GMC 1976 ZEO 6083 Maintenance Manual page 300

up the push-rod to open the valve. The force of the
valve spring which is exerted on the plunger through
the rocker arm and push-rod causes a slight amount
of leakage between the plunger and lifter body . This
"leak-down" allows a slow escape of trapped oil in
the base of the lifter body . As the lifter rides down
the other side of the cam lobe and reaches the base
circle or "valve closed" position, the plunger spring
quickly moves the plunger back (up) to its original
position . This movement causes the ball check to
open against the ball spring and oil from within the
plunger is drawn into the base of the lifter . This
restores the lifter to zero lash . See Figure 23 .
Valve Lifter Diagnosis
1 . Momentarily Noisy When Vehicle Is Started:
This condition is normal . Oil drains from the
lifters which are holding the valves open when the
engine is not running. It will take a few seconds for
the lifter to fill after the engine is started.
2. Intermittently Noisy on Idle Only, Disappear-
ing When Engine Speed is Increased :
Intermittent clicking may be an indication of a
flat or pitted ball, or it may be caused by dirt .
Correction : Clean the lifter and inspect . If ball is
defective, replace lifter .
3. Noisy At Slow Idle or With Hot Oil, Quiet
With Cold Oil or As Engine Speed is Increased:
Insert a .015" feeler gauge between the rocker
arm and valve stem . If noise momentarily disappears
and then reappears after a few seconds with the feeler
still inserted, it is an indication that the lifter leak-
down rate is too fast .
Correction : The lifter must be replaced .
4. Noisy at High Vehicle Speeds and Quiet at
Low Speeds .
VALVE OPEN
VALVE CLOSED
A-1430
Figure 23-Valve Lifter Cutaway
ENGINE 6A- 3 5
a. High oil level - Oil level above the "Full"
mark allows crankshaft counterweights to churn the
oil into foam . When foam is pumped into the lifters,
they will become noisy since a solid column of oil is
required for proper operation.
Correction : Drain oil until proper level is ob-
tained . See Section O in the beginning of this manual .
b. Low oil level - Oil level below the "Add"
mark allows the pump to pump air at high speeds
which results in noisy lifters.
Correction : Fill until proper oil level is obtained .
See Section O in the beginning of this manual .
5 . Noisy at Idle Becoming Louder as Engine
Speed is Increased to 1500 rpm.
a. This noise is not connected with lifter mal-
function . It becomes most noticeable in the vehicle at
10 to 15 mph "L" range, or 30 to 35 mph "D" range
and is best described as a hashy sound. At slow idle,
it may be entirely gone or appear as a light ticking
noise in one or more valves . It is caused by one or
more of the following :
1 . Badly worn or scuffed valve tip and rocker arm
pad.
2. Excessive valve stem to guide clearance.
3 . Excessive valve seat runout .
4. Off square valve spring .
5 . Off square rocker arm pad.
6. Excessive valve face runout .
7. Valve spring damper clicking on rotator.
Correction : Remove valve covers and while lis-
tening with a stethoscope, locate noisy valves by in-
creasing engine speed slightly above idle, about 1500
rpm. With gloved hand, push side-ways on valve
spring . Noise will change, either becoming louder or
disappearing completely . Some noise will be present
in all valve locations. It is necessary to determine
which are actually responsible for the noise.
a . Occasionally this noise can be eliminated
by rotating the valve spring and valve. Crank engine
until noisy valve is off its seat . Rotate spring . This
will also rotate valve . Repeat until valve becomes
quiet. If correction is obtained, check for an off
square valve spring . If spring is off square more than
1/16" in free position, replace spring . See Figure 24 .