The Split BLM Curse
So you've installed the big cam you've always wanted, and the car rewards all the effort with a satisfying lumpy idle. The vacuum and exhaust leaks are all plugged, you remembered to refill the radiator, and all seems fine until you hook up the scan tool to dial in the PCM for the new cam. While idling in the driveway, the PCM reports a left BLM of 142 and a right BLM of 118.
So you start looking at all the possible causes, like intake manifold leaks, exhaust leaks, toasted plug wires, fouled plugs, dirty injectors, a failing O2 sensor, and bad connections in the O2 sensor harness that you stretched to accommodate those long tube headers. Maybe you found a couple things wrong, but fixing them did not cure the split. I feel your pain! Been there, done that, some things twice. Read on for a fix that you may not have thought of.
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.