closed vs open loop

[Ed Wright]

Your son's scan tool should tell him if it is in open or closed loop. Only way that would effect it is if in closed loop the O2 sensors (It has an O2 sensor for each bank) has output/signal voltage pegged every high, causing it to pull all the fuel it can to try to lean it down, or signal voltage stuck very low, causing the computer to pour the fuel to it to fatten it up where it belongs. Both sides do this is very unlikely. One side would make it run very rough, both side's O2 sensors doing the same like all of a sudden is not likely at all. A scan tool would tell you in a heart beat, if you know what to look for.

If he can't see anything on the scan tool, unplugging the O2 sensors, and it no longer shutting off, would tell you if the problem is in that area. It may be an issue of something electrical getting warm before it fails, not whether it's in open or closed loop.

Simply watching the scan tool would tell you easier. O2 milli volts should toggle above and below 450 mv, several times a second. Scan tools up date too slow for you to see it all, but the voltage is up and down all the time. Switching something like 60 times per second.
Takes a very fast digital 'scope to see it all. Staying up or down will prevent going into closed loop. Voltage staying above .450 indicates to the ECU that it's over rich, staying below .450 indicates lean to the ECU.

When it dies,check for spark, fuel pressure and injector pulse.

My experience with parts store reman ECUs is not good at all. I would not just throw a reman ECU at it. I learned to buy ECUs at the Ford dealer, GM dealer, etc.

[Pvt Parts]

Originally Posted by Ed Wright

Your son's scan tool should tell him if it is in open or closed loop. Only way that would effect it is if in closed loop the O2 sensors (It has an O2 sensor for each bank) has output/signal voltage pegged every high, causing it to pull all the fuel it can to try to lean it down, or signal voltage stuck very low, causing the computer to pour the fuel to it to fatten it up where it belongs. Both sides do this is very unlikely. One side would make it run very rough, both side's O2 sensors doing the same like all of a sudden is not likely at all. A scan tool would tell you in a heart beat, if you know what to look for.

If he can't see anything on the scan tool, unplugging the O2 sensors, and it no longer shutting off, would tell you if the problem is in that area. It may be an issue of something electrical getting warm before it fails, not whether it's in open or closed loop.

Simply watching the scan tool would tell you easier. O2 milli volts should toggle above and below 450 mv, several times a second. Scan tools up date too slow for you to see it all, but the voltage is up and down all the time. Switching something like 60 times per second.
Takes a very fast digital 'scope to see it all. Staying up or down will prevent going into closed loop. Voltage staying above .450 indicates to the ECU that it's over rich, staying below .450 indicates lean to the ECU.

When it dies,check for spark, fuel pressure and injector pulse.

My experience with parts store reman ECUs is not good at all. I would not just throw a reman ECU at it. I learned to buy ECUs at the Ford dealer, GM dealer, etc.

Yes, for critical parts that have to be right, OEM (Purchased from the dealer) are the only way to go!

[Stewart Way]

My son drove the car over to my place Sunday AM. Made the trip without shutting off (about 15 miles) but there are 2 bridges with rough entries and it stumbled at each bump. We checked as many wire connectors as we could find and unwrapped a few locations looking for a chafed wire and found nothing. Time to take I to ford and see what they can find. The fuel pressure does not change when it dies and the only code it has shown is vehicle speed sensor. When it dies, the temp light comes on but not the check engine light.
Thanks again for the info.

[Ed Wright]

If wiggling the harness won't make it stumble, or die, try "the slap test". Take the computer out of it's mount, hold it in one have, and slap it with the other, while the engine is running. If it stumbles' misses, etc, you have a broken trace on the printed circuit board. Not uncommon at all.

[Alan Roehrich]

Take a plastic hammer (or small ball peen) and tap lightly but firmly on components such as ECM, MAF sensor, coils, module(s), relays, etc.

Check grounds and harness connections everywhere.

Also tap on and otherwise shake or vibrate the battery.

[Ed Wright]

GM recomends the "slap test", at least on my last trip to the GM Training center. May recomend mallets now, but I would be surprised.

[Alan Roehrich]

Originally Posted by Ed Wright

GM recomends the "slap test", at least on my last trip to the GM Training center. May recomend mallets now, but I would be surprised.

The slap test works okay, on the ECM. The right hammer tap (and no, I'm not advocating beating it to death) will often expose a lot of components that have an internal fault, due to cracked boards, bad solder joints, loose pin fit, and various other "intermittents", especially those that show up on rough road, or under various sharp maneuvers.