Intermittent Symptoms

OVERVIEW
Intermittent diagnostic techniques help find and isolate the root cause of intermittent faults associated with the EEC-V system. The material is organized to help find the fault and perform the repair. There are examples that illustrate the diagnostic techniques. The process of finding and isolating an intermittent, starts with recreating a fault symptom, accumulating Powertrain Control Module (PCM) data and comparing that data to typical values and analyzing the results.

Before proceeding, be sure that:

- Customary mechanical system tests and inspections do not reveal a problem. (Remember, mechanical component problems can make a PCM system react abnormally.)

- Review Technical Service Bulletins (TSBs) if available.

- Quick Test and associated pinpoint diagnosis have been completed without finding a fault, and the symptom is still occurring.

RECREATING THE FAULT
Recreating the fault is the first step in isolating the cause of the intermittent symptom. If Freeze Frame Data is available, it may help in recreating the conditions at the time of a Malfunction Indicator Lamp Diagnostic Trouble Code (MIL-type DTC). Listed below are some of the conditions for recreating the fault.

Conditions to Recreate the Fault

Engine Type Conditions
- Engine Temperature
- Engine RPM
- Engine Load
- Engine idle/acceleration/decel

Non-Engine Type Conditions
- Ambient Temperature
- Moisture Conditions
- Road Conditions (smooth, bumpy)

ACCUMULATING PCM DATA
Powertrain Control Module (PCM) data can be accumulated in a number of ways. Gather as much data as possible when the malfunction is occurring to prevent misdiagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent fault. Reference the known good data values located in the Diagnostic Reference Value Charts.

This will require recording data in four conditions for comparison:
1. Key ON Engine OFF (KOEO)
2. HOT IDLE
3. 30 MPH
4. 55 MPH

Acquisition of PCM Parameter Identification (PID) data using a scan tool is one of the easiest ways to gather information.

ANALYZING DATA FROM STORED PIDs
Look for abnormal behavior or values that are clearly incorrect. Inspect the signals for abrupt or unexpected changes. For example, during a steady cruise most of the sensor values should be relatively stable. Sensors such as Throttle Position (TP), Mass Air Flow (MAF) and Engine Speed (RPM) that change abruptly when the vehicle is traveling at a constant speed are clues to a possible fault area.

Look for agreement in related signals. For example, if TP is changed during acceleration, a corresponding change should occur in Idle Air Control (IAC), RPM and SPARK ADV PIDs.

Make sure the signals act in proper sequence. An increase in RPM after the TP is increased is expected. However, if RPM increases without a TP change, then a problem may exist.

Possible Fault - Table Format:

Table Format: Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values. Notice the major jump in the TP voltage while scrolling through the information. This example would require a smooth and progressive throttle pedal travel during a key ON and engine OFF mode.

Possible Fault - Graph Format:

Graph Format: Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the linear lines showing the transformation of values to the line graph. This example would require a smooth and progressive accelerator pedal with the key ON and engine OFF.

PERIPHERAL INPUTS
Some signals may require certain peripherals or auxiliary tools to aid in diagnostics. In some cases, these devices can be inserted into the measurement jacks of the scan tool or Digital Volt-Ohm Multimeter (DVOM). The New Generation STAR (NGS) is capable of recording the value from the measurement jacks on the NGS while storing the value from other PIDs.

COMPARING PCM DATA
After the PCM values have been acquired, it is necessary to determine the fault area. Typically, it will require the comparison of the actual values from the vehicle to the typical values from the PCM Reference Value Charts.