Detonation/Spark Knock; Excessive Exhaust Emissions (Odors) - GMC G Service Manual

DETONATION/SPARK KNOCK

DEFINITION: A mild to severe ping, usually worse under acceleration. The engine makes sharp metallic knocks that
change with throttle opening. Sounds like popcorn popping.
Check EGR System for proper operation. See Chart
in Section C.
Check ignition timing. See Vehicle Emission Control
Information label.
Check for obvious overheating problems. 85 to 100
degrees C (185 to 215 degrees F) is normal.
— Low coolant.
— Loose water pump belt.
— Restricted air flow to radiator, or restricted water
flow through radiator.
— Inoperative fan clutch.

EXCESSIVE EXHAUST EMISSIONS (ODORS)

Definition: Vehicle fails an emission test. May also have
excessive "rotten egg" smell (hydrogen sulfide).
• If test shows excessive CO and HC (also has exces­
sive odors):
— Check items which cause vehicle to run rich.
• Make sure engine is at normal operating
temperature.
• Visually check hoses for splits, kinks and
proper connections as shown on Vehicle
Emission Control Information label.
• Remove air cleaner and check air filter for
dirt or being plugged. Replace as
necessary.
• Check for misadjusted idle mixture if
plugs are removed.
• Check choke valve and linkage for stick­
ing of malfunctioning operation.
• Check choke, vacuum breaks and fast
idle adjustments.
• Check for stuck PCV valve or obstructed
hose.
• Check for lead contamination of catalytic
converter. Check for absence of fuel filler
neck restrictor.
• Check operation of air management sys­
tem. Refer to appropriate Chart in Air
Injection Reaction section.
• Check carburetor for flooding.
CARBURETOR EMISSIONS 6E1-17
• Check for air leaks at carburetor mounting and intake
manifold.
• Check for poor fuel quality, proper octane rating.
• Remove carbon with top engine cleaner. Follow
instructions on can.
• If excessive carbon in cylinders, check for leaking
valve seals.
• Check for improper operation of transmission (i.e.,
linkage adjustment) and TCC.
• Check for incorrect basic engine parts, such as cam,
heads, pistons, etc.
• Check float level using external float
gage. Refer to CARBURETORS (SEC.
6C1).
• Check canister for loading and check
purge system for proper operation. Refer
to "Evaporative Emission Control System
Diagnosis" in this section.
• Check for incorrect idle speed.
• Check for incorrect timing. Refer to Vehi­
cle Emission Control Information label.
• Check condition of spark plugs, plug wires
and distributor cap.
• If test shows excessive NOx:
— Check items which cause vehicle to run lean, or
to run too hot.
• Check EGR valve for not opening. Refer
to Chart in Section C.
• Check for vacuum leaks.
• Check for inoperative THERMAC.
• Check coolant system and cooling fan for
proper operation.
• Remove carbon with top engine cleaner.
Follow instructions on can.
• Check ignition timing for excessive base
advance. Refer to Vehicle Emission Con­
trol Information label.