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Old 10-15-2008, 01:08 AM   #1
Joe@Trick Tuners
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Default Open and Closed loop Fueling and Fuel Trims

First we need to understand the difference between Open and Closed loop fueling. The vehicle will be in open loop under certain circumstances such as after startup for a minute or so, at WOT (wide open throttle), and sometimes under more rare circumstances like elevated ECT temps. When the vehicle is in open loop the fueling will be calculated based mostly off of the MAF/MAP sensor, but also takes many other things into account like IAT, ECT, TP, etc. There is no feedback in open loop (hence the name “open”). If the fueling is off the only way to really know is to monitor it with a wideband air fuel ratio gauge. Once the vehicle goes into closed loop the fueling is mostly calculated based off of input from the upstream O2 sensors. The O2 sensors measure the combustion gases for a target goal of ~14.7:1 (lambda =1.0) for gasoline. Different fuels have different air fuel ratios, but at their stoichiometric point (not lean, or rich) they will all read at lambda 1. Some people like to use lambda because the readings are the same for all fuels. When you use the air fuel ratio, the numbers will be different for fuel types. Here are a few examples for a lambda of 1 for different fuel types.

Gasoline 14.7:1
Diesel 14.6:1
Ethanol 9.0:1
Methanol 6.4:1
LPG 15.5:1
CNG 17.2:1
Hydrogen ~34:1

These ratios represent pounds of air needed for each pound of fuel to run at the stoichiometric ratio. When the air fuel ratio starts to go lean (higher than stoichiometric) the ecu will readjust the fueling (by injector pulse width) to correct the lean condition and bring the combustion back to lambda 1. This is a constant feedback loop (or a closed loop) and happens fairly quickly so it can alter the fueling over then next few engine cycles. The amount of correction can be viewed as the short term fuel trims (STFT) with an OBDII datalogger. The STFT can be displayed in different forms sometimes. With SCT software the value will be in the form of 0.75 to 1.25 indicating a -25% to +25%, while most other dataloggers will display the actual percent. If the STFT value is negative (or less than 1) the ecu is adding fuel to correct for a lean condition. These STFTs can be viewed in real time with a datalogger and are in effect anytime the vehicle is in closed loop. After a certain period of time, if the STFT is off, the value will be stored in the ECU as the long term fuel trim (LTFT). The purpose of the LTFT is to bring the STFT back around 0% so that it can still adjust +25%. The LTFT can also adjust +25% giving a total of about 50% of possible adjustment. Using SCT software the LTFT will be opposite the STFT (LTFT of 1.25 is adding 25% fuel) while most other dataloggers will read the same way as the STFT. (don’t ask me why!). The LTFTs are stored in the ecu until it is reset by unplugging the battery, or flashing a new tune. Using the STFT and LTFT for engine component diagnosis will really help save some time. If it’s lean, look for a vacuum leak, bad injector or coil, or an exhaust leak upstream of the O2 sensor.
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Old 02-12-2014, 05:02 AM   #2
mooremustangs
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Is the use of carb cleaner in small amounts around potential points of unmetered air still a cheap and easy way of detecting where the air is coming in? Assuming the engine will rev slightly when the combustible mixture enters the engine combustion...
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Old 02-12-2014, 03:35 PM   #3
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Yes. See here for more info on using carb cleaner to find a vacuum leak.
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