Ford 2003 Mustang Workshop Manual page 854

incorrectly functioning valve tappet
air in lubrication system
excessive valve guide wear
low oil pressure
Excessive collapsed valve tappet gap can be caused by loose rocker arm seat bolts/nuts, incorrect
initial adjustment or wear of valve tappet face, or worn roller valve tappets, push rod (6565), rocker
arm (6564), rocker arm seat or valve tip. With valve tappet collapsed, check gap between the valve tip
and the rocker arm to determine if any other valve train parts are damaged, worn or out of adjustment.
An incorrectly functioning valve tappet can be sticking, caused by contaminants or varnish inside the
tappet. The tappet can have a check valve that is not functioning correctly, which can be caused by an
obstruction, such as dirt or chips that prevent the check valve from closing, or a broken check valve
spring. A tappet with a leakdown time out of specification can cause tappet noise. If no other cause for
noisy valve tappets can be found, the leakdown rate should be checked and new valve tappets
installed if found to be out of specification.
Assembled valve tappets can be tested with Hydraulic Tappet Leakdown Tester to check the leakdown
rate. The leakdown rate specification is the time in seconds for the plunger to move a specified
distance while under a 22.7 kg (50 lb) load.
Air bubbles in the lubrication system will prevent the valve tappet from supporting the valve spring
load. This can be caused by too high or too low an oil level in the oil pan or by air being drawn into the
system through a hole, crack or leaking gasket on the oil pump screen cover and tube.