FUEL TRIM DATA: A POWERFUL DIAGNOSTIC TOOL · 2016-06-23 · 22 July 2016 Fuel Trim Data: A...

BY BOB PATTENGALE

OBD II generic data contains more than enough information to solve many diagnostic problems. Monitoring fuel trim data during four different engine loads is just one example.

FUEL TRIM DATA: A POWERFUL DIAGNOSTIC TOOL

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Some things never change. For those who like to think about significant anniversa-ries, 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 mo-bile 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 drive-ability 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 com-plaint, retrieve fault codes, record the information (including freeze frame data), research the fault, make a visual inspection and 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.

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Fuel Trim Data: A Powerful Diagnostic Tool

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 col-lected data. 4. Use the information to target the next di-agnostic steps.

F ig . 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 Sys-tem Lean) present. Typica l serv ice information sug-gestions for those codes might be plugged or dirty fuel fi lter, damaged or worn fuel pump, leaking or contaminated fuel injec-tors, low fuel pressure, evap purge

solenoid leak, EGR system issues, vacuum leaks, PCV issues, damage or contaminated mass airfl ow (MAF) sensor, etc.

Fig. 2 on page 24 shows a typi-

cal 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 ga-rage 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 sen-sor 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 par-ticipating 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 fi rst. This would not be a terrible answer, but the fuel trim data did not sup-port that choice.

Let’s go back to Fig. 1 and take some time to un-derstand the fuel trim data we col-lected 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

Fig. 1 Each step in the four-step fuel trim procedure described in this article is designed to help pinpoint the next steps to take in the diagnostic process. This screen capture shows fuel trim data in four engine management system operating modes.

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.


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rethink the next diagnostic steps. Which of the eight listed compo-nents 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 in-jectors were listed as possible caus-

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 val-ues 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

es for these fault codes, but leaking fuel injectors would create a rich-er-than-normal air/fuel mixture. The fuel trim values would be on the neg-ative side and we would likely have

a P0172 /P0175 (Bank 1 and 2 Rich). Clogged or contaminated in-jectors should be checked, but only after diagnosing the components listed in the previ-ous paragraph.

I t ’ s p o s s i b l e 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

Software is driving advanced sensors and electronic controllers into vehicles. Carmakers believe this will pave the way from fully driv-er-controlled to driver-assisted to semiautomated vehicles, then fully automated driving to, ultimately, the accident-free vehicle.

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Fig. 2 This illustration shows the components commonly found on GDI-equipped vehicles. Which could cause a fuel trim problem?

To check the airflow in all operating ranges, you need to record the MAF sensor data in all operating ranges,

which would include WOT acceleration, then compare the data to a VE calculator.

Canister Purge Valve

Boost Pressure Temperature Sensor

Throttle Device (ETC)

Manifold Pressure Sensor

High-Pressure Pump

PressureSensor

Ignition Coil/ Spark Plug

Fuel Injector

Fuel Pump Module

Air Mass Meter

Oxygen Sensors


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Fig. 1 shows a signifi cant 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 injec-tor 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 specifi cation was taken directly from real-world service information. Unfortunately, the specifi cations 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 airfl ow by approximately 20%, and is the rea-son for the positive fuel trim values at idle.

To check the airflow in all oper-ating 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 volu-metric effi ciency (VE) chart or calcu-lator, which can be found in numer-ous places online.

In our example vehicle, the MAF sensor was dirty and underestimating airfl ow in all operating ranges, which caused the P0171/P0174 codes to set. The MAF sensor was listed as a pos-sible cause, but in order to make an effi cient repair, it’s important to un-derstand how fuel trim data can help point you in the proper direction.

Using fuel trim to help diagnose vehicles takes practice, but you also

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 in-sufficient fuel volume to the GDI high-pressure pump.

The result of not supplying suf-ficient 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 ad-justed for various operating condi-tions. 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 main-tain the desired rail pressure. In-creased 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 specifi cation, what fault code will be set? On a GDI vehicle, the most likely fault code will be a P0087 (Fuel Rail Sys-tem 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 per-form very well.

Once again, the fuel trim data from

Fig. 3 This screen capture of the four engine management system operating modes is an ex-ample of a driveability fault that shows up only at idle speed. The readings under light, mod-erate and heavy loads appear normal; the idle readings are abnormal.


ing on the fuel vapor present in the charcoal canister.

Fig. 5 below shows the change in fuel trim at idle with the purge sole-noid 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—

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need to understand how each compo-nent affects fuel trim when in opera-tion. For example, let’s take a look at the purge solenoid in Fig. 1. In normal operation, the purge solenoid is com-manded closed and the PCM com-mands 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 tem-perature 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, depend-


Fig. 5 This graph shows the fuel trim adjustments made while opening and closing the purge solenoid.

Fig. 4 Shown in 2D here, this image was captured from the Bosch 3D virtual garage. It shows the MAF sensor data at idle.

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or just before 54.6 seconds—STFT begins to move in a positive direc-tion, and within about 14 seconds, no fuel vapors are being purged from the charcoal canister. In es-sence, we have a large vacuum leak. Once the purge solenoid is com-manded 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 infor-mation 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. ST-FT (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 infor-mation is that many European vehi-cle applications use a different fuel trim adjustment strategy. The terms additive and multiplicative are com-monly used with enhanced scan tools. Additive shows fuel trim values at idle or just off idle; multiplicative shows the fuel trim adjustment at

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higher rpm and vehicle speed ranges. When monitoring OBD II generic data parameters, you’ll notice that STFT and LTFT will react inde-pendently in the different operating

ranges considered here.By now you should see

the benefit of monitoring fuel trim values and will be-gin the process of using the four-step fuel trim proce-dure. 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 example, re-move the hose to the purge solenoid and watch what happens in the different op-erating ranges. I would also

encourage you to look at the compo-nents 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.

Fig. 6 This screen capture demonstrates the fuel trim adjustment that occurs with a large vacuum leak. In certain applications, long-term fuel trim does not change.

Using fuel trim data can help diagnose vehicles, but an

understanding of how each component affects fuel trim

also is necessary.

This article can be found online at www.motormagazine.com.

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FUEL TRIM DATA: A POWERFUL DIAGNOSTIC TOOL · 2016-06-23 · 22 July 2016 Fuel Trim Data: A...

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Transcript of FUEL TRIM DATA: A POWERFUL DIAGNOSTIC TOOL · 2016-06-23 · 22 July 2016 Fuel Trim Data: A...