LTFT & STFT on OBD-II scanners

I found the answer to one of my older questions, specifically how do I interpret the meaning of LTFT and STFT data on my OBD-II scanner?

Here is the answer!

http://findarticles.com/p/articles/mi_qa3828/is_200609/ai_n17191947/pg_4

Hint: It shows you how to diagnose problem fuel injectors, like a leaking injector, or slightly clogged injector.....while they are installed and running! Now how cool is that???? It does assume you have a working O2 sensor(s).

Great article...I plan to schedule time this week to see how well balanced my injectors are....Thanks for finding it and sharing it!

McQue said:

Great article...I plan to schedule time this week to see how well balanced my injectors are....Thanks for finding it and sharing it!

Click to expand...

Now the trick is to find affordable scanners that will display continuous fuel trim data. My wallyworld, scan gauge II (IIRC) only displays the fuel trim during a DTC code display for something like a missfire. That was where I ran into it first, but I could not figure out how to use the data.

I picked up a used autoxray scanner on Ebay for around $85 plus shipping...
I am happy with it ...easy to use, it reads out the data realtime, and has a capture mode good for about a 8 sec. snapshot that you can trigger manually to capture and hold the 20 or so datapoints you select just when the motor hicups..
If you go looking for a used Autoxray scanner, be careful not to confuse it with their reader only models...you can visit their site...look at their manuals, etc...Autoxray.com

I also picked up a used 10MHZ scope for around $55 plus shipping...Have been hunting for a dead PCM and cheap engine harness so I can use the 32way PCM connector and harness plugs to make a breakout box....maybe I will fire up the scope and compare a couple of injectors after I do the balance check.

Well, it turns out one of the differences of owning a 2001 XJ with Calif. Emissions is that your XJ has four 02 sensors. So, the bad news is you will eventually spend more money to replace 4 sensors instead of 2 sensors.
The good news is your scanner reports the Short Term Fuel Trim, and Long Term Fuel Trim, for each Bank of 3 injectors, as well as the O2 sensor data for each bank. If you have an injector performing marginally (rich or lean), just comparing the STFT and LTFT between the two banks will start to narrow down a problem.
Low trim percentages indicate that the PCM is not having to make large adjustments to the “preset tables” to get the A/F correct, so if both banks are reading low percentages, and those numbers are relatively close to each other, (within a couple of percent) then it can be a basic indication of balanced performance without having to go through the process of disconnecting each injector, one at a time, and then comparing the net change for STFT and LTFT that the PCM will create trying to makeup for a disconnected injector.
My initial scan today seems to show that the two banks are fairly well balanced.
Trim for both banks was around 2% at most in idle from start up open loop, through closed loop and on up to full running temperature.Once getting to closed loop, STFT and LTFT dropped to zero or went a little minus quite often, as the PCM seemed to be running the show greatly while things were warming up.
So, when I have more free time, I am going to measure and record STFT and LTFT at 1500 RPM and 2500 RPM and then do the injector by injector balance check so that I have a point of reference to work from in the future.

Thanks again for posting that article Mike.