Disassembly: Idle

**Greg E** · 11-12-2012, 08:57 PM

Idle

This was actually the most straight forward area of the disassembly. Too bad in Chrome V1 I had it all wrong.... Nearly all the tables I had listed in that XML didn't do what I thought. There were various flags I didn't understand at the time. Turns out the park/neutral switch was the trigger between each table set. With the P/N switch always being grounded on our cars (because of the MTX) only one set of tables were used and the target idle for the AC was just a single cell, not a whole RPM based 2D table.

In V2, I made a couple branch tweaks so the code never uses the duplicate tables and I tweaked the target idle tables to incorporate the AC function based off RPM.

Turns out, the ISC does quite a bit more than just handle idle. It's got a fixed position for start-up and it changes for cruising as well (suspect it's to help bring the RPMs down slower when you're off the gas changing gears). The starter and idle switch differentiate between the various ISC tables. Best to make sure both these switches work when doing a flash ECU swap. I've labeled the flag bits in EVOScan.

There are 3 other temporary ISC bump up conditions. There's a minor bump up for when the AC is initialy turned on (which then tapers back to the AC ON conidtions), when the ABS is triggered and when you turn the steering wheel.

The last thing I want to go over is the idle timing stability control area of the code. The ISC will get the airflow in the right range for your desired idle RPM but it's ignition timing which is used to keep the idle at the desired speed. You've probably noticed that at idle, your timing values are always inconsistent and constantly jumping around. This is why.

There are two variables in the ROM which the code references. One is the sensitivity variable and the second is a max timing adjustment limiting variable.

Not sure exactly what the sensitivity number translated to English means but I know that raising this value will increase the number of timing corrections per second and the timing limit is + or - 8 deg in the 99 ROM. This is how it's calculated:

TimingCorrection = Sensitivity/64 * (TimingLimit / 500 * TargetIdleRPM - .256 * RPM)

This Timing Correction value is added/taken away directly from the base ignition map. The timing corrections are applied afterwards.

Now when you put the ECU into SAS mode, one of the things the code does is set the timing limit to 0 and fix the ignition timing to 5 deg so you can set the base idle with the BISS. This is one of the reasons why it's important to set the idle with SAS mode and not play with the BISS while the idle timing stability controller is active. You'll get inconsistent idle doing it that way.

**Keyan** · 11-13-2012, 08:42 AM

I love these threads.

**Toni** · 11-20-2012, 02:42 PM

Greg,

I am kind of spit balling here but I'm wondering if my hanging rpm issue can be explained by the fact that my idle switch isn't getting triggered so the ecu continues to hold the ISC open? (if you recall, when I found the idle switch issue and fixed it, the car would stall off throttle. So the real issue is fixed that stall but I am just trying to explain the hanging rpms after the fact).

**Greg E** · 11-20-2012, 02:58 PM

Originally Posted by Toni

Greg,

I am kind of spit balling here but I'm wondering if my hanging rpm issue can be explained by the fact that my idle switch isn't getting triggered so the ecu continues to hold the ISC open? (if you recall, when I found the idle switch issue and fixed it, the car would stall off throttle. So the real issue is fixed that stall but I am just trying to explain the hanging rpms after the fact).

I'm sorry but I don't remember you fixing the idle switch. Too much on my mind lately.

There's a lot of variables with your setup that need to be eliminated. Part of me wonders if the location of the MAF so close to the BOV causes an issue as pre-counted air then gets rushed backwards thru the MAF only to be counted again while the BOV is open.

I can definitely see the throttle hang as a result of the ISC being so far open. Now that the switch is fixed, if my theory is true, we would see an a spike in the HZ readings from the air back washing. This can be fixed by either relocating the MAF or maybe a simple tweak to the MAF smoothing table in that HZ would help.

**Toni** · 11-20-2012, 03:05 PM

Not a problem. The idle switch issue was what caused me to almost not have the meth dyno session. I fixed the switch thinking that was culprit of the hanging rpms and it caused it to stall. Since I was in a rush to get the car dynoed I put the idle switch back to broken.

I guess I don't want to clutter this thread with my issues, I was mainly just inquiring about the details of the idle switch and isc and their affects. Come Spring time I will go all out to try to get this solved, I've had plenty of time to enjoy the car this year. Many thanks!

**Greg E** · 11-20-2012, 03:13 PM

Originally Posted by Toni

I guess I don't want to clutter this thread with my issues, I was mainly just inquiring about the details of the idle switch and isc and their affects. Come Spring time I will go all out to try to get this solved, I've had plenty of time to enjoy the car this year. Many thanks!

On the contrary. You're not going to be the last person to run into this issue and it'd be great to have a public discussion about it in a thread where maybe someone in the future would come to research it.

**Toni** · 04-30-2013, 10:53 AM

Odd thing happened yesterday. Car shifts and did not hang the rpms all day. It was cold and kind humid out so I'm thinking that is why. I haven't started trying to fix this year, have some bigger issues/upgrades to work out first.

**Nater3kgtVR4** · 05-01-2013, 11:31 AM

I am not having the hanging rpm issue but mine does stall when putting the clutch in above 2k RPM. Usually happens when I am also breaking and slowing down, but also if I shift say from 2nd to 3rd @3-4k RPM and leave the clutch the RPM's fall and car dies. Is this also related to the ISC?