Using the Datastream for Failure Analysis

8-05 Illustrate diagnostic steps and datastream analysis.

Anytime you are faced with a MIL-on drivability complaint there are certain steps you should take on every vehicle. Building and following an organized and logical process during any diagnostic scenario will ensure that no steps are skipped. Staying true to this process will result in reaching a conclusion and fixing the vehicle. Highly skilled technicians are able to utilize the information they have available to develop a repair plan and complete the repair on the vehicle. The following information will help out with different situations that may arise when you are diagnosing failures in vehicles.

When Diagnosing a MIL Issue

Generic OBD II Drive Cycle

A Generic Drive Cycle may reset the monitors to complete. The following Generic Drive Cycle may or may not be helpful. It takes about 30 minutes to complete and completes most noncontinuous monitors. The drive cycle may need to be repeated 2 to 5 times depending on the vehicle’s make, model, and year. Once the procedure is started, perform this drive cycle all at the same time to completion, not in segments.

How to run a Generic OBD II Drive Cycle:

  1. Let the vehicle sit for 8 hours before the test without starting the vehicle.

    •  The cold soak is primarily to allow most EVAP monitors to run.

  2. Connect a scan tool.
  3. Start the vehicle and let it warm up to normal operating temperature.
  4. Drive for at least 10 minutes at highway speeds (55 mph or more).

    •  Watch the monitor display for a change to “Ready” or “Complete.”

  5. Drive for at least 20 minutes in urban traffic with at least four idle periods.

    •  Some monitors will run but not update until the key is cycled off and then back on.

    •  Some monitors will not run until the vehicle is shut off, usually EVAP monitors.

No Code or No Published Diagnostics

No code diagnostics or intermittent problems challenge even the best diagnostic drivability technicians. Verifying the condition is essential and sometimes a major part of the repair. Customer descriptions do not always accurately match what a tech feels or experiences. The best way to find the cause of the complaint is to follow a standard diagnostic strategy. Obtain as much information on the primary input sensors and systems as possible using the scan tool.

Before beginning any diagnosis, review the work order. Read the complaint and test drive to attempt to duplicate the concern and obtain some direction. No code concerns can result from a mechanical or electrical fault. Anything from a failing fuel pump, a sticking intake valve, or an intake valve with excessive carbon can cause intermittent no-code failures. Electrical faults include a misreporting sensor that is out of range for the given condition, but still within its normal operating parameters and often results in engine performance concerns that are difficult to diagnose.

Start with the basics during any diagnosis, including no code/no published diagnostics. Low voltage and poor ground concerns are often overlooked causes of intermittent drivability complaints. Loose, corroded, intermittent shorts to power and ground should not be forgotten. Wiring issues are usually the most overlooked fault in any automotive diagnostic scenario. Cycle the vehicle through cold and hot cycles. Use a spray bottle full of salt water and wiggle test components and the powertrain wiring harness as the temperature changes.

Don’t overlook the obvious. The engine should be able to breathe correctly (intake and exhaust systems), have the correct fuel pressure and volume, and be mechanically sound including the correct valve timing.

Every seasoned drivability tech has a list of PIDs that they monitor for most engine performance concerns. Graphing the PIDs draws a picture of the data which aids in catching glitches in the signals. Once you duplicate the concern, record the data on your scan tool. After returning to the shop, you can analyze the captured PID data. Data analysis is much easier to review than attempt to read live.

Take your time when looking at data streams and compare sensors against each other as the PCM does. Remember the more PIDs you select, the slower the scan tool updates the data. Slow update speeds can result in missing faults. Most scan tools divide the data stream into groups such as Engine 1 and 2, Oxygen Sensor, Misfire, EVAP, and so on. Breaking the data into smaller preselected groups is meant to increase the update rate. Use all the features the scan tool provides: graphing, snapshots, triggers, and data plotting. Also, remember the scan tool may auto range the PID minimum and maximum values when in graph mode, making a slight change appear to be a significant shift in the value.

Note: When recording, some diagnostic tools allow you to modify the recording. If the scanner has an adjustable time frame, set the scanner to record 2/3 of the event before the trigger and 1/3 after the trigger. Once you feel or verify the complaint and start the recording process, the event has already occurred. You need the data that caused the event, not the result.

PID’s that may help with diagnosis include:

No Code Diagnosis in the PID Breakdown

Begin the datastream review in the Generic side of the tool. Generic data cannot be substituted. Whatever the sensor or input is providing is actual. The OEM side of the tool can substitute values causing you to think an input is within range when it is not. It is recommended to view data on every engine performance complaint. Become accustomed to monitoring data and determining what is “normal” and what is “abnormal.” Save a log or a picture of what you found for use on the next vehicle. While some data and component operation is manufacturer specific, a negative temperature coefficient sensor works the same regardless of whether it is on an Audi or a Subaru. What changes is the code set criteria and how the sensor is monitored or the actuator is controlled. Experience is a huge asset when diagnosing drivability complaints. The more practice you get the better your skills will become.

PID Definitions

Engine Speed (RPM)—Engine speed will identify when the concern occurs during a test drive. Engine rpm is a good trigger signal. Typically, rpm is from the Crank Sensor (CKP). This also then provides you with an indication of the CKP signal. Be aware of a signal that drops to 0 rpm while the vehicle is still moving. If the engine shuts off and the wheels are turning the CKP should show an rpm signal. A straight line drop off or a 0 rpm signal when the vehicle is moving is a good indicator of a CKP sensor, connector, or wiring concern. A missing rpm signal will not allow the engine to run.

Calculated Load—Calculated load linearly corresponds to intake manifold vacuum and is used as a load indicator. Think of how a vacuum gauge responds to the intake manifold vacuum when the throttle is held wide open. At wide open throttle (WOT), regardless of the rpm range, the calculated load should be near 100% since manifold vacuum drops to 0 in/Hg. In place of calculated load, some technicians use throttle position as the load indicator. After all, your right foot is the load, and both can be used to indicate WOT.

Absolute Load—(LOAD_ABS) Absolute load linearly correlates to airflow into the engine. The peak value of absolute load directly relates to volumetric efficiency (VE) at WOT. Remember volumetric efficiency is the relationship between the amount of air that enters the engine compared to the theoretical maximum. In other words, absolute load can be used to find an airflow/engine breathing issue or an MAF sensor problem. As rpm and MAF increase under acceleration, you should expect a corresponding increase in absolute load. Absolute load on a naturally aspirated engine should approach 80% of the calculated maximum. To calculate the ideal maximum, multiply the engine size in liters by 39. For example, a 3.0L engine at 80% maximum VE, would be 117 gps at WOT. If LOAD_ABS fails to increase proportionately, or drops with rpm, suspect an engine breathing concern. For example, a vehicle with a restricted exhaust slows airflow into the engine, reducing the engine’s VE/breathing ability. As rpm and MAF increase the absolute load, PID will not increase in proper proportion, failing to reach the ideal 80% maximum volumetric efficiency.

Throttle Position—The TP signal is only affected by the position of the throttle. It cannot be influenced by an MAF sensor or airflow into the engine. Conversely, a faulty TP sensor can affect fuel delivery which can adversely affect the MAF reading by limiting engine rpm (performance) and therefore reducing the airflow over the MAF sensor. The TP sensor is also a good indicator of load. A closed throttle to approximately ¼ (under 1.5 volts) throttle suggests light load. Half throttle (1.5 to 2.5 volts) indicates a medium to heavy load. If the TP sensor equals WOT (near 4.5 volts), the vehicle is under full load.

Short-Term Fuel Trim—Short-term fuel trim (STFT) is the correction in percentage the PCM is making to the injector pulse width based on the difference between what the PCM expects to see and the adjustment needed from what the O2 sensor is reporting. In other words, STFT is a calculation based on the O2 sensor’s input to the PCM during closed-loop operation. Short-term fuel trim moves rapidly compared to long-term fuel trim (LTFT). There is a slight delay in its movement, however, due to O2 sensor response time. STFT is affected by all the primary fuel control inputs including O2, MAF, MAP, TPS, fuel pressure, airflow, and a host of other factors. The maximum correction should remain under 10%. Due to multiple fuel calculation tables, STFT should be checked at different throttle openings and loads, known as look up tables in the PCM. An STFT that falls within specification at idle may not be under load, and vice versa. Any fuel trim over 10% is reason for concern and will require diagnosis.

Long-Term Fuel Trim (LTFT)—Long-term fuel trim is the same as short-term fuel trim but has a slower response. LTFT is calculated off STFT and O2 sensor input. The goal of LTFT is to keep STFT as close to 0 as possible. LTFT is a historical correction stored in the PCM’s memory, offering insights into any fuel control issues the PCM has compensated for. STFT and LTFT vary depending on the engine’s operating range. Check fuel trim at idle, 1500 rpm, and 2500 rpm. Different fuel tables exist for various load and rpm ranges. Fuel trim that is okay at 1 rpm may not be at another, changing your diagnostic direction.

Mass Airflow (MAF)—MAF is a critical load input for fuel calculation, ignition timing, variable valve timing, transmission shift points, and pressures. The MAF sensor measures airflow and air density directly. MAF is affected by a variety of faults including, but not limited to, volumetric efficiency (base engine concern), restricted airflow, engine misfire, contamination, and airflow.

Manifold Absolute Pressure (MAP)—Manifold Absolute Pressure is a critical primary load input used to calculate air intake into the engine. Using the manifold pressure, engine size, rpm, and the engine’s volumetric efficiency, the PCM can determine the amount of air entering the engine. The MAP sensor is a primary indicator of base engine and additional concerns. For example, two faults both can trick the MAP sensor into believing the engine is under load under different operating conditions. A vacuum leak will increase the MAP output at idle (higher load) but not off idle at higher rpm. A restricted exhaust, unless severely plugged, indicates to the PCM the engine is under a heavy load (low vacuum in the intake) during normal load and speed. Any Volumetric Efficiency (VE) fault with the base engine will produce a lower manifold vacuum, increasing manifold pressure and tricking the PCM into thinking the engine is under load causing the PCM to over fuel. To quickly check a MAP sensor for accuracy with the Key On Engine Off (KOEO), the sensor should read atmospheric pressure. Compare the MAP to the Barometric sensor (BARO) reading to see if they match.

Oxygen Sensors (O2) or Air Fuel Sensors (AFS)—O2 or AFS are primary inputs to the PCM for fuel control. They report to the PCM everything that is occurring upstream. During a diagnostic, it is imperative that their operation and rationality is checked.

Injector Pulse Width (IPW)—Injector pulse width, if available, is the result and correction after all the upstream sensors report to the PCM. Any defect not already accounted for by the O2 sensor is displayed in the pulse width. Meaning if the engine is running rich, the pulse width will decrease from normal and if the engine is running lean, the pulse will increase from its base.

  • Start the Engine
    • Monitor the datastream.
    • Most PIDs will change once the engine is running.
    • Some will change quickly such as RPM, pressure sensors, load sensors like MAP or MAF, and some position sensors such as CMP or CKP.
    • Others should change gradually (temperature sensors, oxygen sensors) as the engine warms.
    • Compare the KOEO to KOER data looking for any obvious abnormal or inconsistent readings.
    • If there is more than one sensor measuring the same input, such as O2 sensor, CMP sensors, TP or APP sensors, compare them against each other,
    • If a manufacturer supplies a desired PID value and an actual PID value for a sensor, select both and check the difference.
    • This is the same test that the PCM uses; it issues a command and monitors the result,
  • Refer to Service Information
    • If the data points to an obvious problem start there, refer to the code set information and the pinpoint test or flow chart for the code.
    • Compare what the manufacturer’s flow chart is asking for to what you are viewing on the scan tool.
    • Record any test data that you perform for reference.

      a. Are they related?

      b. Is the data causing the fault or the fault causing the data discrepancy?

    • If available, for the code or system you are testing, use bi-directional tests to actuators or control system operation while monitoring the response with the scan tools PID data (SKILL DRILL 8-1 and SKILL DRILL 8-2).
SKILL DRILL 8-1Where to Start with DTC Diagnosis
  1. Gather Information on the vehicle and the customer complaint.
  2. Test drive to verify the customer complaint and to inspect the instrument cluster for signs of a MIL light on.
  3. Perform a simple visual inspection under the hood to identify any obvious faults as often simple faults cause the MIL.
  4. Scan all modules for DTCs, record all DTCs, and pay attention to the common themes of the codes to help direct your diagnostic plan.
  5. Along with looking at the module in question, it is also advisable to run an All Module DTC check.
  6. Next, be sure to read and record the Freeze Frame data before clearing the codes to extinguish the MIL.
  7. Check Service Information for Technical Service Bulletins (TSBs) and Recalls that are code- or symptom-related.
  8. After it is determined that a TSB doesn’t apply to the issue, research the problem and related information so that you can develop a plan.
  9. Use the Trouble Codes as a road map to develop a plan and fix the vehicle.
  10. After completing the repair, document what was done to the vehicle so that if there are issues the technician can backtrack the process.
SKILL DRILL 8-2Where to Start with No Code Diagnosis
  1. Gather required vehicle information and customer description of the problem.
  2. Test drive to verify the customer complaint, or if the vehicle doesn’t start verify that it doesn’t start.
  3. Perform a visual inspection of the affected systems. In case of a no-start, look for loose battery connections, leaking battery, unplugged connectors, pest infestation, disassembled pieces, and anything that might be related to the affected complaint.
  4. Scan all modules for DTCs, check fluids, and refer to vehicle history.
  5. Check Service Information for Technical Service Bulletins (TSBs) that are symptom-related.
  6. Check Service Information for open Recalls. If there is a common issue that you believe may be the source of the complaint, send the customer and vehicle to the dealership so that the repair can repaired at the manufacturer’s cost.
  7. If no DTCs are found, you must attempt to determine and isolate the source of the fault.

    a. Fuel

    i.   Is the engine running rich or lean?

    ii.  Is fuel pressure and volume correct?

    iii. Check at idle and under load.

    iv. Check fuel pump current draw and waveform analysis for possible failing fuel pump.

    v.  Contamination, alcohol content, wrong RVP

    b. Ignition

    i.   Cause of the misfire

    ii.  Mechanical lack of compression

    iii. Electric lack of spark

    iv. Quick tests to check ignition system operation, for example: Spark KV, current ramping all coils, etc.

    c. Mechanical

    i.   Often overlooked until the end, even by experienced technicians

    ii.  Can the engine breathe correctly? Air filter, air intake, exhaust backpressure, etc.

    iii. Compression cylinder or bank

    iv.  Exhaust backpressure for a bank

    v.   Individual cylinder, complete bank, or engine vacuum leak

    vi.  Jumped timing chain or belt, stuck VVT component

    vii. Carbon buildup, borescope check

    d. Emission Systems

    i.   Stuck open Purge Valve, EGR Valve

    ii.  Skewed sensor(s) within DTC setting limits but out of range for operating condition of the engine

    iii. Faulty EVAP system component, Faulty AIR system component, Plugged intake manifold EGR orifices

    e. Electrical

    i.   EMI/RFI faulty ignition coil

    ii.  Alternator bad diode

    iii. Low battery/charging system voltage

    iv.  Loose/corroded battery terminals

    v.   Missing/blown fuses

    vi.  Inoperative relay

    vii. Partially connected electrical harness or component connectors

  8. Once an area is found to be deficient, the technician must use their diagnostic abilities to determine what caused that component or components to fail. This may included finding out how the system works to repair it or doing more in-depth diagnosis.
  9. Complete the repair, using a scan tool to clear vehicle adaptives, and then verify the vehicle operates as it should.
TECHNICIAN TIP

Leaving a DTC stored in memory may prevent a monitor from running. Some monitors will not run with DTCs stored in the PCM’s memory. Consult the vehicle’s service information to verify monitor operation.