LT5 ECM Tuning thread

[XfireZ51]

Originally Posted by ShawnZR-1

Yeah, I misspoke, I know that it had been done but I should have said it wasn't widely available as some of the other ECM code is.

In that you have the in depth knowledge, here is one of my dilemmas that has pushed me into the wonderful world of ECM programing:
I have a PIA surge when I'm coasting down in neutral or clutch in. As I come to a stop, the idle drops very low and sometimes actually stalls the engine. From what I have gathered, it may be related to the IAC follower portion of the code, however, there are no parameters for that in either TunerCat or TunerPro.

I played with the DFCO parameters but ultimately simply disabled it entirely to get rid of the popping in the exhaust (and a nasty surge on decel).

IIRC, it started happening sometime after installing my headers. I can't be absolutely sure though. I also have a Fidenza flywheel so the rotating inertia is obviously less than it was stock.

This is just the beginning of my unraveling of the inner workings of the code. I'm sure I will have even more questions....

Shawn,

Maybe Todd can tell us where the Prop Gain tables are for the $AFA, $AF Mask ID. If you're getting a surge, it may be due to too high a Prop Gain. That could also explain the popping exhaust on decel. You'll want to start by lowering the Prop Gain in the <16gm/sec cell.
I'd agree with Todd that you'll want to check the #of steps at DFCO and not drop the IAC steps too quickly. Had same issue with my old 84. At some rpm points in DFCO, I'd push in clutch and motor would go dead.

Whadya think Todd, would that help?

[ShawnZR-1]

Originally Posted by tpepmeie

In my experience, this is not throttle follower related. What are the IAC counts doing during this surge? The throttle follower routine opens quickly as the throttle is opened, and decays slowly once the delta TPS stops increasing. It would almost certainly be decayed out in the coast down scenario you described.

I have found the consequence of getting too aggressive with the decay rates. What typically will happen is that on a very quick stop, when the clutch is dipped, the revs will drop too fast and the car can stall. IF the throttle blades are adjusted to high single digits at idle, that is usually enough airflow to avoid the stall.

curious, does this situation occur with the stock calibration, or just with a modified one?

Do you have a data recording of this event occurring?

Todd

This seems to occur with both the stock and modified. With the stock one however, it seems worse. I can't say for sure since I am unable to test back to back easily.

My IAC counts are well into the double digits at idle (15 or so). I honestly can't remember what they are during the surge. I'm assuming they run pretty low when it surges.

Question: Why does it only do it until the speed goes below a certain MPH? It seems to settle down around 5 MPH and if I blip the throttle when I'm not moving, the idle drops like a rock right to the set idle RPM and doesn't dip at all.

[ShawnZR-1]

Originally Posted by XfireZ51

Shawn,

Maybe Todd can tell us where the Prop Gain tables are for the $AFA, $AF Mask ID. If you're getting a surge, it may be due to too high a Prop Gain. That could also explain the popping exhaust on decel. You'll want to start by lowering the Prop Gain in the <16gm/sec cell.
I'd agree with Todd that you'll want to check the #of steps at DFCO and not drop the IAC steps too quickly. Had same issue with my old 84. At some rpm points in DFCO, I'd push in clutch and motor would go dead.

Whadya think Todd, would that help?

I have the DFCO completely disabled.

I don't see any Prop Gain table, is it one of the ones that never made it into the masks?

[XfireZ51]

Originally Posted by ShawnZR-1

I have the DFCO completely disabled.

I don't see any Prop Gain table, is it one of the ones that never made it into the masks?

Right. That's why I asked. Is the exhaust popping with DFCO disabled? Is the motor dying with DFCO disabled?

[tpepmeie]

Originally Posted by ShawnZR-1

Question: Why does it only do it until the speed goes below a certain MPH? It seems to settle down around 5 MPH and if I blip the throttle when I'm not moving, the idle drops like a rock right to the set idle RPM and doesn't dip at all.

The idle air PID control takes over at 5 mph to control the idle speed.

While it is tempting to go in and start changing the calibration, such as has been suggested here, I honestly don't think that is the solution. GM didn't miss the tune on 6000 cars, and they all don't surge. Some things to check...

1) if the condition exists only with the modified cal, is the spark advance smooth in the region of low load, low rpm? Has the closed throttle advance been increased?

2) check the MAP hose for contamination. When I had this condition, the MAP reading at coast down was fluctuating wildly, and causing the surge.

3) if you still suspect the IAC movement is responsible, try to reset the IAC motor as outlined in the service manual.

Todd

[rkreigh]

Hi todd,

I have a question, I had the secondaries removed and unfortunately VA has cut the HC standard down to 48. it's made it impossible to pass.

car does have cats (random ss, probably not nearly as good as stock) and I haven't yet had a wideband to see how rich the car is running but it seems WAY rich at light throttle.

I need some help to tune the car to pass emissions. marc is a bit too far away in chicago. I do have EASE, and I could install a wideband or take the car to a shop with a dyno.

the car is an LPE 390 and has the big 234 cams. do you think there is ANY way I can get this car to pass??? I tried doctoring the gas with alchy a few years back when I had the LPE chip in the car (open loop). it was even more rich and I could get the HC down to about 97 running 2/3 alchy, but I don't think that will work anymore, when I last tried the "cocktail" it still was up to 150 HC and I couldn't get the car to run with as much alchy in the gas. I love the ZR-1, but this is a problem

any ideas (besides moving)

[XfireZ51]

Shawn,

I did a search on ThirdGen regarding surging decel. This was in a thread that deals with another ECM, the EBL. I won't go into what ot is but I used it for almost 2 years. Outstanding piece of work by RBob. That ECM in combination with Moates OStrich and Tunerpro RT really changed tuning.

So here's RBob's advice:

"'Option word 3 - bit 5 - OpDcl' is probably set. This will put the ECM in open loop fueling during decel. Once un-set, you may experience surging during decel. If so it may be either the proportional gain at the gms/sec airflow is too high. Or the ECM is switching in and out of async fuel mode.

DFCO, to enter this the vehicle speed needs to be above the MPH parameters, and the RPM above the RPM parameters. This is usually for highway speeds on a thottle lift. Keeps the CAT(s) from melting.

RBob."

Here's the link to that thread.
http://www.thirdgen.org/techboard/di...=surging+decel

Granted EBL code is not the same as our LT-5 stuff but I think you'll find that the controls do pretty much the same thing. Its just that RBob has provided an .xdf which makes all those paramters available to the tuner along with a bunch of tuning aids.
I agree with Todd that first and foremost you make sure everything is "mechanically" correct before turning to the tune.
As a side note, if any of you don't know RBob, he is one of the more highly knowledgeable sources over at 3rd Gen. Heck of a nice guy too. I keep begging him to build a version of EBL for our LT5s.

[bradslt5]

Originally Posted by rkreigh

Hi todd,

I have a question, I had the secondaries removed and unfortunately VA has cut the HC standard down to 48. it's made it impossible to pass.

car does have cats (random ss, probably not nearly as good as stock) and I haven't yet had a wideband to see how rich the car is running but it seems WAY rich at light throttle.

I need some help to tune the car to pass emissions. marc is a bit too far away in chicago. I do have EASE, and I could install a wideband or take the car to a shop with a dyno.

the car is an LPE 390 and has the big 234 cams. do you think there is ANY way I can get this car to pass??? I tried doctoring the gas with alchy a few years back when I had the LPE chip in the car (open loop). it was even more rich and I could get the HC down to about 97 running 2/3 alchy, but I don't think that will work anymore, when I last tried the "cocktail" it still was up to 150 HC and I couldn't get the car to run with as much alchy in the gas. I love the ZR-1, but this is a problem

any ideas (besides moving)

I wonder if corey could burn a couple of different tunes for you to try. he has alot of different tunes in his files. just a thought

[XfireZ51]

Originally Posted by rkreigh

Hi todd,

I have a question, I had the secondaries removed and unfortunately VA has cut the HC standard down to 48. it's made it impossible to pass.

car does have cats (random ss, probably not nearly as good as stock) and I haven't yet had a wideband to see how rich the car is running but it seems WAY rich at light throttle.

I need some help to tune the car to pass emissions. marc is a bit too far away in chicago. I do have EASE, and I could install a wideband or take the car to a shop with a dyno.

the car is an LPE 390 and has the big 234 cams. do you think there is ANY way I can get this car to pass??? I tried doctoring the gas with alchy a few years back when I had the LPE chip in the car (open loop). it was even more rich and I could get the HC down to about 97 running 2/3 alchy, but I don't think that will work anymore, when I last tried the "cocktail" it still was up to 150 HC and I couldn't get the car to run with as much alchy in the gas. I love the ZR-1, but this is a problem

any ideas (besides moving)

RK,

I would definitely put a WB on it. Is the issue you're having at idle or all through the rpm band?

[ShawnZR-1]

Originally Posted by XfireZ51

Right. That's why I asked. Is the exhaust popping with DFCO disabled? Is the motor dying with DFCO disabled?

No, there's no popping with DFCO disabled. Well, there's kind of a burble on decel but with DFCO set to stock, there was a nasty surge and POP as it decelerated down the RPM band. VERY annoying to say the least! The car is almost undrivable with the stock chip.