So what all can cause this?

At idle my car goes into closed loop once warmed up and instantly to Cell 16. The BLM's are 108/156! I also have a split in very light throttle and decelleration (Cell 18) that is similar...

However, all other cells, normal driving, etc. I show no signs of split BLM's with many of them hanging right around 128.

What is there mechanical that can cause split BLM's at idle only? Cam is a 224/230 .600/.600 108+2LSA.

I tried turning the throttle blade stop screw in (So all idle air is controlled through the IAC) and out (So no air is controlled by the IAC) but had little success playing with it tonite ... However, I only gave it 10 minutes between all the way in (where throttle blades felt "sticky" and IAC count was 60's at hot idle) and pretty far out (IAC at 40's hot idle).

And ideas? I am willing to try anything ... Also, for an idea of my datalogged numbers take a look at this thread:

https://www.camaroz28.com/forums/sho...d.php?t=709641

 

There are many posts on this issue. Try a search for "split BLMs". There are tons of things that could be causing this. Yes the method you are using can help but will not if you have issues somewhere else..........ie. VACUUM LEAK/bad .02 sensor/tps sensor/ etc., etc.

 

Normally a big cam is going to cause split BLMs at idle.

 

Are you running an aftermarket TB? Most of them have the IAC passage issue that can cause split BLMs.

http://www.noid.org/~lj/PCM%20Tutorial/SplitBLMinfo.htm

 

Not running an aftermarket TB yet.

TPS checks out fine. Shows around .55V at idle and goes all the way to 100% linearly if I slowly work my way up following it in FreeScan.

02's are fairly new.

There are NO vacuum or exhaust leaks either.

I have chased down all the normal culprits.

Also - when searching for "Split BLM's" it seems that most people have issues across all Cells if it is vacuum or exhaust leaks or faulty sensors. In my case, I am only seeing it at 0% throttle idle conditions, 0% throttle decell conditions, and very light throttle application. It is ONLY in BLM cells 16 & 18. I am sure it is from the camshaft, but now I am scratching my head trying to figure out how to correct it... I can deal with a small split, but one being 108 and the other 158 is too much for me to be comfortable with.

Oddly enough, even if I close the throttle blades entirely with the set screw, the IAC counts on a hot engine stay fairly low (60ish). Will try cleaning the TB blades, IAC, and IAC passage, then thread the TB set screw all the way in again. If I can get all the air to pass via the IAC passage at idle, I hope between that and smoothing out my timing I can get it to stop...

Any one else running a similar set up or have a similar condition on the stock TB?

Thanks,

Joe

 

Cell 18 is the open loop accel operating cell. Cell 18 is also used at WOT under certain operating conditions. If you are in closed loop, you shouldn't be seeing Cell 18 at part throttle. Cell 17 is the decel cell. Both of those are mathematically derived from the other cells, using a weighted average. If you have a bad split in Cell 16 (idle = closed throttle) Cell 18 will have a similar, but not as severe split.

The overlap caused by the 108 LSA is probably causing significant reversion in the intake manifold at idle.

 

Injuneer,

I am looking through my datalogs and notice I tend to spend a LOT of time in cells 16, 17, and 18.

If it were your car, what would you be doing (assuming I ruled out the possibility of O2's, TPS, etc.)

I imagine if that is the case, I am going to have no choice but to keep the throttle blades closed as far as possible, and REALLY work on the timing and MAF tables to smooth out my idle as much as is possible?

Is there anything else you think I should try first?

Also considered changing the BLM cell RPM and MAP boundaries to cater more specifically to the new RPM and kPA ranges the engine sees... But was afraid to change anything until I work some of this split BLM issue out.

I am starting to get frustrated with this one ...

 

If you have also confirmed spark to each cylinder and you don't have any fouled out plugs, which you will shortly running blms like that, and you have ruled everything else the last option my be to run on an open loop tune by locking the blms in. I had to do this but I have a wideband that I use to tune everything in. This smoothed out my idle perfectly and the car actually ran much better and didn't foul out my plugs after this. The main thing is to ensure that you don't have a vacuum leak in any of the cylinders thus masking an actual problem by running an open loop tune. This could also be potentially dangerious if your tune is not already close. I recommend running it on a wideband especially for any WOT runs to get the A/F in prior to going all out ie starting out running it up to 4500rpms WOT then 5000rpms then 5500rpms so on and so forth. I use tunercat for changing PCM values and an excell spreadsheet that I can give you for tuning WOT if you don't already have it. Datamaster is a tool I use for troubleshooting and spark advance tuning while watching the spark retard value.

All this being said; be sure to check all the other culprits before you chalk it up to cam/combo and tune the .02 out of the equation.

G/L!

 

Airbornec507 - I also am running a wideband O2 on the car. As soon as I start to drive it, I stay (for the most part) at 14.7 give or take a little. When I come into PE, my AFR drops to 11.8-12.0 all the way from 2500-6300RPM's.

Is there any way to have it just run in open loop at idle only?

I am fairly new to tuning...

That said, I think I will play around with the throttle body stop screw and my idle timing. PCMforLess has my Closed TPS Spark Advance idle timing set at 27* in 400-800 and 800-1200, 30* 1200-1600, 34* 1600-2800.

I am thinking advancing it a few degrees at idle and smoothing out the transition from 30-34* may improve driveability.

 

Oddly though, when I log it at idle (and it does read TPS closed all the way, 0% .55V) my timing jumps from as low as 25 up to 30. My idle RPM never drops below 800 or above 900. This was at a hot idle.

 

The jumping timing is normal, its the spark advance/retard to control the idle there are tables for it in tuner cat. The lt1 uses the iac and ignition timing to control engine idle speed. Obviously advancing ignition is faster than moving the iac.

If I were you I would look at what the individual cylinder fuel trims are. That is about the only shot you have with fixing cell 16 and 18 outside of some other issue. If you're 100% sure the injectors are in good shape and flow the same, and the ignition is working properly, along with being certain there is no exhaust leaks, then that is where you want to look. I never concern myself with decel or engine braking, its going to do what its going to do there. Only worry about it when you're on the throttle or at idle, because thats really the only part of it you can have an effect on and that is also the most important time.

 

Wow I just read back over some of the posts and noticed you have a 108LSA on that grind. Fred makes a very good point on that LSA/grind causing problems at idle.

To my knowledge I do not know of a way you can cause the PCM to run open loop whenever you drop below a certain rpm. To program out closed loop and lock the blms to 128 you have to go into the constant tables and "max out the min. cool temp. for closed loop" and "min. cool temp. for closed loop, Cold MAT". Then set the min and max blm values just below that in the constant table to 128.

By the values you gave based off of your results while at WOT it sounds like you will be fine to use this tune. You may also need to retune the PE mode so that you aren't so rich. That AFR is good for nitrous but isn't optimal for N/A and is realy quiet rich in my opinion. Shooting for a 12.8:1 to 13.0:1 AFR would be better on an N/A tune. It also seems that with that much fuel you could advance the timing quiet a bit more than what it is. That being said I'd leave the timing where it is and get the AFR ironed out then advance the timing a degree at a time until you get a tad bit of knock, then back it back down one degree and you have found your optimum tune for WOT N/A. If you found a dyno you could still use it to mess with the AFR a bit to see exactly what AFR your combo likes the best.

 

Yea, that one kind of dawned on me when I was looking in Tuner Cat...

So tonite I ran the car for a while datalogging and experimenting.

Running the throttle stop screw all the way in (to point where TB blades hang up -and I just cleaned them and IAC) produces the exact same split. IAC counts were in the 60's.

Running the throttle stop screw all the way out to the point where my hot idle IAC counts were merely 5-10. Produces the exact same split.

Checked that no wires were crossed. No wires were cut/arching with engine running in the dark.

I guess tomorrow I will pull each wire off and put an old plug in it. Arch it across on a good ground and make sure each cylinder IS getting spark. It's actually really really hard to determine a missfire with it loping like it does. Even with an injector harness connector unplugged - the car still is hard/impossible to tell its only running on 7 cylinders. But FWIW - wouldn't I have split BLM's at all RPM's not just idle - if it were a vacuum leak or miss fires on a cylinder or two? I need bed ...

Also, there are no exhaust leaks from the heads all the way to the cut out/cat back.

Joe

 

Airborne - I would rather not go to an Open Loop only tune but I will if I have too...

As for it being rich at WOT - I do plan to bring that back down to the 13.0 AFR range once I iron out the rest of my dilemmas with the car. The car actually will be run on nitrous, but I have a wet kit, and plan to jet accordingly so that my AFR is only around 12.0 when on the spray.

And damn, that's what I thought about only being able to lock it into open loop based off temp... Owell.

No I am not sure where to start ... Guess I will have to really go over the ignition well to be sure I don't have a missfire.

As for my individual cylinder fuel trim multipliers there are as follows:

Cylinder - Off Idle - At Idle
1 - 1.03 - 1.06
8 - 0.98 - 0.95
4 - 1.02 - 0.95
3 - 1.02 - 1.00
6 - 0.99 - 0.98
5 - 0.97 - 0.95
7 - 0.98 - 0.94
2 - 1.02 - 1.08

These are set just as the were when PCMforLess tuned my PCM for me.


Thanks,

Joe

 

What rpm are you trying to have it idle at? It may have to have a higher idle than you're looking for to correct the split. By changing the speed the engine is running you can change what is occurring in the intake manifold. I would guess with that much overlap you need to idle at 850-950 to get it under control with the amount of overlap that you have.

The base individual cylinder trims I use for a cammed car when I start a tune are:

Cyl-Off idle-At Idle
1 1.03 1.03
8 0.97 0.96
4 0.99 0.97
3 1.02 1.01
6 0.97 0.97
5 0.98 0.97
7 0.97 0.94
2 1.00 1.01

You can try them and see if it helps with your issue at all.