LT1 BLMs split behavior - school me

[sherlock9c1]

If this has been discussed before, forgive me; I tried searching and didn't find anything related.

So I'm fuel tuning a stock '94 Roadmaster wagon and a stock '95 Caprice 9C1 and I notice that BOTH of them have the right side BLMs always 1-6 counts higher with open throttle. With closed throttle decel situations or under 5% throttle, the BLMs reverse, and the left side bank is higher. Before I go chasing fuel injectors or the idle air ports underneath the manifold, is this common?

Any benefit to blowing some solvent into the idle manifold to clean it out? (Manifold removal is not in the cards at this time).

[EagleMark]

Stock motor?

[sherlock9c1]

yep, stock motors.

[EagleMark]

That little bit is normal and well within correction of the PCM.

The issue with split BLMs comes into play with larger cams and one side being 128 and the other pegged at 108. It's caused by airflow not being equal to all cylinders, yet all cylinders get approx same fuel.

I don't know what the INT count to make correction to BLM is on LT1, there's lots of paremeters missing in the files. But for instance in $42 INT needs to change by 4 to move BLM by one! So how would you ever get 128?

Don't try to clean intake without removing it! Nasty in there from EGR...

[RobertISaar]

I don't know what the INT count to make correction to BLM is on LT1, there's lots of paremeters missing in the files. But for instance in $42 INT needs to change by 4 to move BLM by one! So how would you ever get 128?

referring to the INT error window? 4 is normal, it's actually what most of the 60V6 stuff runs. in some(all? but usually hidden?) applications, there is another fuel trim that has finer resolution than BLM and INT, which does constant swinging to try and avoid the INT needing to move in the first place.

[EagleMark]

No that is O2 - Counts Err Thres for INT Correction. Movement of the INT takes place whenever the slow filtered O2 error term exceeds this value.

But I was talking about this:
BLM - Intergrator Delta To Move BLM is 4 to move BLM - BLM Delta From INT by 1

Either way none of that is listed in LT1 $EE mask... there's lots of handy things in $42 and $0D to help idle and cammed engines that is not in this mask, probably there just no one made the paremeters.

[RobertISaar]

considering where that XDF came from, i'm not surprised.

[sherlock9c1]

Well I'm back. Ended up with another LT1. And this time I removed the pipe plugs under the intake manifold when I removed it so as to check all the idle air ports.

Low and behold, one of the idle ports is drilled but is significantly smaller than the other 7.

SO FYI - If you're tuning an LT1, be sure to remove the intake manifold, remove the pipe plugs and check to make sure all 8 are drilled and are drilled the same. I'll post up the drill size mine were in a bit.

The pipe plug use a 9/16" hex. Harbor Freight sells a large hex bit set for around $15 that has it, or you can just get a 9/16" bolt and two nuts from the hardware store and put a wrench on that to remove them. Be sure to use RTV or some other good sealant when reinstalling the plugs.