The LIM (Lower intake manifold) is equipped with secondary butterflies. There are a series of holes going through the LIM that allow the air through it and to the engine itself.
Half of those holes are closed (covered) with a metal plate at anything below the ~3700rpm (3200 @ WOT) point. Once you reach that point, the PCM sends a signal to the IMRC (Intake manifold runner control) to open the secondaries The IMRC is located on the radiator (front of car) side of the engine, underneath the plastic cover that says "Duratec" on it. IT is a small silver square box.
When the IMRC opens the secondaries, it does so via a cable (like a bicycle brake cable) that runs out of the IMRC and down to the LIM. When pulled, the other half of the holes in the LIM open up.
When closed the engine has a smaller capacity, increasing velocity and low speed torque (when you need it most). When opened, it has higher volume of air, and aids in increasing horsepower. (Of course other things are needed, too.. like increased timing, more fuel, etc...)
You can physically tell they are working by the "surge" of power at around 3200 RPM in the lower gears.. (1st through 3rd are most noticeable) this is if you are at wide open throttle.. AT less then WOT, they will open at around 3700 or so.
You can VISUALLY verify them to be working by looking at the secondaries being opened while you rev the car past the opening point from within the engine bay. You can see the secondary linkages by following that cable from the IMRC to the LIM, and looking for a brass connecting bar (that goes from one half of the LIM to the other, allowing them to both open at the same time..) when you rev the car, the bar should move back towards the firewall, and away from the radiator...
Normally, you will also receive a CEL on the dashboard that indicates a malfunctioning IMRC. (Scan your codes at Autozone for free, just make sure to get the CODE, and not the description they tell you...)
Ray
