a look into fuel trim

GEARSAugust2007

Driveabilty? Electrical Diagnostics? If youre in the transmission business, youre eventually going to be involved with it, so you might as well embrace it. It didnt seem too long ago that engine, transmission, ABS, and even body con-trollers were mostly independent enti-ties. On modern vehicles, now more than ever, these systems depend on each other for proper operation.

For example, vehicles with the CAN bus communicate through high-speed data lines and share sensor infor-mation that can render the vehicle use-less if the system isnt operating cor-rectly. See for yourself: Short or open a CAN bus line and see if the vehicle will even start.

Or take a look at a Chrysler 300C and try to find the vehicle speed sensor. Youll be looking for a while, since this vehicle uses the CAN bus to share the Electronic Stability Program (ESP) data, including the wheel speed sensor signal, with all modules wanting to know wheel or vehicle speed.

Vehicle technology is increasing by leaps and bounds, but thats no rea-son to shy away from it. Most transmis-sion shops are well equipped to handle electrical and computer diagnostics. If you have a scan tool, a DVOM (better yet, a digital storage oscilloscope), and some general diagnostic tools, you can easily diagnose issues found within many systems of the vehicle.

Electronic engine controls, for example, are so closely related to trans-mission operation, they should always be considered as possible causes for transmission symptoms. Coils, injec-tors, EGR valves, mass air flow (MAF)

sensors, restricted exhaust systems, and skewed sensors are just a handful of components that could cause the vehi-cle to exhibit problems such as shud-dering, poor performance, harsh/soft shifts, and improper shift timing. Even though the root causes of these prob-lems arent transmission failures, they might end up at your doorstep, because of a transmission related symptom.

Maybe nows the time to welcome this challenge and expand your service to include driveabilty diagnostics. Many of the operating principles and concepts found in engine control systems arent that much different than transmission control systems. Fuel management is a perfect example: Fuel control adaptive strategy is very similar to transmission adaptive strategy. To illustrate this, lets compare the feedback system used for shift control (since were pretty famil-iar with this) to the feedback system used for fuel control.

(Figure 1).

On a typical GM transaxle, the PCM will adapt shift pressures to con-trol the shift quality based on pro-grammed values. The PCM checks engine load, engine speed, input shaft

speed, output shaft speed, transmission temperature, etc., to calculate a base shift pressure to use when upshifting the transaxle.

While the shift is occurring, the PCM will monitor the input and output speed sensors to determine the actual shift speed. If the shift took longer than expected, the PCM adapts by adding pressure to the base calibration for future reference. After miles and miles of driving, the transmission adaptive pressures (TAPs) will stabilize to make the shifts as consistent as possible for performance and durability.

For diagnostic purposes, the TAPs can provide excellent insight to how the PCM is controlling the transmis-sion. Positive TAPs indicate the PCM is increasing pressure during the shift to correct for slow shifts. Negative TAPs indicate the PCM is reducing pressure to correct for shifts that are too quick.

These adaptive values can provide hints on whether to look for pressure related problems, sticking valves, or worn/stuck components. This is a basic feedback system where the PCM uses sensors to decide on a base pressure, and then looks to the speed sensors to determine if its calibration was close enough. If not, it adapts to accommo-date the speed differences and stores that information. (Figure 2).

Feedback strategies are used throughout the vehicle, especially in the electronic engine control system. Examples include the idle air con-trol systems, electronic fuel pressure regulation, electronic throttle control, evaporative emissions, and even the age-old closed-loop fuel injection con-trol the major topic for this article.

Symptom Diagnostics: A Look into Fuel Trim by Sean Boyle

Figure 1

GEARS August 2007 9

(Figure 3).Just like transmission adaptive

controls, the fuel system is a fine-ly tuned process that interprets many input sensors to obtain a good base fuel injection pulse width, and then verifies this decision by interpreting the post-combustion lambda sensor (from now on referred to as an HO2S).

There are three main modes of operation: startup, open loop, and closed loop. Interestingly enough, for startup, the PCM mainly looks at engine cool-ant temperature and engine RPM to determine injector pulse width. The ini-tial injector pulse width is rather long, but the goal is to inject a lot of fuel into the cylinder to get it started. The scope image displayed is a 05 Ford F150 with the scope connected to the injector control wire. The startup pulse width is over 90ms at about 50F.

(Figure 4).As soon as the engine starts, the

PCM begins to read input sensors such as crankshaft position, manifold abso-lute pressure, mass air flow, engine coolant temp, intake air temp, and throttle posi-tion to determine the base pulse width to operate the injector.

While the engine is cold, the engine coolant temperature (ECT) has great influ-ence over injector pulse width; as the engine warms up, the MAF/MAP and TP sensors play a greater role. Since these sensors can make such a large difference in the base pulse width calculation, its

very important to check them for ratio-nal and plausible values. During this phase, the engine control is working in open-loop, where it relies on these sensors to determine how much fuel to inject into the engine.

(Figure 5).Once the HO2S warms up, the

PCM will start to make fuel correc-

tions, even while in open loop, but it wont make long-term corrections yet. While viewing data on a scan tool, you can see the HO2S actively switching and the short-term fuel trim making

Figure 2

Figure 3

Figure 4

Figure 5

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10 GEARS August 2007

corrections, while the PCM is still com-manding open loop.

Closed-loop fuel control builds on the open-loop strategy by making long-term fuel trim adjustments to the base calibration. Not only does the HO2S need to be active, but engine tempera-ture and engine run-time need to be adequate for the PCM to initiate closed-loop operation.

The traditional HO2S is a Zirconia type sensor that varies between 0 volt (lean) and 1 volt (rich), depending on the oxygen content in the exhaust. The HO2S is probably the most misunder-stood sensor in the vehicle.

Contrary to popular belief, the HO2S actually works like a miniature hydrogen fuel cell. During rich situations, when there is little oxygen in the exhaust, oxygen will travel through the sensor to combine with hydrogen in the exhaust. This oxygen trans-fer is what creates voltage in the sensor.

During lean situations, there is plenty of oxygen found within the exhaust to complete the chemical reaction with hydrogen. Since no oxygen is pulled through the sen-sor, no voltage is created. For more on this topic, read Steve Bodofskys article, Oxygen Sensors and Fuel Cells: Different Name, Same Technology, in the March 2004 issue of GEARS.

(Figure 6).Even though the HO2S is widely misunderstood, the

general concepts discussed in textbooks still apply. Simply put, when the combustion is lean, theres excess oxygen in the exhaust, and the HO2S sensor produces little voltage. When combustion is rich, there is a lack of oxygen in the exhaust, so the sensor will generate voltage while pumping oxygen through the sensor. For the purpose of this article, expect to see a traditional oxygen sen-sor switch between 100mv and 800mv.

(Figure 7).The pre-catalyst HO2S plays a

strong role in determining fuel control during closed-loop (although this is mostly the case, some vehicles will adjust fuel trims of off the post-cat HO2S to enhance converter efficiency). The PCM monitors the average HO2S voltage and keeps it switching back and forth from the lean extreme to the rich extreme.

While monitoring an HO2S on an oscilloscope, notice that it shifts con-tinuously, about 2-3 times per second. Thats the feedback system in action. The PCM commands a short injector pulse width, and then waits for the

A Look into Fuel Trim

Figure 6

Figure 8

Figure 7

HO2S to shift lean (low voltage); next it commands a longer pulse width and waits for the HO2S to shift rich (higher

voltage). The PCM monitors these rich and lean

trends and adapts to them by adjusting fuel

trim. Fuel trim is an adjustment to the base calibration that the PCM uses to determine the starting point for injector pulse width.


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For example, if the input sensors indicate that the PCM should operate the injectors at a 2.0ms pulse width, but the resulting HO2S stays at 0.8v (rich), the PCM will shorten the injec-tor pulse width until it sees it shift lean. The amount it adjusts the injector pulse width will be displayed as short term fuel trim (STFT) and its typically identified in percent.

So, using the previous example, if the exhaust indicated rich until the injector pulse width was shortened to 1.9ms, the STFT would display nega-tive 5% on the scan tool (1.9ms is 5% less than 2.0ms). The PCM doesnt remember the STFT and its continu-ously updating. In fact, the STFT is what makes the HO2S swing rich to lean and so on. Refer to the snapshot graph and notice how once the fuel trim falls negative, the HO2S falls lean, and

when the fuel trim jumps up the HO2S goes rich. As long as the PCM is in closed loop, it continuously performs this command and observe routine.

(Figure 8, see page 10).If the HO2S voltage remains at

one extreme for too long, the PCM will adapt by adjusting long-term fuel trim (LTFT). LTFT can make major adjust-ments to the base calibration, and store those adjustments in memory, so the PCM can refer to them during closed and open loop operation.

The PCM wants to see STFT switching around zero. If the STFT is staying above or below the zero mark

for a given length of time, the PCM will adjust the LTFT and store the adjust-ment in memory, which will then cause the STFT to shift back toward zero.

For example, if STFT switched around -4%, the PCM will eventually adjust the LTFT to -4% and store that information in memory. So the base calibration will immediately reference -4% LTFT and reduce the base injector pulse width, and the STFT will fluctu-ate around 0% again.

(Figure 9).The system is actually pretty

sophisticated. The fuel control feed-back system has cells, which reference


Figure 9

Positive Fuel Trim Values (+13%) O2 Sensor VERIFIES LEAN running enginePossible Problem Reasoning Isolate and Diagnose

Restricted Fuel System

PCM expects the fuel system to be at a specific pressure when commanding an injector pulse width. If its too low, it wont deliver adequate fuel to the cylinder.

Fuel Pressure Test. Fuel trims might be normal at idle, but increase under load.

Vacuum Leak

Unmetered air is entering the engine. The PCM makes base PW calculations based on sensor input values. Since a vacuum leak allows unme-tered air to enter the engine, it can only detect it through the lean O2 sensor condition.

Use propane around intake gaskets, manifold, and hoses while observing the short-term fuel trim. When you come across a possible leak, the short-term fuel trim will go negative. Also, a smoke machine will pump smoke into the intake and pinpoint vacuum leaks. These machines work great for finding leaks in other systems as well.

Clogged/restricted InjectorsAnything that prevents a full charge of fuel to pass the injector will cause either a lean condition or a complete misfire.

Activate the injector (with a special tool) while observing pres-sure drop and compare the results to neighboring injectors. Observe the injector on an oscilloscope to determine if the pintle is physically opening and closing.

MAF (dirty)

The mass airflow (MAF) sensor informs the PCM of the density and volume of air entering the engine. If it misrepresents this information, the PCM will have to compensate based on O2 sensor values. A dirty MAF sensor typically over-estimates the amount of air at idle, and underes-timates the amount of air at cruise. This will result in negative fuel trims at idle and positive fuel trims at cruise.

Snap-throttle tests while observing the waveform on an oscil-loscope. Clean the sensor using electrical contact cleaner and compare pre and post waveforms to determine improvement.Volumetric efficiency calculators can be downloaded from the internet to help determine if your MAF reading is plausible.

Figure 10


GEARS August 2007 13

different engine speeds and loads. Each cell will have its own value identified in a lookup table that contains the LTFT adjustment to the base calibration.

Although it would be nice to see this table, most scan tools only display the current LTFT and STFT at the given load/RPM the vehicle is operating at. Some tools might indicate what cell

theyre in at a particular time, but many wont.

Make sure you review fuel trim values when a problem is occurring. For example, checking fuel trim values at idle on a vehicle with a weak fuel pump will probably indicate acceptable fuel trim values. But place that vehicle under load, where it cant deliver the

necessary fuel, and youll see fuel trim values start climbing positive.

As with transmission adapts, fuel trim can offer a piece of the puzzle while diagnosing driveabiltiy problems, but it wont pinpoint a specific failed part. Its important to fully understand the fuel control loop concept, where the PCM commands an injector pulse

Positive Fuel Trim Values (+13%) O2 sensor TRICKED LEANPossible Problem Reasoning Isolate and Diagnose

Exhaust Leak

Any leak before or around the O2 sensor could cause outside air to be drawn into the exhaust stream. The flow of exhaust gasses comes in the form of pulses. The pulse itself will create a positive pressure, but after the pulse, the exhaust system might contain a low pressure, which can draw air through an exhaust leak. This air can trick the O2 sensor into thinking the combustion was lean.

Smoke test the exhaust system. Look and listen for evidence of a leak. If youre lucky, you might find carbon around the culprit.

Engine MisfireAn ignition or fuel misfire will allow the engine to pump air directly into the exhaust system. The O2 sensor will misinterpret the excess O2 as lean combustion.

Misfires fall into fuel, ignition, or engine mechanical categories. After determining the cylinder at fault, use a process of elimina-tion narrow the field of potential failed components. Swapping injectors, plugs, wires, and even coils to different cylinders will help determine if the problem follows a component or stays with a cylinder. If the vehicle has multiple banks, check to see if the fuel trim for one bank is higher than the other.

Secondary Air Failure

Yes, its back! Secondary air systems are showing back up on vehicles. A secondary air system pumps air into the exhaust stream before the vehicle goes into closed loop to aid in converter efficiency. If this system fails or leaks, unintended air will be drawn into the exhaust stream, trick-ing the O2 sensor into thinking combustion is lean.

Disconnect and plug secondary air components to isolate the system. Smoke test for leaks.


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width based on the sensor information, then waits for the results of that com-mand by watching the HO2S.

The fuel trim is going to indicate the accuracy of the PCMs guess. Since the HO2S is the only component that reports the results of combustion, there are many different scenarios that result in positive or negative fuel trims.

And there are some situations where it would be normal to find fuel trims to be on the high side (either posi-tive or negative). For example, when the PCM activates the EVAP purge solenoid, which allows engine vacuum to pull canister fuel vapors, expect to see the fuel trims go more negative than with no EVAP purge.

(Figure 10, see page 12).

Even though a lean-reporting HO2S will cause positive fuel trims, and a rich reporting HO2S will cause negative fuel trims, there are some situ-ations where the HO2S can be tricked into thinking the engine is running rich or lean, when its actually not. Refer to the following tables to see possible problems and diagnostic checks for actual lean and tricked lean situations, and actual rich and tricked rich situa-tions.

Just as with transmission adaptive pressures, fuel adapts cannot diagnose any one failed component, but it offers you a look at what the PCM is doing to correct for a problem. The purpose of adaptive controls, whether theyre for fuel or transmission control, is to

correct for minor changes that appear through normal driving. Once these adaptive values go beyond the normal ranges, the PCM cant sufficiently cor-rect for the failures and driveability symptoms surface.


Negative Fuel Trim Values (13%) O2 Sensor TRICKED RICHPossible Problem Reasoning Isolate and Diagnose

Contaminated Sensor

O2 sensors are miniature fuel cells that pump oxy-gen ions to complete chemical reactions (see Steve Bodofskys article, GEARS March 2004). In many cases, when the sensor becomes contaminated, the sensor continuously pumps oxygen ions and gener-ates a voltage on the signal wire. This will trick the PCM into thinking the engine is running rich.

Note repair history. Engine work as a result of oil consumption, head gasket leakage, etc., can cause O2 sensors to fail. Force the engine to run lean by creating a large vacuum leak and observe the O2 waveform for correct operation. KOEO should show low voltage on HO2S once the sensor warms up.

O2 Heater Circuit Failure

If voltage from the O2 heater circuit bleeds into the signal wire, it will register a high voltage at the PCM. The PCM will falsely recognize this as a rich condi-tion.

Force the engine to run lean by creating a large vacuum leak and observe the O2 waveform for correct operation.KOEO should provide low voltage on HO2S once the sensor warms up. Visual inspection might reveal moisture or oil contami-nation.

Negative Fuel Trim Values (13%) O2 Sensor VERIFIES RICH running enginePossible Problem Reasoning Isolate and Diagnose

High Fuel Pressure

PCM expects the fuel system to be at a specific pressure when commanding an injector PW; if pressure is too high, it will deliver too much fuel to the cylinder.

Fuel Pressure Test.Restrictions on a fuel return.Clogged/restricted fuel pressure regulator.

EVAP Purge Solenoid is Always Purging

The EVAP system collects fuel vapors from the tank and stores them in a canister. Most systems purge the canister under cruise, but if the solenoid is faulty, it might purge when its not suppose to, causing a rich running engine.

Disconnect the purge line from the intake manifold and check fuel trim. OBD-II vehicles have monitors that check the purge and vent solenoids for correct operation.

EGR Stuck On

During cruise, the EGR system mixes inert exhaust gases with the air and fuel charge to lower combustion temperatures. If the EGR is coming on when it shouldnt, it lowers manifold vacuum, which will make the PCM think its operating under more load and compensate by adding fuel.

Fuel trim excessive at idle and closer to normal at cruise. Disconnect EGR to see if theres a change. EGR shouldnt come on at idle.Remove and block EGR.

MAF (dirty)

The mass airflow (MAF) sensor informs the PCM of the density and volume of air entering the engine. If it misrepresents this information, the PCM will have to compensate based on O2 sensor values. A dirty MAF sensor typically overestimates the amount of air at idle, and underestimates the amount of air at cruise. This will result in negative fuel trims at idle and positive fuel trims at cruise.

Snap-throttle tests while observing the waveform on an oscilloscope. Clean the sensor using electrical contact cleaner and compare pre and post waveforms to determine improvement.Volumetric efficiency calculators can be downloaded off the internet to help determine if your MAF reading is plausible.

Contaminated Fuel

Fuel vapors from contaminated oil will be drawn through the PCV system. Normally, the crankcase ventilation doesnt supplement the fuel system.

Vehicle history might reveal poor vehicle maintenance. Disconnect and plug the PCV valve and obverse fuel trims.


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a look into fuel trim

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