Anyone know what causes split BLM's at idle? At cruise and part throttle, they are right where they should be, in the 120 range, give or take a little, usually within 10 or 12 numbers of each other. At idle, they start to float around, and the right will drop in the 80 range, and the left will shoot into the 140's to 150 or so. My long term fuel trims are 128 for the right, and 160 for the left. To give you a little background about the car, its a 97 SS with an OBD1 PCM with a PCM for Less mail order tune, 396 with 11:1 compression, 242/252, .575/.575 113 Comp Cam, AFR 210 LT4 race ready heads, heavily welded and ported LT4 intake, AS&M 58mm TB. IAC's are around 80 or so at idle warm. Any suggestions? Thanks in advance.



Nick

 

its beyond me, butim highly curious to find out the answer to that one myself..

 

You appear to be a little confused on the terminology.... BLM's are the long term fuel corrections. When you start talking about the BLM's floating around, and going as low as "80", you appear to really mean the "integrators", which are the short term corrections. The BLM/long terms can not go below 108.

You really only need to look at the long terms.

And, if your long terms are 128/160 at idle (you do mean Cell 16, right?) you don't have a "split" in the classical sense... .when you get into cam reversion problems and IAC flow problems, it usually results in one side dropping toward 108, and the other rises to 160. Your right side appears normal at 128, so its only the left side (160) is max'd out. Might be something that only affects Bank 1, a minor exhaust leak, a misfire on that side, a vacuum leak on that side, or a faulty O2 sensor or wiring/extension problem.

 

wow sounds like you know what your talking about....thats all WAAAAY above me lol.. i wish i could understand it better.

 

I don't know Fred. I've see the BLM split idle act in many different ways.

It's very common problem with a large cam because of the reversion and the LT1 intake manifold's short runners. I don't recall if the AS&M throttle bodies have the IAC bleed drilled in it and IAC system isolated from the main plenum. For some really good information you can go here http://para.noid.org/~lj/PCM%20Tutor...litBLMinfo.htm

Another thing that seems effective in tuning idle is the closed throttle spark advance table.

Of course it could be another problem but if it works fine everywhere but idle that does tend to narrow it down. One thing you might want to try is to switch the O2 sensors from side to side just to verify their operation. Exhaust leaks, vacuum leaks, improperly lashed valves, etc. will all aggravate the situation. What are the MAP readings at idle?

Even with everything running correctly if you have a very large cam it's sometimes better just to run open loop idle. This is a last resort, IMO.

 

wow thats a great link you posted! thumbs up

 

how is your low RPM driveability, any surging or bucking? on my car with a tiny edelbrock performer I have had mild to extreem surging below 3,000 going to be on my third reburn soon

 

I'd open up the TB some to get those IAC counts down (maybe down to the 10-20 range at hot idle). That should help bring your BLMs (or integrators...) together.

 

While that brings the IAC counts down it usually makes a split BLM much worse. The reason for the split is the reversion and the short intake runners. Fuel for one cylinder will be blown back into the plenum of the intake manifold. Then an adjacent cylinder will suck that in on an intake stroke with addition of it's own fuel. You get one cylinder running too lean and another running too rich. This shows up as a lean condition in the exhaust so the PCM richens it up some. It becomes one big downward spiral.

The IAC passages use their own plenum that's underneath the main plenum. There are small holes in the runners just in front of the cylinder head intake ports. The air/fuel mix doesn't get blown back up into these holes very effectively.

By opening up the throttle body you are depending more on the main plenum, which is what causes the distribution problem. You want ALL the idle air to go through the IAC system as it isn't near as sensitive. The "proper" way to bring the IAC counts down is to enlarge the bleed hole around the IAC valve.

 

I haven't been keeping count but I'm well past 3 reflashes just to get the VE tables alone tuned in (I run speed density). If you haven't already, look into Joe Georger's VEMaster program. Sure beats the old way of exporting the data to a spreadsheet! http://home.comcast.net/~jgeorger/vemaster/ Make sure everything is mechanically sound and the O2 sensors are working properly of course.

After the fueling is taken care of the ignition tables are really where it's at to improve drivability of a cam'd car. Unfortunately you don't have any electronic feedback to help you out, just gotta go by feel. Make note of where the problems are at (RPM and MAP) and make changes slowly until you've massaged them out. The biggest changes for my car has always been the low RPM stuff where reversion again makes life difficult. I've had to give the mixture a lot more lead time in many places to make up for the dirty mixture. Kind of hard to describe but surging wants more timing, bucking means too much.

It's only as good as the effort you put in. My car is running almost too smooth for a cam, but I've invested a LOT of time.

 

Hey guys, thanks for all of the replies. Fred, thank you for correcting my terminoligy. Yes, the events I am talking about are happening in cell 16. I am going to go over a few things on the car, as I only have about 700 miles on the new motor. I will check for vacuum leaks and the wiring to the 02's as well, but I use 02 extender harnesses which worked great before. I also sprayed the intake with water and a little soap in it to listen for changes in RPM to detect a leak, all checked out OK. I am going to also try switching the 02's from side to side to see what happens, and I also have 2 new ones in the spare parts room. Also, i will try the trick on LJ's website using the 1/2 piece of copper pipe, as I run an AS&M throttle body as well. I will post my findings in the next day or tow, and let you guys know what happened. Thanks again, and keep the good ideas coming!



Nick

 

My Long term fuel trim counts are running 119/139 (L/R). Any thoughts?

 

Joebeemish I have dealt with split BLM in the past and research and many e-mails helped me fix it, Injuneers scanmaster write up helped me a lot and shed a lot of light on the problem for me. http://members.aol.com/InjuneerZZ/ScanMast.htm

 

Thanks, I have been looking for a write up like this for a while.

 

Hey guys, I have completed the mod on LJ's website concerning the brass tube that allows the IAC to pull air from the "circuit" cast in the intake. I am using the same TB as LJ, and AS&M 58 MM with a heavily welded and ported LT4 intake. LJ is correct about the "major short circuit" between the AS&M TB verses the stock one. I havent had the chance to fire the car up yet, but I will this weekend. I will post my findings after I do.



Nick