Stoopalini

I recently had a chance to run the 1/4 mi with EFI Live monitoring, and I have a few questions I hope someone can answer for me.

When comparing the Long term fuel trims, the right and left values are never the same. For example, Bank 2 shows a value of 5.47 for 50 % of the run, while Bank 1 showed 0 for a while, then -5.47. As a matter of fact, Bank 1 was always a negative # or 0. Is this normal? Does this mean that Bank 1 is adding more fuel?

Second question is the ADV *BTDC reading. This value was anywhere from -16 to -20 at idle, then changed to -30's during the run, then went to a +4 during the slowdown for the turnoff. Is EFI Live reporting negative numbers as advanced timing? In other words, when I see -19*, is this telling me that the timing is running 19* BTDC? You would think since the field is ADV *BTDC, the numbers would be positive and not negative?

Thanks for any advice here, I'm new to the OBD II world. Why couldn't all PCM/ECMs be as easy as my old '92 TBI

Thomas.

Injuneer

Yes..... -20deg = 20deg advance = 20deg BTDC.

Do you have the Cell references to go with the long term values? That will show under what condition (load and rpm variations) the specific values occur.

What the values mean is that Bank 2 would be running lean, but the PCM is correcting it by adding 5.47% extra fuel (BLM = 135) to the standard calculated values. And Bank 1 would be running rich under some conditions, but the PCM is pulling out 5.47% fuel (BLM = 121)from the standard calculations to keep it at 14.7:1.

If those are the largest corrections you saw, they are not all that extreme, and within normal tolerance.

All of this ASSUMES your O2 sensors are providing accurate feedback. A poor extension or poor ground on one sensor could be throwing it "off" on one bank. So could a misfire, exhaust leak or leaking injector, which only affects on bank of the engine.

Stoopalini

I do have the complete chart, but I have no place to host it on the web

The largest correction was -13.3 for bank 1 (this at idle), and +5.47 for bank 2 (at WOT). Is this still within tolerance levels?

Thanks for clearing up the data for me, I appreciate it. It sure does help to know what you're looking at

The only O2 extensions I am using are for the rear O2s (RT Y pipe). I know I'm having some sort of misfire/O2 problem (as you know per previous thread), so I guess it's troubleshooting time to figure it out. I suppose I'll replace the wires and swap the front O2s from side to side, then scan it again to see the difference(s).

Thanks again, very valuable info,
Thomas.

Injuneer

For more info on scan data, you might want to look at this.

It is "normal" for long term fuel corrections to up to maybe +/-5%. Beyond that, something may be wrong. What makes it even worse is the case where one side of the engine is adding fuel (your +5.47%) and the other side is subtracting fuel (your -13.3%). That is call a "split BLM" and the cause needs to be tracked down. Can result from radical camshafts, the idle air on some aftermarker throttle bodies, etc.

Swapping the O2 sensors left to right is an excellent idea.

Stoopalini

Thanks Fred, I've actually read your pages a few times now, and I learn more and more every time I read them

I climbed underneath the car last night to swap the O2s, and I think I found my problem. Upon inspection of Bank 1 Sensor 1, I noticed that plug wire #7 was resting against the O2 sensor.

Seems to me this could cause both the misfire I was having and the strange O2 readings from that bank. I decided not to swap the O2s and just repositioned plug wire #7 out of the way. I drove it to work this morning, and it seems to be running much better!

Tomorrow I am going to scan it again with EFI Live and see what the long term fuel trims read. Any idea how long it takes for the LTFTs to settle out?

I wonder what a spark plug wire arcing to the O2 sensor does to the PCM's fuel correction

Thomas.

Injuneer

Very good catch!!! The high voltage pulses in the plug wire can induce voltage in the O2 sensor wire. Will not take much to upset a system that relies on 1/1000ths of a volt.

Let's see what happens. The PCM will start to alter the long term corrections as soon as you operate in a particular Cell. So.... the Cell 16/idle values will start to change as soon as the PCM goes into closed loop, and will stabilize in a matter of a minute or two. The other cells, which you may not operate in very frequently, can take a while longer to sort out. I always think back to the comment Kieth at WS6.com made about his seeming 16HP gain with a reduced fuel pressure, and the fact that the gains disappeared over a period of about 2 weeks. Can't understand why it would take that long, unless he just didn't happen to operate it near WOT very often in that period.

Stoopalini

Hmm, interesting. So if the spark plug wire was resting against the O2 for any length of time, the tables could be pretty screwed up by now.

Would disconnecting the battery reset the tables to defaults? If so, would it be advisable to do that .... would it put the PCM's tables back into a workable state without having to operate in the table for a length of time?

Thanks for your input, I do appreciate it!

Thomas.