GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

Ignition Timing

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GENERAL INFORMATION



Late model vehicles, both 4 cylinder and 6 cylinder engines, have Distributorless Ignition Systems (DIS). Because the reluctor wheel is an integral part of the crankshaft, and the crankshaft sensor is mounted in a fixed position, timing adjustment is not possible.

Basic ignition timing is critical to the proper operation of the Electronic Spark Control (ESC) system. Always follow the Vehicle Emission Control Information (VECI) label procedures when adjusting ignition timing.

Some engines will incorporate a magnetic timing probe hole for use with special electronic timing equipment. Consult the manufacturer's instructions for the use of this electronic timing equipment.

Ignition timing is the measurement, in degrees of crankshaft rotation, of the point at which the spark plugs fire in each of the cylinders. It is measured in degrees before or after Top Dead Center (TDC) of the compression stroke.

Because it takes a fraction of a second for the spark plug to ignite the mixture in the cylinder, the spark plug must fire a little before the piston reaches TDC. Otherwise, the mixture will not completely ignited as the piston passes TDC and the full power of the explosion will not be used by the engine.

The timing measurement is given in degrees of crankshaft rotation before the piston reaches TDC (BTDC). If the setting for the ignition timing is 5° BTDC, the spark plug must fire 5° before each piston reaches TDC. This only holds true, however, when the engine is at idle speed.

As the engine speed increases, the pistons go faster. The spark plugs have to ignite the fuel even sooner if it is to be completely ignited when the piston reaches TDC.

If the ignition is set too far advanced (BTDC), the ignition and expansion of the fuel in the cylinder will occur too soon and tend to force the piston down while it is still traveling up. This causes engine ping. If the ignition spark is set too far retarded, after TDC (ATDC), the piston will have already passed TDC and started on its way down when the fuel is ignited. This will cause the piston to be forced down for only a portion of its travel. This will result in poor engine performance and lack of power.

Ignition timing for this engine should be accomplished using the averaging method in which the timing of each cylinder can be brought into closer agreement with the base timing specification.

The averaging method involves the use of a double notched crankshaft pulley. When timing the engine, the coil wire, instead of the Number 1 plug wire, should be used to trigger the timing light. The notch for the No. 1 cylinder is scribed across all three edges of the double sheave pulley. Another notch located 180° away from the No. 1 cylinder notch is scribed only across the center section of the pulley to make it distinguishable from the No. 1 cylinder notch.

Since the trigger signal for the timing light is picked up at the coil wire, each spark firing results in a flash from the timing light. A slight jiggling of the timing notch may be apparent since each cylinder firing is being displayed. Optimum timing of all cylinders is accomplished by centering the total apparent notch width about the correct timing specification.

There are three basic types of timing light available. The first is a simple neon bulb with two wire connections (one for the spark plug and one for the plug wire, connecting the light in series). This type of light is quite dim, and must be held closely to the marks to be seen, but it is quite inexpensive. The second type of light operates from the car's battery. Two alligator clips connect to the battery terminals, while a third wire connects to the spark plug with an adapter. This type of light is more expensive, but the xenon bulb provides a nice bright flash which can even be seen in sunlight. The third type replaces the battery source with 110 volt house current. Some timing lights have other functions built into them, such as dwell meters, tachometers, or remote starting switches. These are convenient, in that they reduce the tangle of wires under the hood, but may duplicate the functions of tools you already have.

Because your car has electronic ignition, you should use a timing light with an inductive pickup. This pickup simply clamps around the Number 1 spark plug wire (in this case, the coil wire), eliminating the adapter. It is not susceptible to crossfiring or false triggering, which may occur with a conventional light due to the greater voltages produced by HEI.

INSPECTION & ADJUSTMENT



1.8L and 2.0L OHV Engines
1982-86 VEHICLES
  1. Refer to the instructions on the emission control sticker inside the engine compartment. follow all instructions on the label.
  2.  
  3. Locate the timing marks on the crankshaft pulley and the front of the engine.
  4.  
  5. Clean off the marks so that you can see them. Chalk or white paint will help to make them more visible.
  6.  
  7. Attach a tachometer to the engine as detailed previously.
  8.  
  9. Disconnect the 4-terminal EST connector at the distributor so that the engine will switch to the bypass timing mode (please refer to Driveability & Emissions Controls for more information).
  10.  
  11. Attach a timing light as per the manufacturer's instructions. Clamp the inductive pick-up around the HIGH TENSION COIL WIRE (not the No. 1 spark plug wire) at the distributor. Before installing the pick-up on the wire, it will be necessary to peel back the protective plastic cover which encases the wire.
  12.  
  13. Loosen the distributor clamp bolt slightly so that the distributor may be rotated as necessary to adjust timing.
  14.  
  15. Check that all wires are clear of the fan and then start the engine. Allow the engine to reach normal operating temperature.
  16.  
  17. Aim the timing light at the marks. A slight jiggling of the notch on the pulley may appear due to the fact that each cylinder is being displayed as it fires. The apparent notch width cannot be reduced by a timing adjustment.
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  19. Center the total apparent notch width about the correct timing mark on the indicator by rotating the distributor housing. This will insure that the average cylinder timing is as close to specifications as possible. Once again, the apparent notch width cannot be reduced by timing adjustment.
  20.  
  21. Turn off the engine and tighten the distributor lock bolt. Start the engine and recheck the timing. Sometimes the distributor will move a little during the tightening process. If the ignition timing is within 1° of the correct setting, that is close enough; a tolerance of up to 2° is permitted by the manufacturer.
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  23. Turn off the engine and disconnect the timing light and the tachometer. Reconnect the 4-terminal EST connector.
  24.  

2.0L (VIN 1) 2.2L, 2.8L and 3.1L Engines
1987-94 VEHICLES

The ignition timing on engines with distributorless ignitions, is controlled by the Electronic Control Module (ECM) or Powertrain Control Module (PCM), as applicable. No adjustments are possible.

2.0L (VIN K and M) Engines
  1. Refer to the underhood emission control label and follow all of the timing instructions if they differ from below.
  2.  
  3. Warm the engine to normal operating temperature.
  4.  
  5. Place the transmission in N or P . Apply the parking brake and block wheels.
  6.  
  7. Air conditioning, cooling fan and choke must be OFF . Do not remove the air cleaner, except as noted.
  8.  
  9. Ground the ALCL connector under the dash by installing a jumper wire between the A and B terminals. The Check Engine light should begin flashing.
  10.  
  11. Connect an inductive timing light to the No. 1 spark plug wire lead and record timing.
  12.  
  13. Connect an inductive timing light to the No. 4 spark plug wire lead and record timing.
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  15. Add the 2 timing numbers and divide by 2 to obtain "average timing". For example: No. 1 timing = 4 degrees and No. 4 timing = 8 degrees; 4 + 8 = 12 / 2 = 6 degrees average timing. If a change is necessary, subtract the average timing from the timing specification to determine the amount of timing change to No. 1 cylinder. For example: if the timing specification is 8 degrees and the average timing is 6 degrees, advance the No. 1 cylinder 2 degrees to set the timing.
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  17. To correct the timing, loosen the distributor hold-down clamp, adjust the distributor and retighten the hold-down bolt.
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  19. Once the timing is properly set, remove the jumper wire from the ALCL connector.
  20.  
  21. If necessary to clear the ECM memory, disconnect the ECM harness from the positive battery pigtail for 10 seconds with the key in the OFF position.
  22.  

 
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