Hi All
I've been reading about fuel trims on the forum and can see how useful this data can be. My question is: How do I access this data? I have an icarsoft i930 but don't recall seeing anything on it about fuel trims.
Any advice most welcome
Thanks
Pete

 

PDMiller, think of fuel trims as an error signal. If you have zero error, then what the program is trying to do is matching what it should be doing. If you have an error, then things are not lining up like they should. An example. If you have a pump connected to a computer that is regulating its flow and you program it for 5 gallons liters per minute flow. You measure this flow and you find it is giving you 5 liters per minute, then the error is zero (it is doing what it should). If you measure and you find that it is only flowing say 4.5 liters, then you would want to input an error signal of 0.5 liters per minute to make the pump move more water to get the output up to 5 liters per minute.

In the case of your car. it knows how much air is coming into the engine. It knows based on that amount of air that it has to allow a finite amount of fuel to use up all that air. After the air and fuel burn in the cylinder, the O2 sensor measures how much oxygen is left in the exhaust (should be zero) along with no extra fuel. If this is the case, your fuel trim is zero. If it starts seeing too much oxygen, then it goes positive (as I recall). If it sees too much fuel, then it goes negative. This gets fed back to the computer to change how much fuel is being added to make the cylinder not run lean (leads to the cylinder running too hot) or running too rich (leads to poor fuel economy and harms the environment).

As how to access this information on your icarsoft reader, I cannot help you there. Most readers I have seen, have a display function that you access that gives you a list of parameters to look at and you simply scroll down to something that mentions "fuel trim Bank 1 (or 2)". Then it will display a value for you.

 

Thanks for that Thermo. Great,understandable explanation. I'll plug the reader in next time I have 5 mins and see if I can find fuel trims on it. If not the elm 327 seems to be the way to go. The car is running great at the minute (touch wood) but I like to try to anticipate the next " challenge".
Thanks again.

 

PDMiller, the other value that you may find interesting (while not quite as extensive since it is a single parameter) is the O2 sensor voltages. That can tell you a lot too. If you are getting 0.000 VDC, then you are running rich as there is no oxygen left in the exhaust or the sensor is failed. In an ideal world, it should be around 0.5 VDC (proper amount of oxygen left in the exhaust), get much above say 1.000 VDC, then you know the car is running lean.

On this, you may find them abbreviated as STFT or LTFT (Short Term Fuel Trim and Long Term Fuel Trim respectively). Short term is more of an instantaneous reading based on the last few cycles of the engine where the long term is more over a period of time, say minutes to hours. Your reader may have shortened it down to just these few letters which would make it hard to spot.

 

Many Jags (such as X-Type & S-Type) use newer tech sensors which report current not voltage... Not that we know the OP's car.

Vital to know...

 

Had a play with the icarsoft today and sure enough the long and short term fuel trims are there. The car is an X350 XJ6 and I've had the icarsoft for a couple of years but didn't notice the trims. Probably because I didn't know what they were for until I read about them on this forum.