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EEC-V, OBD II, and Urban Legends

(Electronic Engine Control 5, On Board Diagnostics 2)

By Tim Wright

The Powertrain Control Module (PCM) comes with basic "maps" for ignition timing and other outputs based on coolant temperature, throttle setting and a zillion other inputs. As you drive the computer fills out the fringes of the maps using the 4 heated oxygen sensors to keep the emissions within specifications and optimize performance. Big changes in intake and exhaust systems can cause the chip to burp for a heart beat, then catch its self and recover. It notes the error and continually improves the maps.

The data sets are called "maps" because they used to look like topo maps back in the first days of EEC. Given throttle setting on one axis and coolant temperature on the other axis, you could read ignition timing as an "elevation." The result was an easy to visualize shape. The results of the simple fixed models were crude but functional. Now with dozens of inputs and outputs you can no longer shape a model in a simple 3D-space shape but the term "map" is still used. Once the maps were unchangeable from the factory, now to a greater degree they can be created on the fly as required using feedback information to tailor variables to each individual car and the circumstances. This is not an urban legend, just good 90's engineering.

If your AWA drives your SHO like a hearse for a year then maps for the many conditions not experienced will not exist and you will have to create whatever you need. Every time you drive the car reads values and add to its data. A tank of poor quality gasoline can also record sub-optimal data to your maps. The Octane Adjust Plug and Knock Sensors also will tell the PCM to ease off and if the car runs fine that way even with better gas, too bad.

When in doubt, or if your AWA has been driving your SHO messing up your maps, disconnect the battery to erase the PCM memory, reset your dash clock, then flog your SHO. Several sporty WOT runs should rebuild the maps in a half hour or so. Next morning when the ambient temperature is different your SHO will lack maps for that condition and will need to learn again. Before long you will have covered most ambient conditions and driving situations and your PCM will have a Ph.D. again. If you have never driven a V-8 SHO on a frosty morning you are in for a thrill. Not only that but it will pull stronger every morning for a week. Next week it gets colder yet and the process continues. It is hard work but someone has to do it. {grin}

Most drivers have noted an increase in hp after the first 10,000 miles, after engine break-in. Quarter mile and 0-60 times as reported in car magazines may be not representative of stock SHOs on the street because of engine break-in and MAP learning. It takes several 1/4 mile passes before the ET's quit improving and performance stabilizes. The improvement in ET's between runs can be significant, one or two tenths per run until the car drops from low-mid 16's to mid-high 15's if stock and at sea level.


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