Dec 082016
 

1998 Chevy SilveradoExperiencing a miss in engine at idle and at speed it misses and sounds like popcorn popping in the muffler. — Replaced plugs and wires, distributor cap and rotor, finally replaced entire distributor. “NOT FIXED”… Replaced Fuel injection spider. “NOT FIXED”…Help Please!!!


RESPONSE

Enhanced Ignition System Description

The ignition system initiates combustion by providing a spark to ignite the compressed air and fuel mixture at the correct time. In order to provide an improved engine performance, fuel economy, and control of exhaust emissions, the control module controls the distributor spark advance, or timing, with the ignition control (IC) system.

The ignition system uses a primary and secondary sub system in order to accomplish the timed spark distribution. The primary system consists of a low voltage trigger device which determines the base timing. This signal is modified by the ignition control driver (ICD) module. The signal travels to either or both the engine and transmission processor, or control module, for base timing reference. Another signal is sent back to the ignition control driver (ICD) module, which has been adjusted by the control module, advanced or retarded, in order to trigger the coil, according to the requirements of the engine.

The secondary system consists of the ignition coil which has primary (low voltage) windings and secondary (high voltage) windings. The secondary side of the ignition coil generates a high voltage which high tension spark plug wires deliver to the spark plugs.

The control module controller now controls the ignition control (IC) and bypass functions.

In order to properly control the ignition or combustion timing, the control module needs to know the following things:

System Operation

 The enhanced ignition system used on all OBD II engines somewhat resembles the current distributor ignition (DI) system described in the Ignition Systems. However, the system has been greatly enhanced in order to make it compatible with the new OBD II regulations. The enhanced ignition system provides a spark at precisely the correct time in order to ignite the air and fuel mixture for optimum performance and fuel economy. The system consists of the following components:
  • Vehicle control module (VCM)
  • Distributor
  • Ignition coil driver module
  • Ignition coil
  • Crankshaft position (CKP) sensor

This system does not use the ignition module used on the DI systems in the past. The VCM controller now controls the ignition control (IC) and bypass functions.

The crankshaft sensor, located in the front engine cover, is perpendicular to a target wheel attached to the crankshaft. The target wheel is equipped with slots situated 60 degrees apart. As the crankshaft rotates, the target wheel rotates past the crankshaft position sensor. The rising and falling edges created by the slots cause a signal to be sent back to the VCM. This signal occurs three times per crankshaft revolution and is referred to as the 3x signal for V6 applications. The signal occurs four times per crankshaft revolution and is referred to as the 4x signal for V8 applications.

The VCM then utilizes this 3x (V6) or 4x (V8) signal in order to provide the correct spark to the engine by way of the single coil driver module. The single coil driver module is basically an electronic switch that when commanded by the VCM, causes the primary coil voltage to breakdown, energizing the secondary coil and providing a spark via the coil wire to the distributor cap. The distributor consists of the following components:

  • Cap and rotor
  • Camshaft position sensor
  • Gear drive and shaft

The camshaft drives the distributor shaft which rotates providing a spark to the correct cylinder by way of the cap and rotor. The camshaft position (CMP) sensor functions much like the crankshaft sensor previously described but provides only a 1x signal to the VCM. That is, for every 2 rotations of the crankshaft, there is 1 rotation of the camshaft. Note that the camshaft position sensor will not affect driveability. The sole purpose of the camshaft position sensor is to provide the VCM with the necessary information for the misfire diagnostic trouble codes (DTCs).

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