When i first fired it up, my fuel trim was pretty close to zero on the short term and the long term was zero which it should have been. I have a 2002 toyota corolla 1zzfe engine and i am trying to understand the fuel trim values i am getting in real driving conditions from my scan tool. Fuel trim is a twopart adaptive strategy that adjusts fuel injector pulse width open time to adapt to wear, defects, or changes in the system. The fuel trim ratios represent the amount of fuel the pcm is adding or removing in reference to a baseline to keep the mixture at optimum based on feedback from the o2 sensors, temperature sensors, etc. Fuel trim diagnosis maintenancerepairs car talk community. Jan 26, 2008 scanning obdii fuel trims explains the fuel control pid obd2 scanner parameter.
These are stepbystep procedures that some pencil pusher thinks is the best way to troubleshootthey are never written with speed in mind. Scannerdanner forum scannerdanner book and lecture video questions search. I dont doubt it, if you see long term fuel trim pid but dont see short term trim it is likely a software hole. If fuel trim has been reset, fuel trim learning is accelerated for 2 engine warmup cycles. Engine performance diagnostics by scannerdanner scanner danner. Changes in o2 sensor voltages cause a direct change in fuel.
Here is an explanation of fuel trim and what it does for us. The two parts of a computer fuel trim adaptive strategy are short term fuel trim stft additive and long term fuel trim ltft multiplicative. If the ecm sees it has to keep adding fuel for a period of time, it basically assumes that is the new normal and increments the long term trim by that amount. Sorry man, just read more and what youre describing is bad upstream o2s sensor. It is meant to be a field manual, one that they will. Positive fuel trim means the computer is adding extra fuel too lean. Being able to interpret your scanner fuel trim data requires a basic knowledge of the interaction of short term fuel trim and long term fuel trim data.
The check engine light is not on yet but im getting a constant high stft reading on bank 1, sensor 2. An obdi1 trip has been completed successfully, and all monitored sys tems passed. The ecu controls air fuel mixture in order to maintain power, efficiency, and emissions. Fuel trims are the deviations from the correct air fuel mixture. I would have see how these fuel trim numbers act while watching it a while at different rpm ranges and while driving, to determine if there is slight problem causing this. Additionally, links to over 50 scannerdanner videos are included which explore the content of the book in more detail. The longterm fuel trim ltft is a stored value which accounts for changes in engine condition, wear and tear, etc. May 08, 2006 understanding fuel trim some of the most common fault codes dtcs pertain to fuel trim rich mixture, lean mixture, etc. Fuel trim question poor mileage scannerdanner forum. This book is the printed version of scannerdanners ebook. Find the prices for normal auto service activities including alignment, tuneup, timing belt replacement and more. The truck was recently tuned up with new plugs, wires, cap, rotor, and coil.
The ecu controls airfuel mixture in order to maintain power, efficiency, and emissions. Fuel trims are the deviations from the correct airfuel mixture. Fuel trim is the amount of fuel added or subtracted by the pcm. Advanced engine performance diagnosis edition 6 by james d. It has to do this a minimum of 2 to 5 times a second. The short term fuel trim stft refers to immediate changes in fuel occurring several times per second. For more information on this topic, i have written a field manual called engine performance diagnostics which is available as an ebook or paper book. The oxygen sensor o2 sensor is the only detection for the pcm onboard computer or power train. Understanding fuel trim some of the most common fault codes dtcs pertain to fuel trim rich mixture, lean mixture, etc. Many of the tuning software screenshots and other referencesimages will be highly recommended for anyone diving into a megasquirt project, and highly educational for someone diving into any efi project.
Engine performance diagnostics by scannerdanner scanner danner scanner danner, paul danner, paul danner on. Chapter 4 oxygen sensors and fuel trim an sd premium. This change in fuel being added or taken away is called fuel trim. This book was originally written for my engine performance class at rosedale technical college and is now being used in classrooms, shops and backyards worldwide. After repairs, use the scan tool fuel trim reset function to reset the longterm fuel trim to 128 0%. Long term trim ratios vary less thus they are long term.
When the engine operates at normal or higher load or at higher engine speeds, larger volumes of fuel and air are needed. I used this opportunity to discuss a very difficult topic from chapter 5 in my engine performance diagnostics book. Learn more about scannerdanner s favorite products. How to quickly identify a vacuum leak using fuel trim and water. Short term fuel trim stft multiplicative this is the control of the injection pulse time open over the mid to upper range of engine operation. Advanced engine performance diagnosis robs audi world. The overall fuel trim is an additional compensation applied evenly to the main fuel tables. This book is the printed version of scannerdanner s ebook. Note that the injector size will automatically reduce the overall fuel and cranking fuel even if the trims are set to zero. Use the information to target the next diagnostic steps.
Advanced engine performance diagnosis edition 6 by james. Once that happens, the short term trim is again bouncing around 0%. The actual list of available parameters is vehicle specific. Diagnostic trouble code dtc p0171 fuel trim system too. Understanding fuel trim is an efficient in automotive emission troubleshooting. This code is triggered by the first downstream front o2 sensor. Chapters 4 and 5 fuel trim and oxygen sensor testing. Performance fuel injection systems is not a megasquirtspecific book but does speak from our experience. Technician a says you can switch sequentially through display groups by pressing the r or c button. Using fuel trim to identify type of misfire can it be done. Fuel trim is the internal mechanism within the pcm that switches the fuel mixture from rich to lean. How could they be when half of them start off with disconnecting the computer and the sensor and checking the wire for opens and shorts. Engine performance diagnostics by scannerdanner scanner danner pdf for free, preface. Chapters 4 and 5 fuel trim and oxygen sensor testing chapter 6 temperature sensor, thermistor circuit tests chapter 7 potentiometerposition sensor tests tps, evp, app, vaf etc chapter 8 pressure sensor testing, map, baro, dpfe, fuel tank and rail pressure sensors chapters 9, 10 and 11 the 5v reference and signal circuit quick tests.
If you arent running rear o2 sensors or cat converters anymore you will need to disable those codes and cot cat over temp in the tune. Understanding short term and long term fuel trim an sd premium. Fuel trim spread is greater than 10 points, bank 2. Fuel trim corrections are learned after the engine is warmed up, however, extremely high engine coolant temperatures, ambient air temperatures or excessively high canister purge rates will temporarily disable long term fuel trim learning. Emission control system devices, operation, and diagnosis 28. My specialty is engine performance diagnostics, and i have written a text book that i use for my students.
Chapter 1 misfire id, compression test, vacuum leaks, fuel trim, head gasket, restricted exhaust, low power, dvom chapter 2 switch inputs, digital signals, module communication, hall effect testing chapter 3 transistors, solenoids, computer controlled output testing chapters 4 and 5 fuel trim and oxygen sensor testing. Fuel trim diagnostics using fuel trims during a diagnostic plays an important function in plotting a trend, be it either a rich or lean trend. No matter what the driveability issue happens to be, checking the pcms fuel trim decisions first can get you pointed in the right direction, and may end up cutting your diagnostic time in half. Monitor and record the fuel trim values in the four common operating rangesidle speed, light load 20 to 30 mph, moderate load 40 to 50 mph and heavy load 60 to 70 mph. Diagnostic trouble code dtc p0171 fuel trim system too lean. There will be a deflection in the adaptive terms which is capable of identifying a slow default emerging in case the adaptive value threshold is permanently exceeded. Fuel trim flips to negative to reduce fuel accordingly, fuel to compensate for the leak. Scanning obdii fuel trims explains the fuel control pid obd2 scanner parameter.
How to use fuel trim to identify the cause of a p0171 and p0174. Short term fuel trim is what the computer is doing at that time to control the fuel mixture. Engine performance diagnostics by scannerdanner scanner. It runs fine when driving except takes a couple of seconds longer than it should on initial acceleration. Basically this means that an oxygen sensor in bank 1 has detected a lean condition too much oxygen in the exhaust. Short term fuel trim this is an immediate correction. All other readings across a variety of throttle ranges etc while driving seem good. Jan 16, 2010 a fuel trim which is highly positive may indicate the ecms attempt to correct a lean conditions, and a higly negative trim is an attempt to correct for a rich condition. It is almost like there is a vacuum leak, but i have sprayed. The pcm starts out with a baseline fuelair mixture that it believes will achieve a stoichiometric ratio. A fuel trim which is highly positive may indicate the ecms attempt to correct a lean conditions, and a higly negative trim is an attempt to correct for a rich condition.
Since the standardization of obdii, fuel trim is found on most cars. Fuel trim is broken down to two types, short term and long term. Really, the oxygen sensors are what drive the fuel trim readings. Both the ebook and printed book will serve as your guide to electrical and electronic systems trouble shooting. As the vehicle runs the trim ratios will vary based on fuel mixture wintersummer blends, temperatures, etc. The ltft is also influence by the short term fuel trim stft. The close loop system is designed to rapidly change the airfuel ratio from slightly rich to slightly lean to maintain a. Fuel trim spread is greater than 10 points, bank 2 tacoma world. The rest of this series on oxygen sensors and fuel trim can be. By changing the airfuel mixture from lean to rich is called close loop. Obd auto doctor supports the following standard obdii parameters pids as defined in sae j1979 aug2014. The pcm needs this information for adaptive strategy known as short term fuel trim and long term fuel strategy.
The ecm can only correct for a certain amount, then the check engine light will come on. The cranking fuel trim is applied when starting the engine. You have a ol fault so its stuck in open loop forever with no fuel trim corrections until you have get working o2 sensors again. Fuel pressure an excessively low pressure will make the system lean.
The two parts of a computer fuel trim adaptive strategy are short term fuel trim stft additive and. Ive owned the truck for two years, and it has consistently gotten 460480 kms per tank about 16. A dirty fuel filter and bad fuel pressure will increase trims as well. All monitored obdi1 systems are currently working correctly d. I basically have the same problem with trim values on my truck. The purpose of the ltft is to get the airfuel ratio back to stoichiometric in the event of a faultnormal wear so the stft can again cycle between rich and lean operating conditions. When the feedback from the ho2 sensors shows a deviation, the pcm adjusts the amount of fuel the injectors deliver, in an effort to hit the stoichiometric goal. Here i offer automotive diagnostic training with an emphasis on engine performance, computer controls and electrical systems troubleshooting. Scannerdanner forum scannerdanner book and lecture video questions. Only the parameters supported by the car will be shown in the software. Adjustable insulation piercing test clip with bplug inputs. Nov 11, 2011 fuel trim is the amount of fuel added or subtracted by the pcm.