ULTM8Z

So I was reading through some of the material in the archives - particularly 69Ghost's responses in one of my old thread's.

Found out today that I definitely have split BLMs (even though the car idles pretty smoothly now).

I went back today and ran a test since I have two O2 sensors (only one works with the ECM at any one time though). Note, both O2 sensors are Delco AFS-74. With the drivers O2 plugged in, I was getting a BLM of 135 (multi-meter on the passenger side O2 showed about 800 mV). When I swapped the ECM over to the passenger O2, BLM was 122 (multi-meter on the driver side O2 showed about .100 mV).

I then tried backing out the idle stop screw all the way (essentially forcing all idle air through the IAC passage). The split BLM improved a little going to about 131 driver side and 124-125 passenger side. Although the Miniram lacks the built-in idle passages of the LT1/LT4 manifolds, it seems that the solution is the same- avoid idle air through the main plenum as much as possible. With the Miniram, this is of course impossible due to the wide open TB flange, but the minor improvement from diverting all air through the IAC seems to point to something similar to the LT1/LT4. This now gives me more impetus to try some different IAC configurations. Perhaps that remote IAC from the V6 cars, or a tube off the back of the throttle body, etc.

branz28

Hrm, now you've got me wondering about my car (LT1 Intake). Guess i'll have to make way for another O2 and just find out.

ULTM8Z

It comes in very handy for diagnostics. I routed an O2 sensor harness from the passenger side O2 into the car. I also connectorized the purple O2 sensor wire on the ECM harness so I can switch back and forth between driver and pass side. While the ECM is using one of the O2s, I can put a voltmeter on the other O2 to see how fuel-trimming on one side of the engine affects the other side.

69 Ghost

iTrader: (1)

Don't know if the ECM's that have dual O2 setups actually vary the banks injector pulse. I think it is worth the investigating to see if code could be put into SAUJP for this.

327_TPI_77_Maro

iTrader: (8)

Huh. I have been told that I may have split BLM's due to my 219/229 @0.05 cam's self egr'ing (its a flat tappet so it has big slow lazy ramps compared to a roller, lots of unfortunate overlap). I was told to open the IAC way up by closing the throttle blades down at idle. I have a stealth ram. Does this have "built in idle passages" like an LT1/Lt4? I have never seen an LT1 apart so I didn't know what you meant by this. The stealth ram is just wide open inside. I'm curious, how does routing most of the idle air through the IAC help out in our situations, as far as equalizing the bank-to-bank AFR?

ULTM8Z

It really doesn't help that much (if at all). I noticed a very minor improvement (a couple of BLM's points). Thing is, the goal is to route air uniformly to all cylinders. Somehow, routing all air through the IAC (even though it dumps right into the plenum too) allows for improved distribution. The IAC port is closer to the centerline of the plenum than the throttle bores (only explanation that I can think of). This is prompting me to revisit that tube idea again- that is, run a tube from the IAC port to the middle of the plenum and have the air exit at that point. I think right now, the #2 and #1 cylinders are hogging most of the incoming air.

Basically, on the LTX manifolds, there's a separate idle air passageway cast into the manifold which terminates at the throttle body flange. There are separate passages that go to each cylinder (almost like a separate intake manifold for the idle circuit, but built-in to the main manifold). It's entry point lines up with the IAC port on the TB. This forces most of incoming idle air to be distributed much more uniformly than what's happening with our intakes. GM must have figured this one after running into the issues we're seeing. The best we can do is to mimic GM's solution through some other means (like the tube idea).

69 Ghost

iTrader: (1)

There are posts on some of the newer sites that also talk about the throttle body itself. I think one guy was running a TPIS monoblade and said the problem was caused by 2 blades in the TB and that the Monoblade did not have a problem. Other posts talk about GM going to the round throttle body on the newer intakes to cure the problem also. I saw a pic of a RamJet setup and you can clearly see a idle hole in the center of the blade. In fact some of the guys are doing that on their LS1 engines. I would not recommend drilling a hole in the blades though. Another thought might be to try to take the back of the TB and cut out the back of the 2 blade holes as much as possible to make it one oval port to mimick a Monoblade setup. The LTX intakes would have to be modified also but with a MiniRam you would not have to.

vernw

iTrader: (2)

I'm seeing a similar problem with my SuperRam 383. I have a WB O2 in each bank, only one of which is tied into the ECM (strictly for datalogging), the other goes to a gauge on the A pillar. My passenger's side is running 1.0 to 1.5 AFR richer than where the heated narrow band O2 (AFS-74) keeps the driver's side via the ECM.

I've been trying to run this problem down for several months now, suspecting a mechanical problem of some type. (see thread https://www.thirdgen.org/forums/alte...ut-trash.html). About all that's left is to check the valve spring pressures and/or start pulling everything off the top of the motor (again ) and keep digging deeper until something obvious shows up. I guess I can give the IAC stuff here a try, but it sounds like it would be a minimal if any gain.

ULTM8Z

I wouldn't go tearing anything down yet. Odds are, you have the same idle airflow issues that I and many others have. I think if we remain patient and resolute, we'll figure out the solution for this split BLM thing.

vernw

iTrader: (2)

I sure hope so. I wonder how prevalent this problem is, I kinda suspect that there aren't very many of us with sensors in both banks of the motor. Maybe the problem exists in ALL (even stock TPI) intakes???

Plus one other tidbit on my particular problem - it's not just at idle. The closer to WOT I get the less the difference is, but the difference still exists (around 1.0 difference) at WOT.

So my problem may still be mechanical (lifter, valve and/or spring, cam lobe, vac leak, etc.), right?

69 Ghost

iTrader: (1)

All I have been stating that my theory is this was unknown to first gen's due to only one O2 sensor being used. There are theories about runner length causing the issue and the dual blades. I think GM found out about this in the mid 80's and may have been the whole reason they went to dual O2's in the first place. I have 2 TPI units. One came from a early Camaro and one a later Vette. Guess what? The Vette unit has air ports in the intake runners like the LTX units the Camaro does not. What does that tell you?

General consensus is that the problem is mechanical in nature and the fix is also mechanical. Some guys say they don't see a problem until they put a cam in. They usually say the larger the cam the bigger the split. Other issues can be leaking exhaust, etc. I have always wondered if the firing order has anything to do with it causing to favor one side over the other.

JP86SS

iTrader: (1)

I've been contemplating the problem may be with the exhaust tracts being different on one side to the other.
All of the theories keep hitting on the intake side of things. Maybe there is slightly better flow from one side due to the "Y" pipe being used or in your case Vern, 2 cats that may flow slightly different. Not so much a maximum flow problem but just a slight differential in the pressures at low RPM.
Exhaust pressure will change VE easily.
Havn't seen any posts from guys with true duals that mentioned this problem.
Might just have missed them though.

vernw

iTrader: (2)

Except my problem is not just at low (idle) RPM levels. AS for my problem, we may need to take it to my post on the problem instead of hijacking this one (sorry!)

And thanks for the suggestions and thoughts!

ULTM8Z

I have true duals. Doesn't get any truer than mine! I have the split BLM problem.

69 Ghost

iTrader: (1)

I have true duals with a h-pipe before any mufflers. My exhaust is a 2.5" flowmaster exhaust with Ultraflow mufflers. I have not logged both sides at the same time -yet. A buddy of mine had a 94 Z28 with gutted cats and a performance Y-pipe. He had the splits. The car from the post also has duals.

JP86SS

iTrader: (1)

It just was hitting me as odd that on some of the setups, closing of the throttle blades and using the IAC passages smoothed or reversed the problem. With exhaust not in the picture it comes back to an intake distribution problem.
Firing order issue? (5 & 7 interactive issues causing pressure variances withing the plenum area) Possibly velocity related pressure drop down the center of the plenum when using the the blades as the control.
There is certainly an amount of inertia present that must be overcome as the air travels into the plenum. Adding IAC air into the distribution may cause additional turbulence so the chaos in the plenum is evenly turbulent resulting consitent pressure zones during each cycle event.
The tubes to individual cyclinders as you mentioned would cure the zoning of pressures if the throttle follower of the IAC was set up good.

OK, final thought (for now)
If the IAC trick works for some apps, why not try some more aggressive throttle follwer values to obtain more air throughout the throttle range.
Might have the same effect as the adjustment at idle and keep the turbulence or pressure more consistent.

69 Ghost

iTrader: (1)

You can try the IAC steps. Here is the link that I believe started all of this and that is what is actually suggested. The genral rule of thumb is to open the IAC TB bleed hole to .125 and slowly keep opening it until you get it to equalize with the IAC steps at around 30 and the blades fully closed. I have a .156 hole in my TB with my car idling at 725. Be aware the newer TB's -even on original TPI units have holes and the old ones don't. I verified this with my old TB and a newer TB. The stock hole size is about .08. Again another reason why I think GM found this out. My TB is ASM 52MM original mod that looks like the pic in the link. I used JBWeld instead of a tube since I have a LT1 intake to seal up the leak and a newer TB gasget style. Note that header leaks, vacuum leaks, etc can affect the split.

PCM Tutorial - Split BLMs

another:

SPLIT BLM'S...SOLVED. [Archive] - CamaroZ28.Com Message Board

another:

Grand Sport Tech Tips: Throttle Body Mod