Cams, Idle, and BISS
Jul 11, 2007, 04:43 PM
#1
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Cams, Idle, and BISS
After installing a set of evo8 S1's in a car, I reflashed it with slightly higher idle settings (about 100rpm). I kept the fuel and ignition map untouched.
The idle was still a little low so I unscrewed the BISS enough to raise the idle to just over 1k.
I was reading over old threads when I found out that you can't reset the BISS without having a mut tool
DOh! I also found out that this will mess with the fuel trims and noticed the idle is no longer stoich. It moves around from low 15's to high 16's. I figured that the car would be in closed loop and would learn its way back to stoich, needless to say I was wrong.
If you haven't dealt with this in the past or don't really know what you are talking about, please don't post as I hate cluttered threads.
I was thinking about reflashing the car with stock idle settings, resetting the idle with the biss to 800-850 and then reflash the car again with the raised idle settings. I was also thinking about leaving it as is, would this be problematic? The car has enough vacuum to where it wont stall even with the ac on and tapping the breaks.
Meh,
The idle was still a little low so I unscrewed the BISS enough to raise the idle to just over 1k.
I was reading over old threads when I found out that you can't reset the BISS without having a mut tool
DOh! I also found out that this will mess with the fuel trims and noticed the idle is no longer stoich. It moves around from low 15's to high 16's. I figured that the car would be in closed loop and would learn its way back to stoich, needless to say I was wrong.If you haven't dealt with this in the past or don't really know what you are talking about, please don't post as I hate cluttered threads.
I was thinking about reflashing the car with stock idle settings, resetting the idle with the biss to 800-850 and then reflash the car again with the raised idle settings. I was also thinking about leaving it as is, would this be problematic? The car has enough vacuum to where it wont stall even with the ac on and tapping the breaks.
Meh,
Jul 11, 2007, 11:53 PM
#6
The MUT3 is the current mitsu tool to connect to the stock computer to read sensors, activate solenoids/relays/injectors/etc.
The problem is that most poeple don't realize that the BISS screw is NOT for adjusting idle speed. It's function is to bypass the proper amount of air to allow the ISC to spend most of it's time in the center of it's range of travel during normal operating conditions. Just like we want fuel trims to be at 0 so the ECU has full range of motion in both directions to cover any changes in AFR, we want the ECU to have full travel in both directions so it has enough range to control idle speed regardless of the circumstances (AC on or off, weather conditions, etc).
The MUT tool allows you to force the ISC to the center of it's travel. Once it's there, you set the idle speed with the BISS, and then release the ISC. Assuming you did this with the motor at normal operating conditions, the ISC will have enough travel to do its job properly.
Another alternative is to use a scan tool or similar to watch the ISC's position, and adjust the BISS to center it. I don't know what the values are on an EVO ECU, since I tossed the stock ECU shortly after buying the car, but on the 2g, which I am sure is not significantly different from the EVO in this regard, there are two relevant trims to watch. One is the Learned Idle Adjust position, which is the "long term" trim. The other is ISC Position, which is a short term trim. Just like fuel trims, short term values are dumped into the long term value over time. The ISC can be very slow to adjust, especially in the closed direction. If it is way out of whack, it's often best to reset the ECU (forcing long term trim, and therefore the ISC, to the center), get the idle speed close (again, normal warmed up operating conditions), and then watch as it slowly adjusts. You should be in the ballpark enough to make any fine adjustments relatively easily.
Another important thing to note is that the ECU is not aware of the ISC motor's actual position, there is no closed loop feedback on it. It simply counts steps positive and negative, and keeps track of where it should be by what commands it's issued. If you've ever unplegged the ISC with the motor running it will become out of sync with the ECU. Whenever you reset the ECU (remove power from it for a bit of time) it will force the ISC past it's range of motion, assume it's made it there, and sync up that way. You'll hear it doing this as a clattering or clicking sound. It's not a bad idea to do this before any lengthy work on adusting the BISS.
I'll have to look into it, but I'm assuming that like the DSMs and all other mitsus, the ECU also needs to see the closed throttle signal which comes either from a closed throttle switch (older cars) or a 4th pin on the TPS (newer cars). Without this signal the ECU will not attempt closed loop idle speed control. On cars with the switch built in to the TPS, it now becomes critical to have the TPS adjusted properly, which should not be attempted until the throttle cable is adjusted properly, but that would seem to exceed the scope of this thread.
As far as AFR goes, the BISS setting itself, as in how much air it bypasses, has no direct effect on AFR. Any airflow changes are read by the MAS and compensated for. What will have an indirect effect on your AFR however is the idle speed and load. The ECU's target AFR for 1000 rpm may not be the same as the AFR target at 800 rpm.
The problem is that most poeple don't realize that the BISS screw is NOT for adjusting idle speed. It's function is to bypass the proper amount of air to allow the ISC to spend most of it's time in the center of it's range of travel during normal operating conditions. Just like we want fuel trims to be at 0 so the ECU has full range of motion in both directions to cover any changes in AFR, we want the ECU to have full travel in both directions so it has enough range to control idle speed regardless of the circumstances (AC on or off, weather conditions, etc).
The MUT tool allows you to force the ISC to the center of it's travel. Once it's there, you set the idle speed with the BISS, and then release the ISC. Assuming you did this with the motor at normal operating conditions, the ISC will have enough travel to do its job properly.
Another alternative is to use a scan tool or similar to watch the ISC's position, and adjust the BISS to center it. I don't know what the values are on an EVO ECU, since I tossed the stock ECU shortly after buying the car, but on the 2g, which I am sure is not significantly different from the EVO in this regard, there are two relevant trims to watch. One is the Learned Idle Adjust position, which is the "long term" trim. The other is ISC Position, which is a short term trim. Just like fuel trims, short term values are dumped into the long term value over time. The ISC can be very slow to adjust, especially in the closed direction. If it is way out of whack, it's often best to reset the ECU (forcing long term trim, and therefore the ISC, to the center), get the idle speed close (again, normal warmed up operating conditions), and then watch as it slowly adjusts. You should be in the ballpark enough to make any fine adjustments relatively easily.
Another important thing to note is that the ECU is not aware of the ISC motor's actual position, there is no closed loop feedback on it. It simply counts steps positive and negative, and keeps track of where it should be by what commands it's issued. If you've ever unplegged the ISC with the motor running it will become out of sync with the ECU. Whenever you reset the ECU (remove power from it for a bit of time) it will force the ISC past it's range of motion, assume it's made it there, and sync up that way. You'll hear it doing this as a clattering or clicking sound. It's not a bad idea to do this before any lengthy work on adusting the BISS.
I'll have to look into it, but I'm assuming that like the DSMs and all other mitsus, the ECU also needs to see the closed throttle signal which comes either from a closed throttle switch (older cars) or a 4th pin on the TPS (newer cars). Without this signal the ECU will not attempt closed loop idle speed control. On cars with the switch built in to the TPS, it now becomes critical to have the TPS adjusted properly, which should not be attempted until the throttle cable is adjusted properly, but that would seem to exceed the scope of this thread.
As far as AFR goes, the BISS setting itself, as in how much air it bypasses, has no direct effect on AFR. Any airflow changes are read by the MAS and compensated for. What will have an indirect effect on your AFR however is the idle speed and load. The ECU's target AFR for 1000 rpm may not be the same as the AFR target at 800 rpm.
Jul 12, 2007, 01:38 AM
#7
Evolving Member
I have been fighting with this as well. Following worked for me to get back to "stock" BISS and Idle speed:
1. warm up the engine
2. disconnect the battery for couple minutes - then connect - but do not start the engine
3. flash the ECU with a map where you modify all the values in the IdleLookup Table to value 12 (standard Idle steps should be 2-25 for a stock car at idle when warmed up - based on workshop manual).
4. start the engine and start the logger to observe your idle rpm - adjust using the BISS screw to the desired value (I was aiming to stock 850rpm) - if this procedure takes you too long - you might need to disconnect the battery again (as the ECU is "learning" during that time as you are playing with the screw - which is not wanted until we are not finished.
5. flash back the MAP with the correct Idle Steps table values
6. Then after you start the engine you should leave it idle for 10 minutes - after that you should see in the logger ISC value between 2-25 during idle (850rpm)
I am not quite sure what if you aim to higher idle rpm - you will get to it but then also the "descired RPM" table needs to be adjusted - otherwise the ECU would like to get back to 850.
During the above procedure all fans, A/C, ligths etc should be sqitched of - not to produce additional load on the engine.
1. warm up the engine
2. disconnect the battery for couple minutes - then connect - but do not start the engine
3. flash the ECU with a map where you modify all the values in the IdleLookup Table to value 12 (standard Idle steps should be 2-25 for a stock car at idle when warmed up - based on workshop manual).
4. start the engine and start the logger to observe your idle rpm - adjust using the BISS screw to the desired value (I was aiming to stock 850rpm) - if this procedure takes you too long - you might need to disconnect the battery again (as the ECU is "learning" during that time as you are playing with the screw - which is not wanted until we are not finished.
5. flash back the MAP with the correct Idle Steps table values
6. Then after you start the engine you should leave it idle for 10 minutes - after that you should see in the logger ISC value between 2-25 during idle (850rpm)
I am not quite sure what if you aim to higher idle rpm - you will get to it but then also the "descired RPM" table needs to be adjusted - otherwise the ECU would like to get back to 850.
During the above procedure all fans, A/C, ligths etc should be sqitched of - not to produce additional load on the engine.
Last edited by evo828; Aug 8, 2007 at 05:33 AM.
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