ENGINE DIVISION SERVICE MANUAL
ADENDUM NO. 1
FUEL SYSTEM VAPOR LOCK
Vapor lock occurs when fuel system components
become sufficiently heated to cause fuel to vaporize within
fuel lines, filter, or fuel pump at a rate which prevents liquid
fuel from reaching the carburetor. Vapor lock is generally
associated with hot ambient conditions but can still occur in
cooler climates when vehicle or engine is operated under
heavy load. Vapor lock conditions are aggravated at higher
To trouble shoot a suspected vapor lock condition,
proceed as follows.
1. Do vapor lock conditions prevail? Look for hot fuel system
components. Does engine run OK when cool?
2. Remove carburetor air cleaner and pump throttle by hand.
Observe accelerator pump discharge. Absence of pump
shot or weak discharge may indicate vapor lock. Check
for presence of fuel in tank. Check for fuel system
3. Check fuel pump discharge pressure by removing fuel line
from carburetor inlet, connect fuel pressure tester to open
end of fuel line (*) and crank engine. A vapor locked fuel
necessarily specified pressure. Observed pump pressure
as low as 1 to 2 psig indicates that the pump is probably
OK but should be checked again when engine is running
(*) NOTE: For this test, do not "tee" pressure
tester into fuel line between fuel pump and
carburetor. No pump discharge pressure would be
observed because fuel vapor would be vented to the
carburetor bowl through the open needle valve.
If fuel pump has measurable discharge pressure but does
not pump liquid fuel when system is very hot, vapor lock
is a likely cause.
4. If vapor lock is suspected as the cause for rough engine
operation during cruise or acceleration it may be checked
"Tee" fuel pressure tester fitting into fuel line between
fuel pump and carburetor. Place test gauge in
vehicle within view of driver or passenger. Connect
to fitting with flexible fuel line. Operate vehicle on
highway until rough operation occurs.
Note fuel pressure during rough operation. If
pressure drops to zero, vapor lock may be the cause.
(Check fuel lines and filter for obstructions.)
Remove fuel pressure tester and fitting from fuel line
and allow fuel system components to cool; then
check to see that normal engine operation has been
restored. If engine does not run, refer to Trouble
Shooting Charts 4 or 5.
CGES-215 - Page 45
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