Intermittent Diagnostic Techniques

Intermittent Diagnostic Techniques

Intermittent diagnostic techniques help find and isolate the root cause of intermittent faults associated with the Electronic Engine Control System. The information is organized to help find the fault and perform the repair. The process of finding and isolating an intermittent starts with recreating a fault symptom, accumulating PCM data and comparing that data to typical values, then analyzing the results. Refer to the scan tool users manual for functions described below.

Before proceeding, be sure that:

- Customary mechanical system tests and inspections do not reveal a concern. (Remember, mechanical component conditions can make a PCM system react abnormally.)
- Technical Service Bulletins (TSBs) and OASIS messages, if available, are reviewed.
- Quick Test and associated Diagnostic Subroutines have been completed without finding a fault, and the symptom is still present.

Recreating the Fault

Conditions To Recreate Fault:

Recreating the fault is the first step in isolating the cause of the intermittent symptom. A thorough investigation should start with the customer information worksheet located in the Introduction. 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 DTC). Listed below are some of the conditions for recreating the fault:

Accumulating PCM Data

PCM data can be accumulated in a number of ways. This includes circuit measurements with a DVOM or scan tool PID data. Acquisition of PCM PID data using a scan tool is one of the easiest ways to gather information. Gather as much data as possible when the fault is occurring to prevent improper diagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent fault. Compare this data with the known good data values located in the Typical Diagnostic Reference Values. This will require recording data in four conditions for comparison: 1) KOEO, 2) HOT IDLE, 3) 48 km/h (30 mph) and 4) 89 km/h (55 mph).

Analyzing Data From Playback of Stored PIDs

Figure 1: Table Format:

Figure 2: Graph Format:

Look for abnormal events 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 TP, MAF and 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 IAC, RPM and SPARK ADV PID.

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 fault may exist.

Table Format (Figure 1): Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values. (Refer to the following TP example). Notice the major jump in the TP voltage while scrolling through the information. This example would require a smooth and progressive accelerator pedal travel during a key on and engine off mode.

Graph Format (Figure 2): 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. (Refer to the following TP example.) This example would require a smooth and progressive accelerator pedal with the key on and the engine off.

Peripheral Inputs

Some signals may require certain peripherals or auxiliary tools for diagnosis. These tools include the Auxiliary Adapter and Pressure/Vacuum Adapter. In some cases, these devices can be inserted into the measurement jacks of the scan tool or multimeter. For example, connecting an electronic fuel pressure gauge to monitor and record the fuel pressure voltage reading and capturing the data would help find the fault.

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 Typical Diagnostic Reference Values in Section 6. The charts apply to different vehicle applications (i.e., model, engine, transmission, etc.).