Originally Posted by
Jaxom
That paper has some good information in it, but some bad information as well. The explanation about why EFIE's sometimes set a check engine light makes perfect sense, but they're a little bit off on their ideas of how the PCM monitors the O2 sensors, and how the fuel trim feedback loop works.
" In looking for Horsepower, the O2 signal is fixed to whatever level the tuner needs,by using a fixed resistance in the early O2 models,or a fixed voltage signal in the later O2 models"
This is crap. Sending a fixed signal to the PCM in place of an O2 signal will do more harm than good. The PCM expects to see a signal switching back and forth from ~.2-.8v, and if it doesn't see that switching signal it will decide that the O2 is bad and set the appropriate trouble code. Without the O2 signal, the PCM will use a default fuel curve, which may be too rich or too lean for the current conoditions. If the fixed O2 signal is low, the PCM will increase the injector pulsewidth until it reaches the max. allowed in the programming. The converse is also true. Unless the O2 signal changes in response to the fueling changes, the system will either max out lean or max out rich. This is true for nearly all closed-loop fuel control systems.
Also, the PCM doesn't just add or subtract fuel based on the O2 voltage. There is a very complex set of algorythms that the PCM uses to calculate the injector pulse width based on a number of engine parameters. I could write chapters on the O2 feedback PID (proportional, integrative, and derivative) controls alone, without even brushing against the MAP, MAF, CTS, IAT, Barometric pressure, RPM, Accel. Enrichment, Decel. Enleanment, Highway mode spark and fuel control, Power Enrichment (a.k.a. wide open throttle,) Overheat mode, Startup Enrichment, and a number of other parameters.
I hate it when bad information like this gets circulated.