Some things never change. For those who like to think about significant anniversaries, we are now in 2016, and that makes the introduction of OBD II 20 years old. Many articles have been written about the power of using an inexpensive OBD II generic scan tool as a diagnostic starting point, but I’m amazed at how few technicians embrace the concept.
In 2014 and 2015, Robert Bosch sponsored mobile training vehicles with the goal of educating technicians about new technology and testing their knowledge in a full-immersion 3D virtual garage. Technicians were presented with common driveability issues and had the opportunity to compete with other technicians. The initial diagnostic data presented for decision-making was recorded using an OBD II generic scan tool and followed a solid diagnostic approach: Verify the customer complaint, retrieve fault codes, record the information (including freeze frame data), research the fault, make a visual inspection and finally baseline the fuel trim data to help in the decision-making process.
Baselining the fuel trim data is a simple four-step diagnostic process: 1. Connect an OBD II generic scan tool, preferably with recording capabilities. 2. Monitor and record the fuel trim values in
the four common operating ranges—idle speed, light load (20 to 30 mph), moderate load (40 to 50 mph) and heavy load (60 to 70 mph). 3. Analyze the collected data. 4. Use the information to target the next diagnostic steps.
Fig. 1 above shows the fuel trim data collected on a vehicle with the Check Engine light on and fault codes P0171 (Bank 1 System Lean) and P0174 (Bank 2 System Lean) present. Typical service information suggestions for those codes might be plugged or dirty fuel filter, damaged or worn fuel pump, leaking or contaminated fuel injectors, low fuel pressure, evap purge solenoid leak, EGR system issues, vacuum leaks, PCV issues, damage or contaminated mass airflow (MAF) sensor, etc.
Fig. 2 on page 24 shows a typical gasoline direct injection (GDI) system layout with common engine management sensors and actuators. Note that the subject vehicle has two banks, which means we have multiple upstream oxygen sensors present.
Now, just like the technicians on the mobile training vehicle, you have a decision to make. Based on the fault codes and information from the four-step fuel trim recording, what component or system would you test from Fig. 2? In the 3D virtual garage we narrowed this down to eight possibilities—purge solenoid, GDI high-pressure pump, low-pressure pump in the fuel tank, GDI injector, ignition coils, intake leaks, MAF sensor and upstream oxygen sensors.
So what did you decide? If you said let’s check the intake area for leaks with a smoke machine, you were in the majority of the technicians participating in the Bosch challenge…and no leaks would have been found. How do we know this? Unbeknownst to the participants, we were tracking the decisions they were making in the virtual garage, with the goal of understanding how technicians solve driveability issues.
Here’s the million dollar question: What made you decide to check for intake leaks? The answer from most technicians was simple: This is a common problem, and they wanted to eliminate it first. This would not be a terrible answer, but the fuel trim data did not support that choice.
Let’s go back to Fig. 1 and take some time to understand the fuel trim data we collected during the road test. In this example, the fuel trim data was failing in both banks and every operating range—idle speed, light load, moderate load and heavy load—which means the lean condition was present in all driving conditions.
What are your thoughts about an intake air leak now? Fig. 3 on page 26 shows the values you’ll likely see on a vehicle with air intake leaks. The fuel trim values will be high at idle and would clean up in higher operating ranges. It’s possible to see slightly elevated fuel trim values at higher operating ranges on MAF sensor-equipped vehicles, but this would be due to the introduction of unmeasured or false air. In the Fig. 3 example, the fuel trim values are not high enough to set a P0171 and/or P0174.
Now let’s go back to Fig. 2 and rethink the next diagnostic steps. Which of the eight listed components can affect fuel trim values in all operating ranges? If you answered MAF sensor, low-pressure fuel pump and possibly the GDI high-pressure pump, you’re on the right track.
Leaking or contaminated fuel injectors were listed as possible causes for these fault codes, but leaking fuel injectors would create a richer-than-normal air/fuel mixture. The fuel trim values would be on the negative side and we would likely have a P0172/P0175 (Bank 1 and 2 Rich). Clogged or contaminated injectors should be checked, but only after diagnosing the components listed in the previous paragraph.
It’s possible to narrow down which item should be tested next by looking at the fuel trim data again, but this requires some understanding of the three possible components. Let’s start with the low-pressure fuel pump. The list of possible causes above suggests that a plugged fuel filter, worn fuel pump or low fuel pressure might create a P0171 and/or P0174. If any of these occurred, the most likely result would be insufficient fuel volume to the GDI high-pressure pump.
The result of not supplying sufficient fuel volume to the GDI high-pressure pump would be that the powertrain control module (PCM) would set a target pressure that should be maintained in the high-pressure fuel rail, which is adjusted for various operating conditions. The fuel pressure sensor (FPS) provides the actual rail pressure data back to the PCM. At idle and light load ranges, less volume is required from the low-pressure pump to maintain the desired rail pressure. Increased vehicle loads demand greater rail pressure and more volume from the low-pressure fuel pump.
If you’re wondering where this discussion is going, there’s another question for you to ponder: If the PCM determines that the desired pressure is not within specification, what fault code will be set? On a GDI vehicle, the most likely fault code will be a P0087 (Fuel Rail System Pressure Too Low) and not a P0171/P0174. For additional GDI information, check out “Service Tips for GDI Engines” in the December 2015 issue of Motor.
We just discussed a GDI example, but would the results be different for a port fuel injected vehicle? The answer is yes. The low-pressure fuel pump volume discussion is basically the same—idle requires less volume to the fuel injectors and higher loads require greater volume. Depending on the condition of the low-pressure fuel pump, the fuel trim values may be only slightly elevated at idle, but get worse with greater load. If the fuel pump is in really bad shape, the fuel trim values will be high enough to set a fault code and would not perform very well.
Once again, the fuel trim data from Fig. 1 shows a significant adjustment in all operating ranges, which leaves us with the MAF sensor. The MAF sensor communicates the volume of air entering the engine, which is used by the PCM to determine the correct injector pulse width. In this example, the PCM needs to increase the injector pulse width to reach the correct air/fuel ratio.
How do we test the MAF sensor? Again, an OBD II generic scan tool will provide the data we need. Fig. 4 on page 28 shows the MAF sensor data at 2.0 grams/second (g/S) at idle, with a specification of 2.00 to 5.00 g/S KOER at idle. This specification was taken directly from real-world service information. Unfortunately, the specifications provided in service information are not always precise, and there are times when you must use alternative methods for greater accuracy. The example vehicle in the 3D virtual garage had a 2.5L GDI-equipped engine, which means the more precise target should be 2.5 g/S at idle. In this example, the MAF sensor is underestimating the airflow by approximately 20%, and is the reason for the positive fuel trim values at idle.
To check the airflow in all operating ranges, you need to record the MAF sensor data in all operating ranges, which would include a wide-open throttle (WOT) acceleration, and then compare the data to a volumetric efficiency (VE) chart or calculator, which can be found in numerous places online.
In our example vehicle, the MAF sensor was dirty and underestimating airflow in all operating ranges, which caused the P0171/P0174 codes to set. The MAF sensor was listed as a possible cause, but in order to make an efficient repair, it’s important to understand how fuel trim data can help point you in the proper direction.
Using fuel trim to help diagnose vehicles takes practice, but you also need to understand how each component affects fuel trim when in operation. For example, let’s take a look at the purge solenoid in Fig. 1. In normal operation, the purge solenoid is commanded closed and the PCM commands it open to purge fuel vapors from the charcoal canister. At idle, intake manifold vacuum is present on the engine side of the solenoid and the gas vapor side should be close to atmospheric pressure. Depending on recent refueling events, ambient temperature and a few other variables, the charcoal canister may be full or storing only a small amount of gas vapors. If the purge solenoid was stuck open, in which operating range would the fuel trims have a greater effect? If you answered at idle you’re right on the money. Keep in mind the fuel trims might be positive or negative, depending on the fuel vapor present in the charcoal canister.
Fig. 5 below shows the change in fuel trim at idle with the purge solenoid commanded open (blue trace). At approximately 13.65 seconds, the short-term fuel trim (STFT, green trace) begins to go negative and quickly goes to −30%, which indicates that the charcoal canister is rich. In approximately 40 seconds—or just before 54.6 seconds—STFT begins to move in a positive direction, and within about 14 seconds, no fuel vapors are being purged from the charcoal canister. In essence, we have a large vacuum leak. Once the purge solenoid is commanded closed, STFT returns to zero.
The key point is this: If your four-step fuel trim recording shows an issue mostly at idle, the purge solenoid could be stuck open and the STFT values might be positive or negative. The purge solenoid will have less of an impact at higher load ranges.
Two additional pieces of information are useful in this discussion. First, not all vehicles operate in the same manner. For example, on some vehicle applications, as soon as STFT reaches a specific limit, long-term fuel trim (LTFT) would begin to adjust and then STFT would drift back to near the zero point. On some vehicle applications, it will take quite a bit of time for LTFT to make a shift. The screen capture in Fig. 6 above is from a late-model Dodge Charger, where we created a large vacuum leak. STFT (red and green traces) flat-lines at +32.8% and eventually, after four minutes, STFT adjusts back to zero. LTFT did not move at all, which you might think is an issue, but this is how this vehicle normally operates.
The second useful piece of information is that many European vehicle applications use a different fuel trim adjustment strategy. The terms additive and multiplicative are commonly used with enhanced scan tools. Additive shows fuel trim values at idle or just off idle; multiplicative shows the fuel trim adjustment at higher rpm and vehicle speed ranges. When monitoring OBD II generic data parameters, you’ll notice that STFT and LTFT will react independently in the different operating ranges considered here.
By now you should see the benefit of monitoring fuel trim values and will begin the process of using the four-step fuel trim procedure. One of the best ways to learn is on known-good vehicles and then see what happens when you generate a few faults and monitor the reaction. For exa
mple, remove the hose to the purge solenoid and watch what happens in the different operating ranges. I would also encourage you to look at the components listed in Fig. 2 and take the time to understand how fuel trim would be affected if any of these components were not operating correctly.
It’s amazing that after 20 years, OBD II generic continues to help technicians solve common driveability issues.
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