I first noticed that the left bank BLM's were generally significantly lower than the right side. Then, with my LM-1 wideband, I was getting readings in the low 11's on the air/fuel on the left at WOT and not nearly that low on the right. The last time it was on the dyno, the lowest a/f number was 12.5. Looking at the tune, the commanded WOT a/f is around 12.3 to 12.1. Would you suspect a bad (leaking) injector? Would a leaking header gasket do this?

 

is one above 128 and the other below 128? How big of a difference? A leaking exhaust could do this.

 

It fluctuates--sometimes 4-5 apart then other times 10 or more apart. More difference at idle than cruising.

 

If your cam is allowing reversion at idle, that would explain it. This is a very common problem with cammed EFI motors.

 

This link may help:

http://para.noid.org/~lj/PCM%20Tutor...litBLMinfo.htm

 

I tried that on my car, but it didn't make a difference, unfortunately... But maybe something else is wrong with it. I'm still investigating it...

 

I tried closing the throttle blade as described in that article, but all that it did was cause the car to die when coming up to a stop.

 

I recently bought a wideband so I can investigate this issue further on my car. There's a trick for setting up PE mode to come on during idle, then commanding the closed loop fuel mixture (14.7) that should at least prove whether the problem is an issue with closed loop being unable to properly maintain fuel mixture under low engine speed and idle conditions.

Consider the following PE table settings:

% TPS Threshold for WOT vs RPM

RPM TPS
0 0
400 0
800 0
1200 0
1600 0
...

% Change to Fuel/Air Ratio vs RPM vs WOT

RPM %change
0 -15.6
400 -15.6
800 -15.6
1200 -15.6
1600 -15.6
...

With these settings, the PCM should be in PE mode at any thottle position at or below 1600 RPM. With a -15.6 % change, this would command a 14.7 AFR (assuming the stock % change vs coolant temp tables are being used).

I actually tried this on my car and it sort of worked - the main issue being it didnt seem to stay in PE mode consistently. I didn't have a wideband at the time so I couldn't establish actual AFR - of course, there's no guarantee that a wideband would be any more accurate under these "reversion" conditions then a narrowband would be.

 

That's an interesting experiment. You are considerably more experienced than I, so I think I'll sit back and watch on this one. Going back to my original post and expanding on it a bit, my LM-1 readings at WOT were in the low 11's and I just have a hard time believing that it is really running that rich. I went through the calculations and commanded a slightly leaner AFR but on the next run, rattled the valves so bad I could hear it plus got about 15 degrees of knock retard. Obviously too lean. But, the wideband readings were not overly lean--maybe came up a few tenths of a point. So I was wondering what would cause the wideband to register too rich.

 

At WOT, the wideband should be pretty accurate unless there is an upstream leak or something like that going on.

Are you seeing low 11s on the car described in your sig or are you talking about a different vehicle?

 

The one in the sig. Would a header gasket leak cause the wideband to read rich? If I have a leak, it is very small because I can't really tell it by listening. I did find one loose header bolt where my shop failed to replace the anti-rotation key on the Stage 8 bolt, so I guess a leak is a possiblilty.

FYI, the water/methanol injection wasn't even working when I made these runs and I didn't realize it. Good thing I had a good bit of timing pulled out. I think I will just re-calibrate the LM-1 and try again and see if I get the same readings.

 

A gasket leak upstream of the O2 sensor would generally cause the wideband (and narrowbands, for that matter) to read leaner then actual due to the introduction of additional oxygen. During closed loop, this could cause one side to have a lower L-term count then the other. It might be interesting to use your wideband on the other bank to see what it is reading at WOT, and compare the two.

While low 11s at WOT would be typically be considered too rich for naturally aspirated setups, makers of forced induction setups generally want to see a WOT AFR of low 11s, so it sounds like you're where you need to be. If leaning it out is causing spark knock, then I'd say you've found the ideal AFR, although to be thorough, you really need a dyno also to see how much power it is making, as well.

 

OK, thanks. I might try running some Seafoam through just to see if I see any smoke coming from a leak and then repair it if there is. Actually the car is running pretty good as it is now, so I doubt that I will mess with it very much. I'll also hook to the other side and get a few runs to compare. Thanks for your help.