GMC chervolet camaro 1999 Service Manual page 802

Engine Engine Mechanical- 3.'8 L 6-19
Base Engine Misfire Diagnosis {cont'd)
Checks Action
Intake Manifold Leaks An intake manifold that has a vacuum leak may cause a misfire.
Inspect for the following:
• Improperly installed or damaged vacuum hoses
• Faulty or improperly installed lower intake manifold and/or gaskets
• Cracked or damaged lower intake manifold
• Improperly installed MAP sensor
The sealing grommet of the MAP sensor should not be torn or damaged
• Improperly installed throttle body or damaged gasket
• Warped intake manifold
• Warped or damaged cylinder head sealing surface
Coolant Consumption Coolant consumption may or may not cause the engine to overheat.
Inspect for the following:
• External coolant leaks
• Faulty cylinder head gasket
• Warped cylinder head
• Cracked cylinder head
• Damaged engine block
Oil Consumption Oil consumption may or may not cause the engine to misfire.
1. Remove the spark plugs and inspect for an oil fouled spark plug.
2.
Perform a cylinder compression test.
3. If the compression test indicates worn valves or valve guides, inspect the
following:
• Worn, brittle, or improperly installed valve stem oil seals
• Worn valve guides
• Worn valve stems
\
)
• Worn or burnt valves or valve seats
4. If the compression test indicates worn or damaged piston rings, inspect the
following:
• Broken or improperly seated piston rings
• Excessive piston ring end gap
• Excessive cylinder bore wear or taper
• Cylinder damage
• Piston damage
Abnormal Internal Engine Noises
1. Start the engine and determine if the noise is timed to the engine camshaft
speed or the crankshaft speed.
2.
Using a timing light, two knocks per flash is the crankshaft speed and one
knock per flash is the engine camshaft speed.
3. If the noise is timed to the engine camshaft speed, inspec, the following:
• Missing or loose valve train components
• Worn or loose valve rocker arms
• Worn or bent valve pushrods
• Faulty valve springs
• Bent or burnt valves
• Worn engine camshaft lobes
• Worn or damaged camshaft timing chain and/or sprockets
Important: A slight COLD knock or piston slapping noise could be considered
normal if not present after the engine has reached normal operating temperatures.
4. If the knock is timed to the crankshaft speed, inspect the following:
• Worn crankshaft or connecting rod bearings
• Piston or cylinder damage
• Worn piston or piston pin
)
• Faulty connecting rod
• Excessive carbon build-up on the top of the piston