I've gone through all SORTS of things! Upon tweaking the throttle plates, I wasn't getting any response.....BLMs were still split. Since this didn't jive with what other folks had seen, I started looking elsewhere. I had ruled out everything but the Optispark was still a question in my mind. I changed that out with a new GM unit and then started to see some affect!

I have plugged the IAC bypass hole and have tapped in a seperate route for the IAC bypass air (tapped into the intake tubing, after the MAF, ran it througha valve to the PCV intake - this passage feeds the IAC passage). So now, I can tweak the throttle plates as well as the IAC bypass hole size. This got me close but things still weren't how I wanted them. Trying to remove all variables, I removed the Ford SVO 30# injectors and re-installed the stock injectors/constant/offsets. The car is running much better and showing the data I posted earlier. What confuses me though.....previously I had swapped the Ford SVOs from side to side and did not see a change in my split.

So now, the car runs pretty good at part throttle but I'm RPM limited at WOT 'cuz the stock injectors just can't keep up! I haven't had the time to play with things lately.

Rob
95Z28

 

Let me make sure I have this straight:

With a big cam, the BLMs will often be split because the PCM thinks one bank is running lean. But in reality, the bank isn’t running lean? Intake air is going into the exhaust on that bank so the O2 reads a lean condition, but the actual combustion in the cylinders is not lean? Is that correct?

I’m contemplating going with a PE idle and this would get rid of all the extra fuel that’s being dumped in that bank so I want to make sure that those cylinders don’t really need that fuel before I do so.

 

Split BLMs, or How do you adjust the IAC and why?

This split BLM that the Big Cam guys complain about - so what? Suppose you're idling in the staging lanes at the drag strip, and your engine has split BLMs at idle. When it's your turn to run and you hammer the gas, one side of your engine might be fueled with a 118 BLM, but the other side may be fueled by a BLM of up to 160. Why do we care? Because max power is obtained only with correct air/fuel mixture. There's no way to get max power in this scenario, because all of the PCM fuel control tables are applied to both sides of the engine. Lean out the rich side and the lean side gets leaner. Fatten up the lean side and the rich side gets richer. You can't win..

What can you do to fix it? If the problem is not something that can cause false O2 numbers, or something that can really cause one side of the engine to be different than the other (check out the first paragraph for a list of suggestions), what can it be? It's been a puzzle! Split BLMs suck big time, but read on for a possible bandaid.

Recall that IAC is what the PCM uses to control idle speed. Zero is closed (no air gets in, really slow idle!), and 160 is wide open (faster idle). Generally speaking, you want to set it to a fairly low value (40 seems like a popular setting for a warmed-up engine) so that the PCM can raise the idle when you switch on the a/c, or when it's way cold outside first thing in the morning and the engine doesn't want to idle.

I wondered if my split BLMs were caused by my long duration cam disrupting the contents of the intake manifold, which is essentially just one big cavern (plenum) with 2 holes in front for air to get in and 8 holes spread out along the sides for air to get out. Ordinarily, the plenum only contains air, but the extra valve overlap (both intake and exhaust valves open at the same time) caused by longer-than-stock duration cams can create reversion, which means that some of the air/ fuel mixture from the combustion chamber flows backwards through the intake port in the head and ends up in the manifold plenum. If fuel is in the plenum, the cylinder that draws it in will be too rich (because the PCM thinks all the fuel is coming from the injector).

But idle air coming through the IAC has its own small passageways in the manifold - it doesn't use the plenum (unless you have an aftermarket throttle body, at least some of which have a big "leak" between the IAC outlet and the main plenum - see below). I figured if I could get all, or almost all, of the idle air to go through the IAC and those small passage ways, big cam reversion would have less effect. You can't "adjust" the IAC, but you can trick the PCM into adjusting it for you by messing with the throttle body stop screw. This screw holds the throttle body blades slightly open when you're not touching the gas pedal.

If you close the throttle blades almost completely by adjusting the stop screw, the PCM will compensate by opening the IAC (remember that the PCM has a programmed idle speed that it tries to hit). I started the engine and connected my scan tool (Diacom back then) to the PCM. The IAC was probably in the 40s or 50s, and the split BLMs were something like 125 and 155. As I closed the throttle blades, the IAC climbed and the BLMs got closer together. By the time the IAC got above 140, the BLMs had actually reversed, meaning that the went from 125/155 to 125/120 (or something like that - I've forgotten the exact numbers). Try out this trick to see if it solves your split BLMs.

OK, now for the aftermarket throttle bodies. My AS&M 58mm throttle body had a major "short circuit" between the IAC outlet and the main plenum as shown in the pic below left. Even if I close the throttle blades completely, this will allow idle air into the main plenum. What to do?

Fortunately the IAC outlet is exactly 1/2" in diameter, and it turned out that a piece of 1/2" brass tubing from the local hardware store fit perfectly. Some careful work with a carbide burr in a die grinder was needed to create an opening in the wall of the tube that matched up with the IAC port. The pic below right shows the new tube completely inserted. The IAC opening in the intake manifold is a bit larger than 1/2", but once again the hardware store came through, this time with an o-ring from the plumbing department that seals the gap between the tube and the intake pretty well.


would attatch pictures of how I did it but don't know how??? someone help me so I can show you how to deal with this problem.


here are the pics I attatched them on my home message board
scroll to the bottom of the page....
http://www.ksfba.org/forums/showthre...7875#post57875

 

I have never figured this out... now I hardly drive my car enough to care and I dont check out my scanmaster 24/7 so out of sight and out of mind is my only solution. It could very well be an exhaust leak causing all my stress but driving on the highway the drivers side is always 10 pts higher than the pass side so actually that is reverse of what im seeing at idle. So even if i have a leak.. its not affecting my idle fuel trims.

I have already read all that information about the IAC on the web page of the original author... it did nothing for me.