Note that as soon as you change the table scaling, you need to rework the entire table to ensure that for example, the spark advance at 180 percent load is not the same as that previously called for at 90 percent load (that would be bad). Also note that Ford starts counting rows and columns at zero, so the leftmost column for example, would be con-sidered column zero.
Idle Control
Idle control could easily be an article in itself, so we'll just cover the absolute basics at this point. Engine intake airflow at idle is controlled by the Ford EECs, using a bypass air channel around the throttle body butterfly. The total amount of air that gets in is the amount squeaking by the closed throttle blade, plus the amount going through the bypass channel. The Idle Air Bypass, IAB (aka IAC - Idle Air Controller or ISC - Idle Speed Controller) is the actuator which allows a specific amount of air into the engine, via the bypass channel, based on the desired idle speed.
The IAB actually pulses open/closed rapidly, so the amount of air bypassed is determined by the duty cycle of the IAB. For a given desired idle rpm, the EEC is programmed to bypass a specific amount of air, in addition to what it knows is getting by the throttle body blade. A scalar value exists in the programming for the amount of air that gets through the butterfly valve of the closed throttle body. Functions also exist in the EEC for the total intake airflow (i.e., the TB airflow plus IAB airflow) versus rpm for both Neutral and Drive gears (for automatic trans cars). There are also many modifiers that act on the idle rpm, for things like low ECT, A/C on, and so forth.
Of course, the amount of idle air to achieve a given idle rpm is highly dependent on the engine combination. If you change things like throttle bodies, camshafts, cid, and so on, the idle bypass air needs to be reprogrammed. Certainly, for a new throttle body, the TB airflow scalar should be reprogrammed.
Idle speed control is its own closed loop system. That is, the EEC will command a certain airflow for a specific rpm it's trying to achieve. It then monitors the rpm to see if it's correct. If not, the EEC will adjust airflow up or down to hone in on the target idle rpm.
And this is where things can go sour. If your ISC airflow function and TB airflow scalar values are miles off what your engine needs, the EEC can start hunting the idle up and down in a desperate attempt to get the idle speed to the target value.
Idle fuel will be added according to the desired A/F ratio (14.64 during closed loop idle, or whatever the appropriate OL fuel table-and active modifiers-are commanding for open loop idle). So as long as your MAF transfer function is correct at the low flow end, you should be getting the correct idle A/F ratio when in open loop.
For closed loop idle, it gets a bit more involved. Since the O2 sensors are used to control the A/F ratio, it's imperative they be at the correct temperature to give proper readings. Relocating the O2 sensors into the collectors of long-tube headers for example, causes all kinds of CL idle problems when the O2 sensors get too cold at idle. Thermal-coated headers sometimes help the situation, but otherwise it's often necessary to retune the vehicle to use OL at idle.
Most often, a hunting idle is the result of excessively rich idle fuelling. This can be caused from a MAF transfer function that's way off on the low end (common with many aftermarket MAF sensors and/or CAI kits), or O2 sensors that get too cold during idle.
Well, there's the groundwork. If you've followed along so far, you're ready for some actual custom tuning, so let's do it next month. Stay tuned.