Been tuning with my ZT-2 WB. Great tool! Reading RBob's article on IAC tuning I ran into his comment that he wasn't certain what the N-bias was. The article was qwritten sometime ago and I was wondering if there was an update to it particularly addressing
N-bias.

 

It is one of the stupidest things I've seen for a long time. Setting it to the highest possible value will help. Unfoutunately it is MPH * 3.2, so can't ever make it high enough to disable it.

It's so stupid it makes the kickdown steps look like a good idea. . .

RBob.

 

RBob,

Does it have anything to do with the re-set of the IAC's? By increasing the setting, any idea how that shows up in the performance or driveability of the vehicle? How do you tell if lowering or raising it makes a difference?

 

During the time between kick down and closed loop idle there is an upper RPM limit. To bring the idle RPM below this limit a variable is incremented (stored in RAM). The value stored in this variable is then subtracted from the IAC steps count. This brings the idle down.

Whenever the vehicle speed matches or exceeds the N-Bias term this varibale is set to zero (0). Now, when the vehicle comes to a stop the idle speed is again too high. And again, this same variable is increased in order to bring the idle down.

This stupidity continues until the engine is up to the temperature threshold for closed loop idle. The ECM will then discontinue this increase and zero action and the idle will settle down.

To observe this in action set the N-Bias value (LD5F4) to a low value, say 5 MPH (set the term to 16). Now do a cold start and let the engine warm up for a few minutes. Notice how the idle RPM is brought down to match the cold RPM limit term.

Move the vehicle enough to trip the 5 MPH N-Bias term and come to a stop. Notice how the idle speed is high again. The longer the engine warms up prior to closed loop idle temperature the worse the idle up is at each stop.

Best thing to do is to set the N-Bias term as high as possible. And let the code handle the idle without this interruption.

RBob.

 

RBob -

What masks are known to have this bias?

 

$42 & $61

RBob.

 

RBob,

WOW! You hit the nail on the head with that one Bob. The last bin I burned, I played with the N-bias and used the number from the APAB bin which had a term of 6. My previous bins have used 5mph or term of 16. As I warm up the car in the garage, the idle finally drops to where its supposed to be, around 850. BUT THEN, as you described, I'll take the car out for a scan. When I get to the first stop, I need to sit there and let the idle drop down, as if something was re-setting the IAC position, and idle down. Usually, this is accompanied by an oscillating idle for several seconds until it settles down. After that, I have no more problem with the idle UNTIL the next time I start the car and go through the whole process again. With the term at 6, the oscillation got much worse. If I don't stop that first time the idle will not be set until I do. Guess I'll try it at 255.

 

Is there any way to change that multiplyer from 3.2 to 1 or lower?

 

Only by going ROMLess(tm). The code for this is in ROM. May be able to patch the current code to force the MPH*3.2 term to MPH*1 for that particular major loop. That would also work but may affect other 'stuff.'

RBob.

 

I got a good couple of laughs out of that one!

If you set the term to 255, and the the term equals MPH*3.2 (255=MPH*3.2) wouldn't this make the N-Bias = 79.6875 MPH?

Wouldn't this pretty much be disabling it? I'm speaking for myself here but I'm not gonna go more than 80 MPH on a cold engine.

OR could you just set the RPM limits higher than where you want it to idle? I'm guessing you're talking about the idle rpm limits @ $5E1 or $5E2?

 

Me too.. I can relate to that!

 

Ya know what? This jogged a few gray cells and it can be disabled. Its been so long since I've worked with a MPH*3.2 term I forgot about something:

The MPH*3.2 term is limited to a max of 60 MPH. So setting the N-Bias term higher then 60 MPH (192) will disable this 'feature.'

Yes to the $5E1 & $5E2, they are both post-kick down, pre-closed loop idle max idle RPM limiters.

RBob.

 

This has turned into a pretty fruitful thread. Anytime you can "jog" a few of RBob's gray matter cells, you get more info.
Thanks RBob!.