GM Service Manual Online
For 1990-2009 cars only

Worn or dirty plugs give satisfactory operation at idling speed, but under operating conditions they frequently fail. Faulty plugs are indicated in a number of ways poor fuel economy, power loss, loss of speed, hard starting and general poor engine performance. Spark plugs may also fail due to carbon fouling, excessive plug gap, or broken insulators. Fouled plugs are identified by black carbon deposits. The black deposits are usually the result of slow-speed driving and short runs where sufficient engine temperature is seldom reached. Worn pistons, rings, faulty ignition, over-rich air/fuel mixture and spark plugs which are too cold will also result in carbon deposits. Excessive gap wear, on plugs of low mileage usually indicated the engine is operating at high speeds or loads that are consistently greater than normal or that a plug which is too hot is being used. Electrode wear may also be the result of plug overheating due to insufficient spark plug torque or too much spark plug torque. An excessively lean air/fuel mixture will also result in excessive electrode wear.

Broken insulator are usually the result of improper installation or carelessness when regapping the plug. Broken upper insulators usually result from a poor fitting wrench or an outside impact. The cracked insulator may not show up right away, but will as soon as oil or moisture penetrates the crack. The crack may not be visible. Broken lower insulators usually result from carelessness when regapping and generally are visible. This type of break may result from the plug operating too hot, which may happen in periods of high-speed operation or under heavy loads. When regapping a spark plug, always make the gap adjustment by bending the ground (side) electrode. Spark plugs with broken insulators should always by replaced.

Resistor type spark plugs with tapered seats are used. No gasket is needed on these plugs. These spark plugs are a High Efficiency (HE) spark plug that has a fine wire electrode, nickel plated shell and a platinum pad across from the electrode.


Object Number: 1053  Size: SH
  1. Normal service is a mixture of idling, low speed, and high speed operation.
  2. • Occasional high speed driving is essential to good spark plug performance.
    • Increased combustion heat burns away excess carbon deposits, which accumulate from frequent idling or stop-and-go driving.
  3. The heat resistant, insulating boot covers the spark plug terminal.
  4. • The boot extends over a portion of the spark plug insulator.
    • These boots prevent engine misfiring.
  5. Corona discharge is a steady blue light appearing (in darkness) around the insulator, just above the shell crimp.
  6. • Do not mistake this for a shorted insulator.
    • This is visible evidence of a high-tension field.
    • This has no effect on ignition system performance.
    • This discharge repels dust particles.
    • This leaves a clear ring on the insulator just above the shell.
    • This is not evidence that combustion gases are escaping.

Spark Plug Diagnosis

Step

Action

Value(s)

Yes

No

DEFINITION: A visual inspection of the spark plug firing end.

1

  1. Remove the spark plug from the engine.
  2. Note which cylinder the plug is from.

Do you see a brown to grayish tan deposit with a slight electrode wear?

--

Go to Step 2

Go to Step 3

2

  1. This is normal wear clean, regap and install the plug.
  2. Tighten the plug, refer to Fastener Tightening Specifications .

Is the repair complete?

1.524 mm

(0.060 in)

System OK

--

3

Do you see dry, fluffy, black carbon deposits?

--

Go to Step 4

Go to Step 6

4

This indicates poor ignition output, refer to Spark Plug Wire Resistance .

Is the ignition coil(s) to the spark plug connections OK?

--

--

Go to Step 5

5

Repair or install new parts as necessary.

Is the repair complete?

--

System OK

--

6

Do you see wet, oily deposits with very little electrode wear?

--

Go to Step 7

Go to Step 8

7

  1. This results from either the break-in of a new or recently overhauled engine or from failed piston rings.
  2. Degrease, gap and reinstall the spark plug.

Is the repair complete?

1.524 mm

0.060 in

System OK

Go to Step 11

8

Do you see red, brown, yellow and/or white colored coatings on the insulator?

--

Go to Step 9

Go to Step 11

9

These are the by-products of combustion.

Is the insulator heavily coated?

--

Go to Step 10

Go to Step 2

10

  1. Gap and install new spark plug(s).
  2. Tighten the spark plug, refer to Fastener Tightening Specifications .

Is the repair complete?

1.524 mm

(0.060 in)

System OK

--

11

Are there heavy deposits of dark colored coatings, especially on the side of the plug facing the intake valve?

--

Go to Step 12

Go to Step 13

12

This is the result of failed intake valve seats or from excessive valve stem to valve guide clearances.

This may not occur in all cylinders.

Repair or install new parts as necessary.

Is the repair complets?

--

System OK

--

13

Is there a shiny, yellow glaze coating on the spark plug insulator?

--

Go to Step 14

Go to Step 15

14

These are the melted by-products of combustion.

  1. Avoid wide open throttle after long periods of low speed operation.
  2. Gap and install new spark plug(s). Refer to Fastener Tightening Specifications .

Is the repair complete?

1.524 mm

(0.060 in)

System OK

--

15

Are the insulator tips blistered or burned?

Are the electrodes badly eroded?

--

Go to Step 16

--

16

This is the result of overheating.

The spark plug heat range may be too hot.

The fuel octane rating may be to low for the application, refer the customer to the Owner Manual for correct fuel rating.

Is the plug heat range correct?

--

Go to Step 17

Go to Step 20

17

Check the torque value to ensure good spark plug seat contact.

Are the spark plugs tightened according to specifications? Refer to Fastener Tightening Specifications

--

Go to Step 18

Go to Step 10

18

Check for a misfiring engine, refer to DTC P0230 Fuel Pump Relay Control Circuit for V6 or DTC P0300 Engine Misfire Detected for V8 in Engine Controls.

Is the vehicle operating properly?.

--

Go to Step 19

Go to Step 10

19

Check for a lean/rich fuel mixture, refer to Fuel System Diagnosis for V6 or Fuel System Diagnosis for V8 in Engine Controls.

Repair or install new parts as necessary.

Is the repair complete?

--

System OK

--

20

Gap and install the correct spark plug(s). Refer to Spark Plug Usage .

Is the repair complete?

--

System OK

--