382 Auto Engine Performance and Driveability
Copyright by Goodheart-Willcox Co., Inc.
Excessive Hydrocarbon Reading
Overview
Excessive HC is usually caused by a problem that results in an incomplete burning of fuel. This is
sometimes accompanied by a rotten egg odor.
Possible
Problems
• Poor cylinder compression
• Leaking head gasket
• Ignition misfi re
• Incorrect ignition timing
• Defective input sensor
• Defective output device
• Defective ECM
• Open EGR valve
• Sticking or leaking fuel injector
• Improper fuel pressure
Leaking fuel pressure regulator
• Oxygen sensor contaminated or responding
to an artifi cially lean or rich condition
• Fuel fi ller cap improperly installed
Excessive Carbon Monoxide Reading
Overview
Excessive CO is caused by a problem that results in
a rich air-fuel mixture. However, it is often created by
an insuffi cient amount of air or too much fuel reaching
the cylinder. A high CO reading sometimes coincides
with a high HC and/or low O2 reading.
Possible
Problems
• Plugged air fi lter
• Engine carbon-loaded
• Defective input sensor
• Defective ECM
• Sticking or leaking fuel injector
• Higher-than-normal fuel pressure
• Leaking fuel pressure regulator
• Oxygen sensor contaminated or responding
to an artifi cially lean or rich condition
Excessive Hydrocarbon (HC) and Carbon Monoxide (CO) Reading
Overview
When both HC and CO are excessive, this often
indicates a problem with the emission control system
or an on-going problem (usually indicated by a rich
air-fuel mixture) that has damaged an emission-con-
trol component. Check all of the systems mentioned
above along with the systems listed here.
Possible
Problems
• Plugged PCV valve or hose
• Fuel-contaminated oil
• Heat riser stuck open
• Air pump disconnected or defective
• Evaporative emissions canister saturated
• Evaporative emissions purge valve stuck
open
• Defective throttle position sensor
Excessive Oxides of Nitrogen (NO
X
) Reading
Overview
Excessive NO
X
readings are created when combus-
tion-chamber temperature becomes too high or by an
excessively lean air-fuel mixture.
Possible
Problems
• Vacuum leak
• Leaking head gasket
• Engine carbon-loaded
• EGR valve not opening
• Fuel injector not opening
• Low fuel pressure
• Low coolant level
• Defective cooling fan or fan circuit
• Oxygen sensor grounded or responding to
an artifi cially rich condition
• Fuel contaminated with water
• Pinging
• Detonation
Figure 18-10. These are the most common causes of high emissions readings. (continued)