GM Service Manual Online
For 1990-2009 cars only

Step

Action

Value(s)

Yes

No

Engine Performance diagnostic procedures are covered in Engine Controls and should be consulted for diagnosis of any Driveability, Emissions, or Service Engine Soon lamp complaint.

    • Refer to Engine Controls - General Information.
    • Refer to Engine Controls - Fuel Injection (MFI). Check for any related service bulletins or service manual updates prior to extensive engine diagnosis.

Notice: The following diagnosis information covers common problems and possible causes. When the proper diagnosis is made, the problem should be corrected by adjustment, repair or part replacement as required. Refer to the appropriate section of the manual for these procedures

This diagnostic table will assist in engine misfire diagnosis due to a mechanical problem such as a faulty camshaft or leaking headgasket. Base engine timing is assumed to be correct and, that acceptable fuel is being used. This table will not identify a crossed injector wire, faulty injector, coil, driver, etc., or any other component failure or perceived misfire with no driveability symptoms (i.e. chipped reluctor ring with a DTC set or Service Engine Soon Light illuminated). The On-Board Diagnostic System Check must be performed first in order to ensure the correct table is followed for diagnosis of a misfire. Refer to Engine Controls first, then, if necessary, refer to this table.

1

Measure the engine oil pressure at the intake camshaft carrier.

Is the oil pressure at operating temperature in excess of 586kPa (85psi)?

--

Go to Step 2

Go to Step 3

2

Be aware that oil viscosity affects oil pressure.

Is the recommended oil viscosity being used based on the ambient temperature?

--

Go to Oil Pump Replacement

Go to Step 3

3

Are there any obvious mechanical noises?

--

Go to Engine Noise Diagnosis

Go to Step 4

4

Visually inspect valve train and timing components spark plugs, coils, integrated ignition components, and hoses for proper hook up or damage.

Is there any damaged component or improperly connected wires or hoses?

--

Go to Step 5

Go to Step 6

5

Repair or replace the parts as necessary.

Do the symptoms still exist?

--

Go to Step 7

System OK

6

Remove and inspect the spark plugs.

Are any spark plugs contaminated with the engine coolant?

--

Go to Step 15

Go to Step 7

7

Are the spark plugs contaminated with engine oil?

--

Go to Step 9

Go to Step 14

8

Check valve stem oil seals and valve guides for wear.

Is the misfire corrected?

--

System OK

Go to Step 9

9

Repair or replace valve stem oil seal and valve guides as necessary.

Is the misfire corrected?

--

System OK

Go to Step 10

10

  1. Remove and inspect the intake manifold and the intake manifold gasket. Refer to Intake Manifold Removal .
  2. Replace the parts as necessary.

Do the symptoms still exist?

--

Go to Step 15

System OK

11

Remove and inspect the intake manifold and the intake manifold gasket. Refer to Intake Manifold Removal .

Was there any damage noted on the intake manifold or the gasket?

--

Go to Step 12

Go to Step 13

12

Replace the parts as necessary.

Do the symptoms still exist?

--

Go to Step 15

System OK

13

Remove and inspect the cylinder head. Refer to Cylinder Head Cleaning and Inspection .

Are the valve guides or the seals damaged?

--

Go to Step 12

Go to Step 14

14

Remove and inspect the piston, the piston rings, and the cylinder for wear.

Are the piston components damaged?

--

Go to Step 12

Go to Step 13

15

Perform a cylinder leak test.

Does the engine pass the cylinder leakdown test?

--

Go to Step 16

Go to Engine Compression Test

16

  1. Install a vacuum gauge to the intake manifold.
  2. Measure the manifold vacuum with the engine at the idle and record the measurement.
  3. Disconnect the vacuum lines from all of the openings of the manifold and plug the connections.
  4. Inspect the manifold vacuum again with the engine at idle.

Are both vacuum readings the same?

--

Go to Step 18

Go to Step 17

17

Inspect each vacuum operated accessory/vacuum line and repair any open or leaking components.

Do the symptoms still exist?

--

Go to Step 19

System OK

18

Using a spray bottle, lightly spray the ignition coil cover and housing while listening to the engine.

Did the engine stumble or stall?

--

Go to Step 19

Go to Step 20

19

Replace the ignition coil, coil housing, or connectors.

Do the symptoms still exist?

--

Go to Step 21

System OK

20

Remove the intake manifold and carefully inspect the intake manifold gasket.

Is there any apparent damage or problem with the gaskets or the sealing surface?

--

Go to Step 21

Go to Step 22

21

Repair or replace the damaged components.

Do the symptoms still exist?

--

Go to Step 22

System OK

22

Measure the camshaft lobe lift.

Is the camshaft within specifications?

--

Go to Step 24

Go to Step 23

23

Replace the camshaft. Refer to Camshaft And Lifter Replacement.

Does the symptom still exist?

--

Go to Step 24

System OK

24

If the problem is still suspected to be mechanical, remove the engine and perform a complete disassembly and inspection.

Is the symptom repaired?

--

System OK

Go to Engine Controls