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Tecchie EFI Idle question
Question:
Often, (but not always), when EEC-IV engines are modified with bigger meters, lumpier cams, freer flowing heads and intakes, basically hi-po induction parts, an idle problem develops. This I have gathered from my own experience, since I have all these mods (from air intake to tailpipes), as well as from this news group and from my local guru, who builds all the fast mustangs in my area (Northern San Diego County, CA). It is acutely obvious that this problem does not occur on every similarly modified EEC-IV engine, nor does it occur with equal severity in these cases. Mine was really bad after modification, and eventually, after a trouble code scan revealed a bad oxygen sensor, with subsequent replacemet with a new one, it cleared up for the most part,(BUT NOT ENTIRELY)! Here is the maddening part, and the reason for this posting. I can’t get anybody to tell me how or why this occurs. I’m looking for the "nitty gritty" on this. Dammnit I’m an engineer, and all I can get from the people who should know is, "IT’S A CALIBRATION PROBLEM, MAN". I am appealing to the people who really know the EEC-IV, and how it controls idle to explain to me why these problems develop in modified cars. What is a "calibration problem"? Can it be solved with more computing power? I’M TOLD THAT THE METER CAN’T CALIBRATE RIGHT; could someone please explain to me what should be happening in that meter, and what IS happening there that is causing people’s idle to oscillate rhythmically between 400 and 1500 rpm and then die if left alone? Please don’t be afraid to give it to me straight. I am well versed in Thermodynamics, Fluid Mechanics, Heat Transfer, and computer systems and logic. I really can go beyond "it’s a calibration problem, man". Here’s my guess at what the problem is. With modified cars and their larger diameter intake tracts, coupled with lumpier cams, the air flow through the system is not as consistent as it was when stock. The larger diameter intake tract, including the meter, combined with the low vacuum signal from the high lift and overlap cam combine to make the airflow through the meter slower and more turbulent than it was. I’m picturing air at idle meandering slowly through the meter, in a non-laminar fashion, low Reynolds number, flow separations and curling eddies everywhere around the sample tube. The hot wire sensor in the sample tube is just unable to get a good "lock" on the mass flow rate of the incomong idle air. Stockers might have a much smoother vacuum signal (stock cam), and higher flow velocities from the narrower intake tract, a higher Reynolds number situation, much more laminar in the boundary layer near the wire/tube. Is this theory of mine right? Still, why do some idle well when others idle so poorly. Oh yeah, and some people refer to a car idling rich or lean when it’s idling poorly. How can either be the case when the computer is actually hunting up and down, through rich and lean, to find a good idle mixture. Also, the idle air bypass solenoid, what does this do? Is it actually the source of idle air for the EEC? If so, my wonderful theory is right out on its ear. Thanks in advance, PR San Diego, CA