Re: Fuel Cut Eliminator for ST - On-Off Throttle Issue Resolved
Joe, please delete or move this post if you feel it doesn't belong here. I have been working on this a while and may be fitting here. Thanks Tom
EFI 101
Hi everyone,
There have been quite a few posts on the ?operation? of the ST EFI system. I would like to offer my insight into the systems as to what they can and can not do.
I have been programming automotive computer controls since 1986, most of in the GM product line. I have the software to do 1987-1995 OBD I and 1996-2007 OBD II systems. I have programmed 1000?s of ?chips? in that time. All of them ?custom? applications. I also have the ?actual source codes? (the programming bible) for some of the applications, so I do know a little more than most in this area.
The ST system is a VERY simple basic injection system. It would be classified as an OBDI type system even though it is later than 1996. It has a very basic trouble code reporting system (flashing the light). The ?check engine light? will report if an unexpected voltage occurs from one of the sensors. It can not tell if it is a problem with the sensor, the wiring, or a malfunction in the engine. It just knows what the voltage (standard operating parameters) should be for that sensor.
The computer does have the ability to make small corrections in the fuel curve. These have limits that are set in the program. (Usually +/- 3-4 %) The computer can not make large changes. The ST system uses the O2 sensor to make these corrections up to the ?limits?. If the computer were capable of making unlimited changes, there would be no need for modified programs, modified fuel pressure regulators, or any other type fuel control device. You would be able to drive it long enough and the computer would correct the fuel curve to perfect.
The ST computer does have the ability to tell changes in altitude, just not while it is running (many report drivability issue at altitude). The system uses the MAP sensor voltage when the key is turned on / before starting as the altitude setting until the next time the engine is restarted. That is why the reported drivability issues improve after a couple of starts at the different altitude.
The idle speed is not controlled by the computer. It is a completely mechanical system. It uses a cold start valve to mechanically open ports on the TBI to increase the idle speed on cold start. At altitude, the engine will naturally idle slower (less air). If you have the idle speed set at the lower end of the scale, the engine may continually try to stall at altitude.
ST EFI modifications:
The ST has a speed density type EFI system. It uses a calculated fuel table based on ? throttle position, RPM, MAP voltage (engine load). The fuel table is modified with engine temperature, altitude (on start up), and O2 sensor feed back. The fuel table in the computer is ?carved in stone?. It can not be changed without re programming. To this day, no one outside of Honda has? broken the code? to allow programming. As more bikes are built with EFI, the demand for this will grow and someday it will happen. It can take 100?s of hours to ?hack? a system (I have done it in the past).
The only type of device that can really modify the fuel curve outside of the factory parameters has to wire between the computer and the injectors (power commander, etc.) It would the alter the injector pulse based on the supplemental program (added or subtracted time). With a feed back controlled system (ST). The program becomes a constant? moving target?. It may never be correct because the computer is making changes along with what ever you are putting in.
O2 sensor controls:
The O2 sensor has a very limited range of perimeters that it is functioning in the program. The O2 has no control at cold start. (It takes a while to warm up)(The ST O2 is a heated type for quicker functioning). The O2 sensor also loses control when engine load or throttle position is too great (determined in the program) and goes to power enrichment mode. The ST computer also monitors the heaters on the O2 sensor to verify correct operation.
Fuel Pressure Regulator:
The basic fuel table is built around a specific fuel pressure; again, the system can alter the curve within the limits. In a stock system the fuel curve will float up and down between the limits. When we raise the fuel pressure, we are forcing the fuel curve against the ?lean limit?. I.E. if we raise the pressure 7% and allow the computer to go to the lean limit (-4%) the net change is a + 3% in fuel flow. The computer can not remove any more fuel and as long as we don?t exceed +5% additional fuel, the computer will not ?throw a code? Everything is within the designed operation parameters. We end up with an increase across the entire fuel table. The increase is non linear because of the limited O2 sensor control. The low end of the curve may be +3% and the upper part of the curve will be 7%. The 7% increase may not be ideal for a stock engine. You have to ask, how much time you spend at higher than ? throttle and is a slight loss in higher rpm power worth the increase in drivability on the low end. With modifications to the air intake and the exhaust, the demand on the fuel curve must go up (if the modifications work), so additional fuel pressure is not bad on a modified ST and the FPR will tend to correct the lean condition created.
O2 Sensor devices:
There are two different types of O2 sensor defeat devises.
The O2 eliminator and an O2 simualtor
In a standard system, the O2 voltage will vary from 0.0 volts (lean) to .99 volts (rich) with the median voltage being .45 volts.
An O2 eliminator is a resistor network that is designed to provide a neutral signal (.45 volts) to the computer so there is no modification to the fuel curve by the computer. These type devices are generally used in systems where programming of the computer is available. The programming can be set for max power, or max economy and the computer will be unable make changes to these curves. The ST factory fuel curve is too lean and creates the drivability issues that we all try to correct, so the O2 eliminator doesn?t work for the ST. There is also the issue of the computer checking the heater operation of the O2 and triggering the ?check engine light?
An O2 simulator would be a device that feeds an altered O2 signal to the computer. This can be very advantageous on a stock system. If we were to send a slightly lean voltage to the computer, the adaptive systems in the computer would head to the rich limit, which would richen the fuel curve the 3-4% allowed. This could promote much better drivability as it would always stay at the rich limit. The disadvantage is that there is no additional fuel, outside the stock program, for modifications and there would be a limited range of operation. The device would also need to provide the proper signal to the computer for the heater current to keep the?check engine light? off.
In any of the programs that I have been involved with, the Deceleration fuel cut off (DCFO) is a programmed function controlled by, RPM, throttle position, and MAP voltage, and can not be eliminated without altering one of those functions outside the trigger point. O2 operation does not affect DCFO in any program I have seen.
As you can see, using a FPR and the O2 simulator , at the same time, would be counter productive. The devices are doing the same function from different directions.
All this is based on my many years of experience modifying EFI systems. I have NO direct knowledge of the inner workings of the Honda computer other than the info /data that is provided in the manual.
Thanks Tom
