Hello All,

Wondering if anyone can help with this issue. I don't think I have a vacuum leak or exhaust leak causing this. The car runs strong but I think this issue may be affecting gas mileage some and it has some popping or backfiring on decel occasionally, usually when cold. The random higher trims are mostly on Bank 2. I've seen threads on here stating that aftermarket throttle bodies and cams can cause this at idle but this occurs when driving as well. I only have information from this cheap bluetooth scanner as I have not invested in a better one so I may need to do this. Looking at the file it happens during decel and idle but not always. I have replaced many parts like fuel filter, PCV valve, EGR, Fuel Pressure regulator, ICM and O2's as well as the old extensions for them chasing this. I have also cleaned the throttle body on the car as well as the intake with some foaming cleaner from Mopar that I read on here worked well. It was not very dirty as the car was well cared for by my mother who was the original owner. She had the majority of the mods in my signature installed back in the late 90's and early 00's. Any help would be much appreciated.

 

I have very similar experiences with my setup. I know my gas mileage isn't going to be good, but that's part of having a sports car. What exactly are you trying fix? I kinda looked for fixes like your mentioning but never found anything really wrong.

 

I’m thinking fuel mileage and maybe performance could be a bit better if it wasn’t dumping more fuel every so often on that Bank 2 side. Why only on that side?

 

I was reading old posts and see a lot of people with different readings on both side. It seems pretty common. I'm curious if you can figure this out.

 

Let's start off by clarifying the use of the fuel trims. A "high" fuel trim does not necessarily mean that fuel is being wasted. A positive fuel addition (BLM greater than 128, or a positive % greater than 0%) means the PCM has detected a lean condition, and is adding extra fuel to correct that lean condition. It means the injectors were not supplying adequate fuel to meet the target A/F ratio of 14.7:!. So it adjusts the injector pulse widths, based on the long term fuel trim to supply additional fuel to eliminate the lean (and possible engine damaging, or emissions increasing) condition.

On the other hand,, if there is a "false lean" condition, and the PCM starts adding fuel, the fuel trims MAY indicate wasted fuel. Typical issues are leaks in the exhaust before the O2 sensors, faulty O2 sensors, and misfires. But you need a lot more data in the log to attempt to identify a "true" vs. a "false" condition, and this data log has nothing in the way of data beyond the MPH and the fuel trims.

The same can be said about subtract9ng fuel (BLM less than 128, or a negative % below 0%). If the PCM didn't reduce the fuel flow, the engine would be running rich, and possibly wasting fuel.

There are two kinds of fuel trims - short term (STFT), and long term (LTFT). A short term fuel trim is a TEMPORARY correction. The PCM sees the O2 sensor reading below 450mV, says "a bit lean", and for one instant, adds extra fuel. Or, "a bit rich" (above 450mV) and for an instant subtracts fuel. This results in the STFT's cycling rapidly back and forth between a slightly rich and a slightly lean condition. It does this to make the catalytic converter work more efficiently.. This data log seems suspicious, because the STFT's are not cycling above and below 0%.

If the PCM starts to see the STFT is always adding fuel, and not cycling above and below 14.7:1, it it bumps up the long term fuel trim (LTFT) and store it in memory, making a (somewhat PERMANENT) change to the fuel flow. In this log, that is not happening for some reason. Again, make me question the validity of the numbers. It's also like "learning" is disabled.

It is normal to see some sort of positive or negative LTFT's under various engine operating conditions. As long as these correctios are within +/- 5%, they are not necessarily cause for concern. You also want to see similar, but not necessarily identical LTFT's on both banks of the engine. Again, if these correctios are within 5% of each other, not necessarily a cause for concern. In fact, the results shown in this data log are highly suspect, because the LTFT's under most engine operating conditions are all "0". Very unusual.

In OBD-2, a data log typically reports both the pre-cat O2 sensors and the after-cat O2 sensors. In this presentation, appears they are calling the after-cat sensors bank 3 and bank 4. There are no fuel trims normally associated with the after-cat sensors, ,so those columns can be ignored.

As mentioned, the LTFT's are based on different engine operating conditions. Typically there are 19 "cells" for LTFT storage. The cells (0 to 15) are defined by a grid with RPM on the X-axis and MAP (as a measure of engine load) on the Y-Axis. There are also cells for idle (16), low loads/decel (17) and normal loads (18). In open loop, only 16, 17, and 18 are used. In OBD-1, the scanner reports the Cell #. But in OBD-2, although the system still uses the Cell system, the typical scanner only reports "engine load" as a %, making it more difficult to correlate the engine load to the typical Cell breakdown. None of this matters here, becasue this data log is lacking virtually everything you need to try to correlate fuel trims to engine operating condition.

Additionally, this log, which appears to cover 41 minutes of driving also has a scan rate of about 1 frame per second, and that limits the usefulness of the data. The PCM is updating the values close to 10 times per second.

For a bit more insight to the PCM scan data, see if this helps:

https://www.camaroz28.com/forums/com...-guide-886891/

 

Thanks for the replies and the link. I suspected this Bluetooth scan tool with the phone app wouldn’t be sufficient. It may not be showing ltft’s because I had a small nick in the Bank 2 pre cat O2 sensor wire that was just repaired and the code was cleared for it. I doubt the car has been driven enough but I see a value of around +4.56 on both engine banks for the ltft’s on that log I attached starting to pop up occasionally. I can no longer find a Scanmaster LT1 available to buy. What is another reasonably priced tool that will show the necessary data to help determine if I have false lean or not?

 

After driving for 41 minutes, some of the more commonly used cells should have started to change. As I recall, there was one area of STFT's that were as high as +14.5%. That alone should have triggered an increase in the corresponding LTFT.

We have a member (GaryDoug) who has written both OBD-1 and OBD-2 scanning software for the LT1 PCM. The OBD-1 version, Scan9495 is IMHO the best available. The OBD-2 version does a good job, but is limited by the existence of “advanced parameters”. GM, or any manufacturer, is only obligated to make available the data required by OBD-2 standards. There are other diagnostics and data that GM reserves for professionals, and they deal with systems or data that operate outside the Federal OBD-2 standards. In order to read GM advanced parameters, GM charges the scan system developer to pay a $50,000 licensing fee. So we are stuck for the most part with limited data, or buying a more expensive scan tool.

https://www.firebirdnation.com/threa...ftware.430932/

Software download is free, requires a laptop and a cable. It can produce a .csv file. I am available to help with data log interpretation, but sometimes I really get backlogged, like right now.