Intermittent Diagnostic Techniques
Intermittent diagnostic techniques help find and isolate the root cause of intermittent concerns associated with the EEC) system. The information is organized to help find the concern and carry out the repair. The process of finding and isolating an intermittent concern 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 manufacturer's instruction service information for the functions described below.
Before proceeding, be sure that:
- Customary mechanical system tests and inspections do not reveal a concern. Mechanical component conditions can make a PCM system react abnormally.
- TSB and OASIS messages, if available, are reviewed.
- Quick Test and associated diagnostic subroutines have been completed without finding a concern, and the symptom is still present.
Recreating the Fault
Recreating the concern 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 MILDTC. Listed below are some of the conditions for recreating the concern:
CONDITIONS TO RECREATE FAULT
| Engine Type Conditions | Non-Engine Type Conditions |
|---|---|
| Engine Temperature | Ambient Temperature |
| Engine RPM | Moisture Conditions |
| Engine Load | Road Conditions (Smooth-Bumpy) |
| Engine Idle/Accel/Deceleration |
Accumulating PCM Data
The PCM data can be accumulated in a number of ways. This includes circuit measurements with a DMM or scan tool PID data. Acquisition of PCMPID data using a scan tool is one of the easiest ways to gather information. Gather as much data as possible when the concern is occurring to prevent improper diagnosis. Data should be accumulated during different operating conditions and based on the customer description of the intermittent concern. Compare this data with the known good data values.
Peripheral Inputs
Some signals may require certain peripherals or auxiliary tools for diagnosis. In some cases, these devices can be inserted into the measurement jacks of the scan tool or DMM. 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 are acquired, it is necessary to determine the concern area. This typically requires the comparison of the actual values from the vehicle to known good data values.
Analyzing PCM Data
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, as well as an RPM that changes abruptly when the vehicle is traveling at a constant speed, are clues to a possible concern area.
Look for an agreement in related signals. For example, if the APP1 or APP2 changes during acceleration, a corresponding change should occur in RPM and SPARK ADV PID.
Make sure the signals act in proper sequence. An increase in RPM after the TP1 and TP2 increases is expected. If the RPM increases without a TP1 and TP2 change, a concern may exist.
The PID values are not always captured from the same execution loop. Depending on the number of PIDs acquired, the sample rate may be 60 ms or longer. For example, the ETC_ACT reading will always lag behind the ETC_DSD reading due to the physical time to move the throttle plate. This is an expected difference between ETC_ACT and ETC_DSD during these events.
Scroll through the PID data while analyzing the information. Look for sudden drops or spikes in the values.