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

Are there any obvious mechanical noises?

--

Go to Engine Noise Diagnosis

Go to Step 2

2

Visually inspect valve train components spark plugs, wires, and hoses for proper hook up or damage.

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

--

Go to Step 3

Go to Step 4

3

Repair or replace the parts as necessary.

Do the symptoms still exist?

--

Go to Step 4

System OK

4

Remove and inspect the spark plugs.

Are any spark plugs contaminated with engine coolant?

--

Go to Step 10

Go to Step 5

5

Are the spark plugs contaminated with engine oil?

--

Go to Step 6

Go to Step 11

6

Check valve stem oil seals and valve guides for wear.

Are the valve guides or the seals damaged?

--

Go to Step 7

Go to Step 8

7

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

Does the symptoms still exist?

--

Go to Step 8

System OK

8

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

Are the intake manifold or the intake manifold gasket damaged?

--

Go to Step 9

Go to Step 10

9

Replace the parts as necessary

Do the symptoms still exist?

--

Go to Step 12

System OK

10

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

Are the valve guides or the seals damaged?

--

Go to Step 11

Go to Step 12

11

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

Are the piston components damaged?

--

Go to Step 10

Go to Step 12

12

Perform a cylinder leak test.

Does the engine pass the cylinder leakdown test?

--

Go to Step 13

Go to Engine Unit Repair

13

  1. Install a vacuum gauge on 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 then plug the connections.
  4. Inspect the manifold vacuum again with the engine at idle.

Are both vacuum readings the same?

--

Go to Step 17

Go to Step 14

14

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

Do the symptoms still exist?

--

Go to Step 15

System OK

15

Using a spray bottle, lightly spray the spark plug wires and the ignition coils while listening to the engine.

Did the engine stumble or stall?

--

Go to Step 16

Go to Step 17

16

Replace the spark plug wires and/or the ignition coils.

Do the symptoms still exist?

--

Go to Step 17

System OK

17

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 18

Go to Step 19

18

Repair or replace the damaged components.

Do the symptoms still exist?

--

Go to Step 20

System OK

19

Rotate the engine and measure the camshaft lobe lift.

Is the camshaft within specifications?

--

Go to Step 21

Go to Step 20

20

Replace the camshaft. Refer to Camshaft Replacement .

Does the symptom still exist?

--

Go to Step 21

System OK

21

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 Electrical