MIL Command ON and Fuel Trims

[98GSX-WI]

Hey All. First thing--I went for an emissions test and failed. The printout says that MIL Command is ON and the guy said that my ECU must have been reset and is not ready to be read yet.

The battery was disconnected recently, but it was about a week before the test. I've been driving in all kinds of conditions--highway and city since then. Does it really take that long for the ECU to readjust or could there be something else going on? I've been checking the MIL Command for 5 days since the emissions test...and it's still "ON". (Using Pocketlogger).

Secondly, my fuel trims also seem weird. I have the keydiver chip to compensate for 650cc injectors and I have a SAFC. After the emissions failed, I adjusted the SAFC to get the Short Term Fuel Trim to "0" at 1000, 2000, and 3000rpms. After driving for five days, my Long Term Fuel Trims are at about "10"--while the Short Term Trims are still at around "0". I thought that after a few days of driving that the Long Term Trims would adjust to around "0" as well. Is this correct--and are Long Term Trims at "10" indicative of a problem?

Thanks for the help. (My registration expires at the end of the month, too).

 

[spyderturbo007]

The OBD-II drive cycles are rediculously difficult to complete. Good luck. Some eclipses even have the problem of resetting the OBD-II monitors every time the car is turned off. If time is a problem you will probably have no choice but to go to Satan and have the ECU set to "increased sensitivity" mode and let them set the monitors. I had this done 2 years ago and it was about $130.

As for the FT's. Your STFT will normally always return to 0, unless your O2 sensor is shot and the FT's becomes maxed out. You will need to adjust the SAFC-II until you start to see the STFT change in the negative direction. The ECU will then recognize that there has been a change and over time (doesn't take too long) incorporate those changes to the LTFT.

For example: The ECU senses a lean condition and begins to increase the STFT to compensate. The ECU sees that this is an ongoing condition and starts to transfer this compensation to the LTFT. The STFT then begins to approach 0.

 

[98GSX-WI]

Hey, thanks for the reply. Do you know if I change the SAFC settings to adjust the fuel trims...will this "reset" the ECU and even take longer to get to a ready state?

One more question...will using the Pocketlogger and taking readings through the OBDII port affect whether the ECU can reset itself or not. I wouldn't think that it would...but thought that I would ask.

 

[spyderturbo007]

No changes that you make on the SAFC will effect the ECU that way. The SAFC works by intercepting the airflow signal from the MAS before it reaches the ECU. When you change settings on the SAFC the intercepted airflow signal is modified before it is passed to the ECU. This change, either increase or decrease in airflow, "tricks" the ECU into injecting more or less fuel depending on the change made to the SAFC.

Wow, that was fun!

The pocketlogger will not effect the ECU, but you will obviously have to unplug it so the emissions machine can be plugged into the OBD-II port.

 

[98GSX-WI]

Thanks again for the reply.

Actually, I do know how the SAFC works and know that it won't "reset" the ECU directly (which is why I put the word in quotes)...but I thought that maybe if I changed the fuel levels that the ECU would then "detect" the change and start the process of adjusting itself all over again...

I'm not sure if I understand what I should be trying to adjust with the fuel trims though. Is the goal to not worry about the STFT, because in closed loop the ECU will automatically adjust these to "0"...and then to only worry about making adjustments on the SAFC to utimately get to the point where the LTFTs stay around "0" after a few days of driving?

 

[spyderturbo007]

Yes, you are correct. The "endgame" is to have both the STFT & LTFT at zero. You will see that the STFT moves very quickly and the LTFT moves slower. If you have a LTFT of +6 you will want to begin the adjustments so the STFT begins approaching -6. As the ECU senses this is an ongoing condition the STFT & LTFT will approach. So the end result will be to have the LTFT at zero. If you do that, you can almost bet that your STFT will be zero also. Once the LTFT is zero, you will see small fluctuations in the STFT but nothing that will effect the LTFT. Unless your O2 craps out or starts getting older.

As the O2 sensor gets older the voltage as seen by the ECU will beging to degrade. This makes the ECU think the car is running lean. You will then notice that your LTFT will begin moving in the positive direction. No biggie, you will just have to compensate on the SAFC. Not like you will be doing this anytime soon, maybe 6 months or a year. But just something to keep in mind.

I hope all of that made sense.

 

[98GSX-WI]

Thanks for the explanation. What you said made sense....except for one thing. Is everything that you said about getting the LTFT to zero, what is to be done as a starting point? I guess my assumption has always been that when the LTFT is at zero that the air/fuel ratio is right in the middle of the range (so to speak)--not too lean and not too rich.

If the goal is to lean out the fuel to make maximum power and have no knock with the EGTs not being too high--I thought that the goal would be to run leaner than the middle of the range an not at a LTFT of zero. Is this a false assumption?

One way that I've read to tune as a starting point is to lean out the SAFC to the point where the ECU can no longer adjust the fuel ratio in closed loop...and then richen it up a couple clicks on the SAFC. I guess I assumed that if you did this...that the LTFT would not be at zero after tuning like this. Does this make sense? Please clarify with any specific tuning advice if you could. Thanks.

 

[98GSX-WI]

Here is what I just found out from a Mitsubishi Master Mechanic. Apparently, the 1995 ECU has some bugs in it. One of them is the fact that even though there are no error codes present...the MIL Command will stay ON. I'm not sure if this alone will cause me to fail emissions--but it might.

The second bug with the 95 ECU is that even though the 3 of 5 monitor modes in the ECU may have been completed which will pass the emissions--when the car is shut off with the 95 ECU--the monitor data may be deleted or reset.

Both these problems are apparently known glitches with the early 95 ECUs.

My plan is to stop by the mechanics shop on the way home from work--he thinks that at least 3 monitor modes should be completed during that length of drive. He will check with a scan tool to assure that they are completed and then I will go straight to the emissions station without shutting my car off. (The only problem being that last time they made me shut my car off).

Does anyone know that if they can do the full emissions test instead on my car instead of checking the OBDII monitors in Wisconsin. Because if they fail me just because of the fact that the MIL Command is ON...then I will have to try something else.

 

[spyderturbo007]

What you said made sense....except for one thing. Is everything that you said about getting the LTFT to zero, what is to be done as a starting point?

This depends on the size of your injectors. The standard is (old injector size/new injector size) - 1 = baseline SAFC-II correction factor. But I always start a couple of percent closer to zero than the calculation result. For example for 550cc injectors the SAFC-II correction would be -18%......I would start around -15%. Better safe than sorry!

I guess my assumption has always been that when the LTFT is at zero that the air/fuel ratio is right in the middle of the range (so to speak)--not too lean and not too rich.

Correct.

I thought that the goal would be to run leaner than the middle of the range an not at a LTFT of zero. Is this a false assumption?

You are incorrect. The purpose of the FT's are to maximize the gas mileage. It is used during idle and cruise speeds. The FT's are not used at WOT and will make no difference in the performance of the vehicle at WOT. During idle/cruise the ECU uses feedback from the O2 sensor to perform it's fuel calculations (it uses other sensors, but I will deal with the O2 for now). The FT's are the ECU's way to adjust the AFR. It does this by monitoring the O2 voltage, 0.50 being the stoichometric point 14.7:1. The stoichometric point is the ECU's goal and will adjust the amount of fuel injected based on the O2's feedback.

At WOT the ECU no longer cares about the O2 voltage or the FT's. It only cares about air mass, intake temperature, coolant temp etc. Air mass being the most important.....coolant temp, intake temp, etc are only used to calculate max timing advance.

One way that I've read to tune as a starting point is to lean out the SAFC to the point where the ECU can no longer adjust the fuel ratio in closed loop...and then richen it up a couple clicks on the SAFC.

I really wouln't go this route. I don't think it's a good idea to lean out the car that much while you are trying to set the FT's. But it's up to you.

Please clarify with any specific tuning advice if you could. Thanks.

Hope some of that made sense. Let me know if you need further clairification.