Engine performance diagnosis procedures are covered in Engine Controls - 5.7L and should be referred to for diagnosis of any driveability, emission, or powertrain diagnostic trouble codes (DTC) problems.
The following diagnosis covers common problems and possible causes. When the correct diagnosis is made, the cause should be corrected by adjustment, repair or replacement as required.
Refer to the appropriate Section of the service information for each specific procedure.
This diagnostic table will assist in diagnosis of an engine mechanical fault such as a faulty camshaft, worn or damaged bearings or bent pushrod.
This diagnostic table will not isolate a crossed injector wire, faulty injector or any other electrical component failure that may cause a misfire. The Powertrain On-Board Diagnostic System checks must be performed first. Refer to
Diagnostic System Check - Engine Controls.
When using this table to perform the Engine Misfire diagnosis, begin with the preliminary information below and then proceed to the specific category.
Checks
| Action
|
Preliminary
| Perform a visual inspection of the following:
| • | A loose or incorrectly installed crankshaft balancer and if required check the flywheel or flexplate for loose or incorrect installation |
| • | Worn, damaged, or misaligned accessory drive system components |
| • | Listen to the engine for any abnormal internal engine noises. |
| • | Verify the engine is consuming coolant. |
| • | Perform a compression test on the engine. |
|
Intake manifold leaks
| An intake manifold that has a vacuum leak may cause a misfire which is more prominent at idle.
Inspect the following:
| • | Incorrectly installed or damaged vacuum hoses |
| • | Faulty or incorrectly installed intake manifold and/or gaskets |
| • | Cranks or damage to the manifold, also inspect the areas between the intake runners |
| • | Incorrectly installed or damaged Manifold Absolute Pressure (MAP) sensor or O-ring seal. The MAP sensor is located at the rear of the Intake manifold. |
| • | Incorrectly installed throttle body or damaged gasket |
| • | Warped or damaged cylinder head sealing surface |
|
Coolant consumption
| Coolant consumption may or may not cause the engine to overheat. However, an engine that has been overheated may cause premature failure of head gaskets and distortion of gasket surface.
Inspect the following:
| • | Faulty cylinder head gasket |
| • | Distorted cylinder head |
| • | Cracked or corroded cylinder head |
|
Abnormal internal engine noises
|
Important:
| • | Camshaft speed is half the speed of the crankshaft. Use a timing light to determine the knock speed. Two knocks per flash is crankshaft speed and one knock per flash is camshaft speed. |
| • | A slight COLD knock or piston slapping noise could be considered normal if it is not present after the engine has reached normal operating temperatures. |
- With the engine running, try to determine the knock speed. That is if the noise is timed to camshaft speed or crankshaft speed.
- If the noise is timed to camshaft speed, inspect the following:
| • | Missing or loose valve train components |
| • | Worn or loose valve rocker arms |
| • | Worn or damaged timing chain and/or sprockets |
- If the knock is timed to crankshaft speed, inspect the following:
| • | Worn crankshaft main or connecting rod bearings |
| • | Piston or cylinder damage |
| • | Worn piston or piston pin |
| • | Damaged or faulty connection rod |
| • | Excessive carbon build-up on the top of the piston |
|
Engine misfire
|
- Remove the valve rocker arm cover on the side of the engine with the cylinder that is misfiring.
- Inspect for the following:
| • | Loose valve rocker arm bolts |
| • | Faulty valve lifters (bleeding down) |
| • | Worn or incorrectly seated valves |
|
