Typically when I see split BLM values it is a result of some engine modifications, throttle body modifications, etc and it is isolated to idle. Today I data logged my car for an hour and found that at a steady cruise I seem to have a fairly consistent 130/118 BLM split.

Trying to think of reasons for such a split I only come up with a few possibilites.

1. Exhaust leaks
2. Dirty Injectors
3. Unequal fuel pressure (early LT1 fuel rail)

Any exhaust leaks my car has are minimal, at the collector gasket. These are downstream of my O2's so I can't imagine they would really affect it that much. It seems to me that at a steady cruising RPM the exhaust flowing out of the pipes would easily keep any air from entering in and backup up to the O2's and fooling them. There are no leaks upstream from the O2's. It's extremely obvious to spot an exhaust leak when you're using copper gaskets. I assure you there are none.

What about fuel delivery? Possible that the injectors aren't flowing as well on one side as the other? I guess I could swap sides with the injectors and see if the split changes sides. What about the early style LT1 fuel rail? It doesn't have the additional front crossover tube that the later styles have. Is it possible that there is enough of a fuel pressure difference to account for the split?

It's not a major problem, the car drives perfectly. It's just a pride thing to get the tune tweaked as perfectly as possible. It's impossible to get the BLM's where they need to be when you're fighting a constant split.

 

Have you varried idle speed to check for that trend in different BLM cells?

I could think of injectors, intake air leaks, valve leaks, valve adjustment, ignition (bad plug or wire) and such as possibly contributing to such a split. If the trend occurs across various speeds or not, that information might narrow the list.

You might even swap O2 sensors.

 

The split seems to be present to some degree regardless of the trim cell that it's in. Interestingly enough I just noticed that the split seems to reverse itself at idle. Now what kind of sense does that make?

 

Maybe swapping the O2 sensors will shed some light; I'm wondering if one is flaky and dropping out at idle.

 

Here's a little bit of reading. I can't vouche for the information. Just stuff I've had filed away:

http://para.noid.org/~lj/
http://www.hashmarks.com/techtips/throttle_body_mod.htm

The first suggests that overlap created by larger cams distrupts the contents of the intake manifold. There are several pages there of info. You might have to dig around for the split blm page. Start with the PCM Tutorial link on the left.

 

Actually both O2's are doing exactly what they are supposed to according to the data log.