Weird stalling problem. ECM??

[caffeine]

The past week or so I've had a weird problem with my 3500 swapped Fiero stalling. I think it's the ECM but I wanted some input.

It only stalls once in a while, pretty much always at low speed and I think always in DFCO. When it stalls, the SES light will come on and then the SES light and the shift light will flicker. If I try to start the car again it won't start but if I turn the key to off, wait a few seconds, and then try starting it it starts fine.

When I did the harness I didn't do any splices in the SES light or shift light wires. The ECM is a remanufactured equivalent of a 7730 (Delphi brand).

[robertisaar]

sounds like it's kicking into limp-home mode for whatever reason.

[caffeine]

Any idea why turning the key to off for a few seconds allows the engine to run again?

[robertisaar]

when you power up the ECM, it attempts to execute code as it's supposed to. if it gets stuck somewhere or starts executing things oddly, the "watchdog" timer(which normally gets reset quite often) will hit a value and then the code will jump to a section of the code that is predefined to cause the processor to quit executing instructions and the FMD chip will control fueling. that chip has it's fueling controlled by the limp-home stuff on the MEMCAL. if there are significant mods(especially engine size and injector size) compared to what the limp-home chip was tuned for, then fueling will be radically different.

now, shutting the key off for a few seconds causes the processor to be allowed to start executing instructions again, which bypasses the limp-home stuff and it starts running off of your tune again.

[Superdave]

Have you datalogged it?

[caffeine]

No not yet. It only happens once every couple of days. Sometimes going wot all the way to redline in 1st will cause it (stalls while I shift), usually not.

I should double check my chip is installed properly still but it seemed fine when I checked a few days ago.

[TGP37]

Maybe broken wire not liking the rock forward when shifting. Try wiggling wires around while idling. Odds are you'll get it to stall if it is a bad connection. That happened to me before. It didn't happen often at first.

Or having the MAF less then 6" from the throttle body may cause issues, as I learned. If you're using a MAF from the swap. Something about circulating air fooling the MAF and adds too much fuel.

Or you could have a bad FPR leaking gas into the intake (more of a guess). Thinking the sudden vacuum during shift (stronger vacumm from higher rpms) drawing fuel into the plenum.

DFCO should no be active below 1000-1200, 1600? rpms IIRC.

[caffeine]

No MAF, no FPR. Wiring is a definite possibility I haven't had much time to test things.
Yesterday I started the car (warm) in a parking lot and it was revving up and down with the SES light on. I turned it off, waited a few seconds, then turned it on again, started up fine but revved to about 4k then gradually idled down, also threw a code.
Just checked the code this morning and its 24; haven't had a chance to check what that means yet.

[TGP37]

No MAF, no FPR. Wiring is a definite possibility I haven't had much time to test things.
Yesterday I started the car (warm) in a parking lot and it was revving up and down with the SES light on. I turned it off, waited a few seconds, then turned it on again, started up fine but revved to about 4k then gradually idled down, also threw a code.
Just checked the code this morning and its 24; haven't had a chance to check what that means yet.

Code 24 is VSS (Vehicle Speed Sensor), which can cause irregular fueling issues. I don't know the exact details how the VSS interacts with the PCM/ECM but......a bad sensor is the first thing I'de fix and hope it was the problem.

Suppose it could send a false speed signal (over the fuel cut limit) and think it is going 254 mph, cuts fuel.

[caffeine]

Well today I just gave a few good yanks to all the wiring in the engine bay and the interior wiring and nothing happened (with the engine running). The VSS thing is really weird because my speedo was reading normally and both the speedo and computer use the same wires for their signal.
So wiring seems to check out. Only thing that could cause an issue with the VSS is if there's something to do with the wires after they're spliced into the ECM harness.
I'm beginning to think its chip/adapter related because of the issue I already had that was similar (chip adapter wasn't seated 100% in ECM and caused a weird part throttle/idle behaviour).

[pocket-rocket]

Try rapping on the pcm case.

Sent from my HTC One X using Tapatalk 2

[TGP37]

Well today I just gave a few good yanks to all the wiring in the engine bay and the interior wiring and nothing happened (with the engine running). The VSS thing is really weird because my speedo was reading normally and both the speedo and computer use the same wires for their signal.
So wiring seems to check out. Only thing that could cause an issue with the VSS is if there's something to do with the wires after they're spliced into the ECM harness.
I'm beginning to think its chip/adapter related because of the issue I already had that was similar (chip adapter wasn't seated 100% in ECM and caused a weird part throttle/idle behaviour).

The thing about the speedometer is, if the VSS is causing the stalling the speedo is gonna drop anyways.

Maybe the VSS wire is too close to strong interference noise. Not sure but if it relies on magnetic signals they are probably weak and can be screwed up with noise from the alternator, spark plugs or coils. See if your VSS wire is routed very close to any source of electrical noise.

Seems hard to believe it is the VSS, especially if new but often times people buy new parts that fail early on. Since you have a code for the VSS, I would concentrate around that first.

[cavalier3400z]

Somewhat related. I had this exact same problem but it was with a 2000 z24. I know its OBD2 but my radiator hose blew one day and then for about a month my car would just randomly stall for no reason while driving even with cruise control. Then all of a sudden my speedometer would stop working after 30mph. Replaced my vss for $90 and presto, problem was solved. Maybe you have a bad sensor like I did. Never hurts to try.

[caffeine]

VSS is a possibility but today I took the centre console off and found that one of the old Fiero ECM pins used for fuel injector power or ground had come un-taped and was possibly touching the firewall at times. I've left the console off since to see if the stalling comes back and so far it hasn't but its too early to tell.

There could still be something up with the VSS as the other day after it stalled the speedo jumped up to 80km/h during cranking for a split second. That's the first time I had noticed erratic behaviour with the speedometer.

[TGP37]

VSS is a possibility but today I took the centre console off and found that one of the old Fiero ECM pins used for fuel injector power or ground had come un-taped and was possibly touching the firewall at times. I've left the console off since to see if the stalling comes back and so far it hasn't but its too early to tell.

There could still be something up with the VSS as the other day after it stalled the speedo jumped up to 80km/h during cranking for a split second. That's the first time I had noticed erratic behaviour with the speedometer.

Speedo can jump during cranking when the volts drop low enough. If you ever have the misfortune to crank a weak battery, you'll see the speedo jump pretty high.

You might have a bad batt connection. I know it doesn't seem to fit the symptoms but it can happen. I had one once and the car stalled at odd times.

Check batt connections and also check resistance across the battery cables when disconnected. Anything over 0.2 Ohms is suspect of cable failure.

Reason is your speedo jumping during cranking. If your battery is in great condition that shouldn't happen. It shows you might have a hard volt drop when engaging the starter which means you may have excessive resistance somewhere in the circuit.