3400/7730 Fiero stalls coming to a stop

[neophile_17]

Hello,

I think I've got everything the way I want it except for this one little issue. The car stalls when coming to a stop. If I catch it in time then I can keep it alive but I want to sort it out. Things start to go south at log point 2540 and again at 5240 in the data-log below. The funny thing is that I have "Idle BPW as a function of airflow" cleared but it still enters this mode. Any thoughts are appreciated.

~sam

http://www.rustytearracing.com/3400_stuff/Tune/try_2.csv

[SappySE107]

try_2 2.csv

Line 2540-2541 you can see the fuel bpw go to nothing, so it is cutting fuel. Probably because there are airflow calculations that were not converted. I am not sure why it is doing this as I have not had a car cut the fuel like that. Im still looking but I wanted to upload a modified/smaller file. Don't need a lot of that data and even what is there now is more than I normally use for tuning.

[Superdave]

yah, don't need 95% of what's in that log... lol


I looked as well, if the idle BPW flag is un-checked the it shouldn't go into that mode. It's a royal pain in the ass to tune it that way, I've done it and will never try it again.

[neophile_17]

Thanks for taking a look guys. I figured that was TMI but didn't want to leave anything vital out. I'll cut down the files in the future.

A couple of other possibly helpful tidbits. If I clutch in as soon as I know I'm going to stop as opposed to engine braking until I'm down to 15mph or so it rarely stalls.

The ECM I have is from an 87 century that had the early MAF system replaced. It's a 7730, but I don't know if there are any differences with the retrofit version.

~sam

[Superdave]

the stalling when pulling up to a stop is an issue that almost everyone with a 3400 swap on OBD1 deals with.

I don't know if it's the same in Nast1 but one thing you can try is to go to the main spark advance table, the first row (400 RPM) and add 2 degrees to the 35 and 40 kpa values. That's the row that's used to calculate idle spark advance in cell 0. The extra 2 degrees might save you from stalling.

One other thing you can try is to change the idle spark disable speed threshold to 6 mph, then change the idle spark disable tps threshold up to about 2.5%

If you mess around with the IAC throttle follower VS Speed it may help as well but don't make huge changes, trust me.

[neophile_17]

I'm running Ben's chip, which is regular A1, or that's what I'm assuming anyhow. I did however install the stock Fiero exhaust which is more restrictive, even after porting, than the 3400 exhaust.

Please forgive my ignorance but how does modifying the spark help what appears to me to be a fuel problem?

Is the assembly code available somewhere. I'm confused that it appears to be entering airflow mode when the flag specifies it should not.

Until I can nail down what's happening I'm thinking I'll lower the IAC follower vs Speed to 1.75 and raise the idle 75 RPM. Is this an effective blunt instrument for a stop gap?

Many thanks.

~sam

[Superdave]

don't use the .ads or .xdf for nast1, use one made for $A1. You may be seeing incorrect data...


adding a few degrees is basically cheating the idle under and overspeed spark correction tables. it works some times and not others.

[SappySE107]

The chip is A1. This isn't spark related, and my chip usually works without stalling or any other drivability issues. Exhaust would cause the fueling to be off a bit if anything but not stalling. Its like it hits DFCO but it shouldn't be doing that either with the RPM point that it does it.

[neophile_17]

I strongly suspect that there is another factor at play here. I have a couple things I want to play with but after that I'm going to swap the ECM/memcal for one out of a 91 Sunbird. Any changes need to be made to the chip to do this? I couldn't find any info on what the BCC affects. Thanks!

~sam

[SappySE107]

The memcal is only used for the cylinder select, knock sensor filter, and limp home mode resistor backs. Nothing needs to be changed unless there is something in the code causing it. I don't know what is setting something off that hasn't come up yet but anything is possible.

[Superdave]

any trouble codes when it stalls?

[SappySE107]

It takes about 10 seconds of decel but its not exact. The only thing I see that might be it is flag 1 bit 1. Mod Select BPW or A/F. Hell, that could be for something else, I don't think I have ever touched it. The stall is normally right after 800 RPM as well. Your battery voltage isn't too good either.

[Superdave]

i thought that's the alpha-n switch... i flipped it once on mine but it didn't make a difference...

[neophile_17]

I was eyeing up that bit as well. Last night I burned a chip with the lb/hr and gph values filled in. This was a blind shot in the dark but it seems better. However I only had the ride into work to test this out. I'll do some more testing this evening.

I'd still like to have the assembly code if that's available.

~sam

[neophile_17]

So it hasn't stalled out in two days and 4 trips. However, I do get an anomoly where odd things occur every couple thousand samples (2 examples below). I've been running without the center console so I have some concern that the ECM gets warm and flakes out. I'm going to replace the console this evening and see how things go.

~sam

http://www.rustytearracing.com/3400_stuff/Tune/data_log_8_26_11.xls