Basic OBD II operating systems for idiots
(by an idiot)
What I am saying here, is I will try to explain how the OBD II system works as far as a DIY person of limited background is concerned. This will be an oversimplification and not necessarily correct for every car, but darned similar. I may (Will!) miss a few points, but you may just learn a few things, and of course feel free to correct me , if I make a mistake.
Ready? Lets start with the TPS (throttle Position Sensor) .The TPS attached to the throttle blade, and varies a signal to the computer depending how much its opened.This is interpreted as the drivers demand for power. There are a lot of other sensors, we wont consider, but lets consider the ECT (Engine Coolant Temperature sensor) Usually located on the crossover at the front of the intake manifold. It tells the computer the coolant temp, and helps determine how long the engine stays in closed loop and how rich to run it at start up. It also in some instances tells the computer its too hot , to pull timing. The IAT (Intake Air Temp. sensor) It is someplace between the MAF and the throttle body, and the computer does much the same with the signals from it. The MAF ( Mass Air Flow meter) is the heart of good engine control, as it measures the mass of air entering the engine any given time, and reports it to the computer. this is a sophisticated piece, which uses two miniscule wire wrapped bobbins which measure flow and mass of air based on the voltage the bobbins will pass in the airstream. This is accomplished by heating the wire, and measuring temps.The electronics involved are more compled than that of a good color TV. A couple asides here ; Never try too fool a mass air meter by changing its shape, or size. You will simply get in trouble. Never change Mass Air Meters to that of a different calibration , say useing a 'stang MAF in a T-Bird. Oh that will work, but not right. there are sophisticated reasons that may have you changing the MAF, but they must include a retune by a competent tuner for the specific replacement MAF.Earlier ,non OBD II systems used a MAP sensor (Manifold Absolute Pressure sensor) in what was known as a speed density system. those systems were not as accurate, and are all but obsolete these days, except for use in some race applications.
OK those are the principle sensors that are used in the Open Loop System. The computer determines how much fuel and timing advance to give the engine based on tables, and verry quickly recomputes these values over and over....The tables are different for several other factors, so the look ups are on different tables as conditions vary. This adds another dimension to the computers search for a perfect fuel air and timing mix The goal after warmup (rich) is to bring the air fuel blend into perfect stochiometric/stoichometric? level. Ford always actually runs a tad rich for safety. When the car is warm, and all conditions have been met , the computer switches to Closed Loop.........Closed loop? yup what the car runs in most of the time, for economy and emissions control.The closed part means the O2 sensors in the exhaust are warmed up and sending a rich (less oxygen) or lean (more oxygen) signal to the computer, which in turn tells the computer to add or reduce fuel by altering the injector open time. Thing is its never perfect, but the computer is quick and almost perfect, so it will end up flip flopping a little on both sides of the rich lean equation alternately. the computer also has something called long term fuel trims , which it learns from operating the engine, and are applicable to get verry close to the perfect. These usually vary about plus or minus 10 percent or less (the smaller the better) 25 percent correction will set off a check engine light. there are also short term fuel trims as an additional thing the computer does to get even better Air Fuel ratios. Thing is , Those O2 sensors, usually are a pair for each engine bank of cylinders these days. The Upstream Sensor, (closest to the engine)reports to the computer and is used in closed loop computations. The Downstream Sensor, only reports the catalytic converter is working OK.....NEVER use anything but OEM upstream O2 sensors of the appropriate calibration as replacements , the downstream ones whatever Autozone ,or whoever, wants to sell you should be ok. Of note here, is the sensors can be cleaned to get some extra life out of them. this especially important if you had to use leaded gas for some reason. To clean them, you need to use a LP torch to burn off the carbon... its ok to see a little glow on the shield for the sensors , just be careful not to overheat, but what do you have to loose? The manufacturers wont tell you this , but it works for a while, and kinda helps the budget...
Ok.... we covered some sensors, open loop 'N closed loop , kinda sorta, but why? To tell you about WOT (Wide Open Throttle) performance. when the TPS signals there is a wide open throttle, and Its time to go, the computer shuts off readings from the O2 sensors, and uses the tables , MAF, speed sensor , and maybe the ECT,IAT, and a few others to run the car, It is a tad rich, and used for max performance. the transmission kicks down a gear , and youre off!.... now if you see a tuner for a chip, or a tuner appliance with some programs, part of the rich slop is trimmed away , running somewhat leaner, thereby generating more power. The computer can also be made to shift in lockup at WOT, and shift faster, harder. If your tuner has a dyno , or street tunes with a A/F meter, it can still be tuned finer, with some closer adjustments that fit your SPECIFIC car and its modifications.
A little Caveat here; in my experience,the dyno is set up with a fixed amount of inertial resistance, which works juuuust dandy for say, a mustang ... However, a T-bird is heavier, has possibly a little xtra rolling resistance, and therefore has a much higher load then said 'Stang in the real world.The thing is , all things being equal, you may end up with a little leaner , or a bit too much timing on that dyno tune,ending up with more detonation than you want. Street tuning then becomes necessary, go figure .
For those of you who know more about this , please correct me, and share the wealth of your knowledge with us, bearing in mind this is a simplification of an electronic car operating On OBD II, biased toward Ford systems.