SPLIT BLM'S...SOLVED.

on my little set up, here is what worked for me. after making sure there were no vacuuum leaks, ignition problems or exhaust leaks i was able to balance my BLM's by adjusting the IAC. On my 52mm factory ported TB, i would insert a torq 15 (i believe) bit into the the throttle stop screw hole on the driver's side top of the TB. I used a 1/4" open end wrench on the bit to turn the throttle stop screw in and out which changes the IAC counts on a scanner. I give the PCM a minute to adjust the BLM's. when things look about right i turn the ignition and engine off. This resets your TPS to zero throttle open. I restart the motor and wait for it to go into closed loop and recheck the BLM's. As little as one turn of the Throttle stop screw makes a big difference on my car. i make a fine adjustment if necessary and turn off the ignition and engine. I keep doing this till i have it right....usually 3x. fyi... the TPS microvolts changes as you screw in and out the throttle stop screw, but on an OBDII car, as long as it it between .3-.9 volts at idle, once reset to zero throttle open, and you have 100% open with the gas petal to the floor, drivability is normal. I hope this works for some...

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

Thanks for the info Carl. I have a bad split at idle and will try that this weekend. I will let you know how it goes...

Kenny

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Black 1996 Camaro SS #0082,6 spd., GTP Stage II heads, GTP6 cam, 58mm AS&M TB, FLP headers w/ ORP, McCleod Street Twin-steel, CC 1.6 pro-mag rr's, CC R-series lifters, GM ED timing chain, Lou's short stick, BMR STB, BMR torq. arm, SLP SFC, LT1 edit OBDII

hey kenny, i see you have the BMR TQ arm. which one do you have? do the poly bushings increase noise vibration or harshness?

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

97 SS what where your BLM's before and after?
And thanks for sharing the info.
Kevin

Carl, I have the "11 second version". I felt no increase in harshness or noise vibration after the install.

[This message has been edited by KenP (edited July 18, 2002).]

kevin, i use an autotap scanner which reports in percentages. it showed at idle, cell 16, passenger side was adding 25% fuel. the drivers side "normal". the plus 25% i believe represents a 160 reading on various softwares. the interesting thing is i can reverse the BLM's via the throttle stop screw, so that the drivers side is adding 25% fuel. On my car it did not take much to completely reverse things. I watch the IAC counts. a 10 count thows fuel to the drivers side and a 35 count throws it to the passenger side. On another car, it might be the opposite. You may need to really crank on the throttle stop screw and see the effects. right now i'm set at 24. As i have been playing with this over the last few weeks, i believe the MAF program has a significant affect on the cell 16 BLM readings. I have played with 4 or 5 MAF settings and while i was able to balance the BLM's via the throttle stop screw, some settings it was easier to do re: less screw adjustments. One other thing that i did in frustration, at first, then i started to do it to learn. I would make vacuum leaks (pull vacuum hoses off) and leaks after the MAF to see the affects on the BLM's. This lead me to believe that perhaps ultimately the MAF is so very important especially in cell 16, idle. fyi...in my case, other cells appear to be normal. I need to thank Bryan Herter at PCMFORLESS for his patients and assitance in working out my issue. I know the pain of these cars, i hope this helps.

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

[This message has been edited by carl97ss (edited July 19, 2002).]

I thought I'd add my latest findings. while I've been trying to fix the high historical misfires in cylinders 1,2,3 and 4 only, i found by playing with my base timing as little as 2 degrees advance or retard, the split BLM's would return and/or go away. I was always able to balance them out by adjusting the Throttle plate stop screw which of coarse affected the IAC counts and TPS MV>...just FYI...

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

OK, so where should someone start to try and get rid of the split at idle?
Start with the idle stop screw or timing at idle?
BTW, what is the acceptable range that the IAC count should be at idle?
Does a larger than stock cam necessitate a different IAC value at idle?

assuming no vacuum or exhaust leaks, i'd start at the throttle stop screw. with after mkt mods, i would think that any IAC count between 5 and 40 counts that get's rid of the split BLM's are ok. factory settings that i have seen have been in this range. After adjusting the Throttle stop screw, you may need to turn the motor off for a few seconds and restart. Adjusting the screw will change the TPS settings and by turning off the motor resets the PCM to 0% throttle open at idle. Make sure your TPS MV at idle are in spec. It made a difference in mine when i went from .52mv to .72mv, both are considered in spec by the way. If the throttle stop screw doesn't bring you in line, I'd set the IAC at 25-30 counts and add a little fuel or take a little fuel out( depending on your mods, adding or subtracting 5-10% is all you should need to see an effect in the splits.)out at idle. If no affect, try leaving in extra fuel or taking fuel out and advance the timing 2-4 degrees or retarding the timing 2-4 degrees. observe the splits and see if they close in. If the split BLM's start to get close, you can do the final adjustment by the throttle stop screw. As you can see it's trial and error and seeing what is going to work for you. On three LT1's that had different than factory cams, all had split BLM's. Good luck.

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

Problem is, doing it that way can cause essentially a vacuum leak (small decrease in MPG), you will see that your MAP is now slightly worsened by cracking the blades. With a 396 I would have gone with a 58mm (I had that on my stock bottom car) or a mono.

Open up the IAC passages a bit, and close those blades (not so much that they stick shut).

This post sounds just like something I've been going through today. Once I'm on the freeway under acceleration the BLM's stay within 10 on the Rin and Lin readings on my scan master 124-134. Come to a red light and one side or the other depending on the cars mood will soar to 160. I crack the throttle and back to 128. I've also noticed that when lets say Rin shows 160 Lin will read 108. Go figure. So I started thinking about the way it goes to 128 with a light tap of the throttle so I raised my throttle stop and the car seems to run better. I don't have it set perfect yet, but this post atleast tells me I'm getting closer. Dr. Mudge you think opening the holes to the IAC will effectively do the same thing and be a better fix?
I just put heads and injectors on mine, but before the project on the way to my dads garage mine threw a code 44 left o2 bank lean. I replaced both o2 sensors, fuel filter, fuel pump, new plugs. I've got SLP 1 3/4 shorty headers still with the original donut gaskets. I don't hear them leaking though. Maybe I'm just not hearing it, damn car is too loud. Anyways it seemed at the time when it threw a code something failed. Now with all the new stuff added I'm not sure whether its just out of tune, or if I'm still tracing an old problem.
According to everything I've read once we see 160 on Rin or Lin we are also supposed to get a SES light?? I don't get a SES light since I put it back together.
Good news is I did it myself and no oil leaks,
Fred

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1994 Z28, A4,
BEST 1/4 12.413@109.55
BEST 1/4 MPH 110.49
BEST 60' 1.668
BEST 1/8 7.884@86.35
BEST 1/8 mph 86.77
Race Weight 3600lbs
No new times with P&P heads Flow 239.4 INT 182.1 @525 lift, or
24lb SVO inj's...

I had an aftermarket 58mm TB (BBK) and had to do a couple things to get it idling correctly. Like the first post, I originally got it idling by cracking the TB blades, but noticed that it hurt my MAP numbers.

After complaining on the LT1 Edit list a few posts came up on the "right way" to do it, as well as a couple sites which show what had to be done.

With a 48mm ported to 52mm most of these problems should not exist, if your car wont idle without cracking the blades you may need to open the IAC opening more (dont push your lucky, drill bits are cheap, do it a little at a time), however your low RPM timing may also be off. My car made 411 RWHP on stock bottom, and idled at 800 RPM if I wanted it too, but I had it at 900 RPM (M6) and idle was just fine, MAP bounced around probably .05, not too much. For an NA car I was fairly happy with how well it turned out with the cam I had, so I feel that most peoples woes with good programming can be solved, fixing the TB worked wonders for my car, from BBK it unfortunately is a POS without fixing it to act like the stocker.

For a 396 a 52mm TB is just too puny IMO, but I have no data to back that up, but that thing is going to require some air! As for someone running a 240/250 approximately or larger cam, then I would also see trouble getting timing right for good idle quality, may require even larger IAC passage yada yada. I believe I used an 11/64" bit but honestly it was long enough ago I am not positive. Measure what you have now, and take it up a small amount.

Are you confident in your current timing setup down low? What RPM? I smoothed the idle timing and surrounding cells somewhat so that it would not be jumpy if it went to another RPM zone, this also helped a good bit. Also make sure your not too lean, that was another problem I dealt with - a lean idle is a weak idle. Some people run PE idle for this reason, since the split BLM possibiilties wont make the O2 sensors wack out and provide lean one side rich other, etc

Hopefully this makes some sense, preferably though you want those blades closed. I was happy with that "solution" too for awhile, but its not optimal, its cheating your way around the issue. I know that there are 396/402s etc etc idling just fine with closed blades.

Edit: The spot to drill is the little pinhole which does not even exist in aftermarket units (depending on brand), NOT THE PASSAGE ITSELF. I can clarify if not understood.

Fred, your issue is something else. Did you reprogram for your new injectors? The post is mentioning IAC counts which has to do with the TB. I had SLPs for OBD I as well, make sure that your gaskets against the heads are still 'good', make sure the headers aren't coming loose (mine liked to back out bolts around #8 primary). Also make sure the Y pipe looks snug, you may even want to put gasket sealant around the donuts on both ends where the Y meets the headers, I had to put my Y pipe on both sides at a time sort of slowly to try to make sure good seal on both sides.

[This message has been edited by Dr.Mudge (edited August 11, 2002).]

Guys I just solved my problem by installing a new IAC this morning. The funny thing was the old one read good with an ohmeter 50 ohms. It was just stuck and wasn't allowing air by the pintle. It cost me $63 but as soon as I installed it and started the car I noticed after the warm up period the Rin and Lin short memory was no longer soaring to 160. I put it in gear and no change. Mine is staying in the 120's now at idle well after the o2's are working. I have programming for the injectors, just need to get a laptop from work tomorrow so I can download it. My long term memory was starting to drop as well. Damn, now I need to get back to the track and see what gains the heads and injectors did for me.
Fred

UPDATE: I just got back from a ride. The problem returned... I saw Rin readings as low as 88 and Lin readings as high as 185 on the scan master.. No SES light though. I'm really starting to think something in my computer has failed. It will suddenly go back to 128 on Rin and Lin for a few minutes then it gets lean on one side and rich as heck on the other. Man I'm pissed. I wish I had another computer to try with my new injector program.

[This message has been edited by Fred94LT1 (edited August 11, 2002).]

DR, I've said many times, the more i think i know about these car, the less i really know. I'm listening to what you have to say and I would agree with you that, possibly the best idle quality will accure w/ the IAC controling the idle 100%....by that I mean, that 100% of the needed air for idle flows thru the IAC. On some cars that may work, but i've never been been able to do it, or have i seen that on any car...when i've looked at factory LT1's and LS1's they all have an IAC count of 5-40 and most in the 25-35 range. On modded cars, simular numbers. My feeling is that as long as the car is idling at a set RPM, that the engine/MAP/vacuum will be the same regardless if 100% of idle air is flowing thru the IAC or the Throttle plates or a combo of each. The MAP/vacuum are measured in the intake manifold after the IAC or throttle plates. In my situation(s), none of those areas change when i've takened the IAC count from "0" to 80 at idle. My split BML's at idle did change. I have read the other posts about cranking the IAC counts to 140-160 to solve the splits, but i've never found it worked for me. I too wondered about going to a 58mm on my application. but, i set my car up as a daily driver, 25,000 mile a year. I wanted strong mid-range TORQUE and off idle drivability. My engine was choosen for that purpose....cam, heads, TB, headers etc. I did not care about Max h.p. bragging rights at 6000 rpms since i don't drive there very often and when i do, i'm litterally there for only a second....(smile) before i shift. I do spend time at 2500-5000 getting to 6000 rpms. My engine builder and programer said the 52mm TB flows enough for my application. Interestingly,Engine dyno numbers and pressure drop tests on my motor, indicated no pressure drop from the intake manifold to the TB to the MAF (stock, screened!). I'm not familar with those tests/numbers but I was told that i had no restrictions in those areas to air flow vs. what the motor needed. I also noticed that at WOT, the MAP is within .5" of barometric pressure. I don't know if that is a good indicator or not, but it would appear the engine is getting all the air it needs thru the 52mm TB ??? At 6000 rpms the super flow engine dyno said the engine only needed 611 cfm. the motor made 485 h.p., 496 TQ. As we know the 52 mm TB flows 750-770 cfm. Some say my cam is too small for the motor also, (222I, 230E, @.544"I, @.566E, 112 LSA) but it pulls hard from where i want it to and is very streetable. At these power levels i've seen countless times the guy w/ a fatter midrange beat the high zinger because the high zinger doesn't pull as well after the shift (in their midrange). However, a real high zinger that runs to 7500 rpms will do in the 6000 rpm redline guys like me...smile...Looks like different avenues to the same end, can work. IMHO...FRED...i had a simular experience when i thru a new IAC on my car. I felt good for about 3 minutes...

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

Fred, your mentioning BLMs but the post is about IAC counts not BLMs, the IAC should not affect your BLMs at all. Am I missing something? Is it your BLMs that are randomly haywire? I have heard about measuring the voltage output of the IAC but never the resistance???

Carl, there is an IAC circuit not just in the TB but throughout the intake manifold, it was there to be used. Some people chat and drill a small hole in the TB blades, or crack them open like you and I did, but that is a hack solution. My cam was big enough that people with smaller cams should not have a problem getting thier stuff to work, I got my IAC counts down to the 40s with some fixing of the TB and no blade opening at all, that is a plenty fine IAC count and nowhere near the cieling of 160. I had 160 IAC counts when the TB was freshly installed and it would not idle on its own without cracking the blades open or revving the engine a bit, so I know of the problems. I will try to hunt down the IAC circuit pictures and post them later.

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94 M6 Koni DA, 600#/140-160#
Shorties, Cutout, K&N
Stock bottom 411.91 SAE RWHP@5900 RPM (dying rotor, fixed)

[This message has been edited by Dr.Mudge (edited August 12, 2002).]

<font face="Verdana, Arial" size="2">Originally posted by Dr.Mudge:
Fred, your mentioning BLMs but the post is about IAC counts not BLMs, the IAC should not affect your BLMs at all. Am I missing something? Is it your BLMs that are randomly haywire? I have heard about measuring the voltage output of the IAC but never the resistance???

Carl, there is an IAC circuit not just in the TB but throughout the intake manifold, it was there to be used. Some people chat and drill a small hole in the TB blades, or crack them open like you and I did, but that is a hack solution. My cam was big enough that people with smaller cams should not have a problem getting thier stuff to work, I got my IAC counts down to the 40s with some fixing of the TB and no blade opening at all, that is a plenty fine IAC count and nowhere near the cieling of 160. I had 160 IAC counts when the TB was freshly installed and it would not idle on its own without cracking the blades open or revving the engine a bit, so I know of the problems. I will try to hunt down the IAC circuit pictures and post them later.

</font>

As far as mentioning BLM's at idle I added that because My BLM's were getting way out of wack at idle. It was running real lean on one side and rich on the other. I thought the two might be related. Carl is adjusting his throttle stop with success for idleing. According to my Haynes manual the resistance on that little step motor(IAC), should be between 40-50 OHMS. OHM readings between the a and b and c and d terminals. That is only to measure the resistance of the motor windings. BTW, My pintle was stuck and wouldn't move.
Fred

Hey DR. As odd as it sounds, as i stated earlier, the IAC counts at idle affect the idle split BLM'S. I know where ya are coming from w/ the idle air passages thru the intake. All i can say, the shutting down of the throttle plates and having all the idle air going thru those little holes at the mouth of the intake manifold runners, has never worked for me.

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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

Well, you do have a 396 and I have a 350 so who knows, but with my cam and 40ish counts, I figure anyone can do this, and I didn't open up my 58 much more than stock.

Here are some goodies, compliments of Christian:
<font face="Verdana, Arial" size="2">I'd say the split BLM and IAC are LT1edit issues for sure, never be afraid to share stuff, because you're rarely alone.

The actual IAC passage that needs to be opened is the very small hole pictured here:
http://carprogrammer.com/Z28/PCM/TB/DSCF0706.JPG
I opened it to look like this:
http://carprogrammer.com/Z28/PCM/TB/DSCF0699.JPG

Don't mess with the actual IAC passage. Several other stock pictures here:
http://carprogrammer.com/Z28/PCM/TB/

You basically want to get the air flowing through the triangle pictured here:
http://carprogrammer.com/Z28/PCM/TB/DSCF0704.JPG
And less air in the intake plenum, since rumpy cams introduce reversion like a mofo.

Keep me posted,
-Christian

Check out Als' updated page on the IAC passages- really good!
http://members.cox.net/chipsbyal/idle.htm
</font>

[This message has been edited by Dr.Mudge (edited August 12, 2002).]

<font face="Verdana, Arial" size="2">Originally posted by Fred94LT1:
As far as mentioning BLM's at idle I added that because My BLM's were getting way out of wack at idle. It was running real lean on one side and rich on the other. I thought the two might be related. Carl is adjusting his throttle stop with success for idleing. According to my Haynes manual the resistance on that little step motor(IAC), should be between 40-50 OHMS. OHM readings between the a and b and c and d terminals. That is only to measure the resistance of the motor windings. BTW, My pintle was stuck and wouldn't move.
Fred</font>

Ooooooh, ok. If you've got a 'biggy cam', this is why many people give up on close loop idle and run PE up to 1200 RPM in the PCM, then make sure the AF is good on the dyno and you have closed loop operation like a normal rig the rest of the time.

<font face="Verdana, Arial" size="2">Originally posted by Fred94LT1:
UPDATE: I just got back from a ride. The problem returned... I saw Rin readings as low as 88 and Lin readings as high as 185 on the scan master. It will suddenly go back to 128 on Rin and Lin for a few minutes then it gets lean on one side and rich as heck on the other. [This message has been edited by Fred94LT1 (edited August 11, 2002).]</font>

Fred,
The car I've been working on that has split blm's at idle does the same thing. For about 30 seconds the blm's will be within 3-4 of each other and then one will start to drop and the other will rise. MAP, idle rpm, and timing doesn't change but the blm's start to go crazy. On one side the long term will drop to 108. About 10 seconds later the short term will rise and go over 128 and then the long term will start to head back to 128. At the same time the other bank will be rising up to 140 or higher and after 10 or 15 seconds the short term will drop below 128 and the long term will start to drop again.
Then they'll both sit at 122-126 or so for about 30 seconds and the process will start all over again.
I guess we're going to try and drill out the passage that Dr. Mudge shows in the post above and see if that helps at all.
Then try to adjust the IAC again.

well at least you know your sensors and PCM are working correctly from the standpoint that the short term and long trerm fuel trims are trying to do what they need to do. I had that issue and in my application it turned out to be slow O2's and/or the non-GM O2's. i replaced w/ GM O2's and that swinging back and forth stopped. FYI...the electrical tests of the O2's were fine. scanner really did not show a slow condition.
Dr mudge, if you get a chance, email me about your mods and cam you use. thanks!
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'97SS, More Performance 396, AS&M cold air, Grotyohann LT, SLP 2ODL muffler, Magnacor, PCMFORLESS PCM, LS1 twin piston calipers, KVR crossdrilled and coated rotors, SLP stage II w/ KYB AGX, 6M, Star Stage II.

[This message has been edited by carl97ss (edited August 13, 2002).]

These are GM O2's. The car is a 93 and we converted it over to heated O2's using all GM parts. Sure wish I could figure out why it does this.
Not sure if it hurts anything or not though. I guess I'll just adjust the idle stop screw until the O2's are about as even as they can get.

Dan, I dont imagine stock speed density cars have that problem, do you have any mods though that might explain it?

Carl, pretty simple, Heads, SLP shorties, K&N intake cutout, proprietary cam similar to cc306 but better lobe profile, without getting into every single part thats it.