Chapter A: General Diagnosis
16
Abnormal CO2 and O2 Readings
Since the catalytic converter reduces HC and CO, these
emissions are unreliable for determining the airfuel ratio.
However, CO2 and O2 readings can be useful, provid-
ed that the air injection system has been disabled.
When air and fuel entering the engine burns with the
least amount of wasted energy, at the stoichiometric air-
fuel ratio, the engine emits the highest amount of CO2.
Look for readings between 13.8 and 15 percent. As the
air-fuel ratio of the mixture leans or enriches, the CO2
level drops. To determine whether a low CO2 level indi-
cates a lean or rich condition, examine the O2 reading.
Levels of O2 below approximately 1.0 percent indicate
a rich-running engine; above 2.0 percent indicates a
lean-running engine.
To perform adequately and operate efficiently, an engine
must be in sound mechanical condition. Therefore, it is
important to determine the overall mechanical condition
of the engine before attempting to isolate or repair the
cause of a driveability or performance problem.
Perform a compression or cylinder leakage test to de-
termine the internal sealing capabilities of the engine.
When test results are marginal and indicate valve seat-
ing problems, performance can often be restored by ad-
justing valve lash or servicing hydraulic valve lifters. If
test results are below specifications, internal engine re-
pairs are required to restore performance. This chapter
focuses on engine repairs that can be performed with-
out disassembling the engine or removing major com-
ponents.
VALVE LASH ADJUSTMENT
Ideally, an engine should operate with near zero valve
lash, or clearance. Under these conditions, valve move-
ment follows the profile of the camshaft lobe exactly to
provide efficient operation. Over the life of an engine,
valve clearance tends to change as a result of wear on
the valve face, valve seat, pushrod, and rocker arm. For
many years engines were designed with provisions to
make periodic adjustments to correct for this wear.
Many late-model engines have no scheduled need for
and therefore no provision for making a valve clearance
adjustment. These engines require precise machining
during overhaul to ensure that the proper clearances are
met. If valve clearance problems surface diagnosis will
reveal a failed component. Follow the Service Manual
procedures for repair or replacement of the component
or components involved.
Valve Lifter Designs
With hydraulic lifters, once engine oil pressure is estab-
lished, the lifters automatically take up all slack in the
valve train to maintain zero clearance, figure 1-17. Hy-
draulic lifters also compensate for metal expansion as
the engine warms up. Hydraulic valve lifters do not re-
quire routine adjustment.
Mechanical lifters must be set with a precise amount of
clearance so that the valves operate with close to zero
lash once the engine is at normal operating tempera-
ture. Mechanical lifters must be adjusted periodically to
compensate for wear in the valve train.
Maintaining the correct clearance is important. Too
much clearance prevents the camshaft from opening
the valves fully. This shortens effective camshaft dura-
tion causing valves to open late and close early, which
reduces engine efficiency and performance. Continued
operation stresses valve train parts and can lead to pre-
mature failure.
Insufficient clearance causes the valves to open too far.
Effective duration is increased so the valves open early
and close late. Once the engine warms to operating
temperature, the valves might not be able to close com-
pletely.
Fig. 1-17.
Typical hydraulic lifter.
Summary of Contents for ASE-A8
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