources of carburetor troubles.
CONDITION
PROBABLE CAUSE
,
Low Fuel Pump Pressure l
Diaphragm stretched or leaking. l Fuel line cracked or broken.
l Fuel pump diaphragm spring is l Fuel pump valves seating
weak.
improperly.
l Cam eccentric worn or undersize.
l Dirt in fuel tank and/or lines.
l Fuel tank vent restricted.
.
l Fittings loose or cracked.
l Fuel pump screen clogged.
l Diaphragm ruptured.
,
Low Fuel Pump Volume with l Fuel filter clogged.
l Restriction in fuel supply line to
Normal Pressure
l
Fuel pump to carburetor inlet line
obstructed, crimped or leaks.
fuel pump.
Fuel Pump Leaks
l Diaphragm defective. Fittings loose.
,
Fuel Pump Leaks Oil
l Fuel pump retaining bolts loose. l Mounting gasket defective.
L
Fuel Tank and/or Inlet Line
Hoses Collapsed
l Fuel tank vent restricted.
Flooding or Leaking Carburetor l Cracked carburetor body.
l High fuel level or float setting.
0 Excessive fuel pump pressure.
l
Fuel inlet needle not seating
properly or worn needle and/or
seat.
l Improper starting procedure causing
0 Incorrect fuel pump pressure.
a flooded engine.
l Dirty air cleaner element.
, l Improper carburetor fuel level.
I
l improper idle adjustments.
l Sticking or incorrectly seating fuel
inlet needle.
l Incorrect idle fuel mixture.
l Engine idle speed too slow.
l Dirt, water or ice in fuel filter.
l Carburetor icing (cold, wet or
humid weather).
l Incorrect throttle linkage
adjustment to carburetor.
I
l Fuel lines restricted or leaking air.
l Clogged air bleeds or idle
I
l Fuel tank vent restricted.
passages.
l Leaking intake manifold or
carburetor gaskets.
l Defective fuel pump.
l Excessive looseness of throttle
shaft in bore(s) of throttle body.
.
L