97 K3500 fresh engine runs great in open loop and horrible inj closed loop.

[suspect racing]

If #5 misfires, what is different on #5 than any of the others?

Which of those differences would be affected by open-loop/closed-loop operation?

I asked about plug wire routing, vacuum nipples on #5 runner, and some other stuff, and never got an answer.

Plug routing is nice a tidy and all new. no crossing wires. Everything that folks have offered so far has been done if it were possible. There is nothing old left to replace. In a number of cases I have replaced things twice. I did find one suspect knock sensor but it did not fix anything...still no codes. The 3 known working computers all thought everything was good. New fresh engine, checking pin connectors show all is working as it should. New exhaust zero leaks fresh intake gaskets now replaced twice. Open loop works great the minute is changes to close loop and try to put it under load it misfires...unless you drive it with little throttle input. It will tip toe around ok...just ask it to pull a load.

 

[Komet]

I wish, I have already tried to have the data recorded to send out for a tuner to have a look. When I told him the year he said, 1996 to 1998 will not allow data to be recorded. I cannot record real time data from this computer. Now I just want to make it stay in open loop and let rip. It runs great in open loop. Runs like **** in closed loop. 3 computers and all the sensor have be replaced and all look good while running. Long and short fuel trim looks correct too. I would have already put a Qjet and HEI on it if it did not have a 4L80E

What I'm telling you is your "tuner" is garbage at what he does and that there is a way to do the thing you need that you haven't discovered yet. I don't do OBD2 but my buddy gets datalogs out of a phone app called Torque I think. You can also put a qjet and HEI on it with a trans controller.

 

[Supercharged111]

What are your O2 mV? Really sounds like a couple O2 sensors that I've had take a dump over the years. One leaned a bank out, the other caused the truck to roll coal. Both caused it to fall flat on its face. Have you ohmed the O2 wiring end to end? If you want to keep it in open loop just unplug an O2 sensor.

 

[Schurkey]

Plug routing is nice a tidy and all new. no crossing wires.

I don't give a crap about "nice and tidy". That was not what I asked you. I asked if the plug wires were routed EXACTLY like the factory service manual illustrates.

"No crossing wires"? That may be part of the problem. Running wires parallel to each other invites cross-fire, and that's especially dangerous on #5--#7.

Everything that folks have offered so far has been done if it were possible.

You aren't data-logging.

Any decent scan tool can record a "movie" on a vehicle, going back to the beginning of computer control in 1980 1/2.

Open loop works great the minute is changes to close loop and try to put it under load it misfires...unless you drive it with little throttle input. It will tip toe around ok...just ask it to pull a load.

Yes, I understand that you haven't found the problem yet, and you're frustrated.

 

[df2x4]

As others have mentioned, there are several options for datalogging OBD2 vehicles and the '96-'98 GMT400s are no exception. I would not trust the tuner who told you that. His particular setup may not work with those years for some reason but there are options. Torque Pro and Dashcommand are two popular phone apps that can datalog a '97 with a Bluetooth module.

 

[L31MaxExpress]

The 97 PCM can definitely be datalogged. Around 2006 I had an ancient DB9 serial cable OBDLink interface and GM Enhanced software.

The first tuning on my L31 was a MAF T+ translator designed for a LT1. It connected between the MAF sensor and PCM as well as the Ignition Control Module and PCM. Thus it could alter the fueling and timing without having to flash the PCM. Later on I found someone local with TunerCatsOBD2 that could write to the 97 PCM and did a rough base tune based off the changes the MAF T+ had programmed into it. I already knew how much the fuel/timing was being manipulated and where. Even after tuning the Black Box, I left the MAT T+ on the thing and used it for small adjustments in fuel and timing until I 0411 swapped it In 2013. Similarly my 03 Dakota had a Superchips flash and I ran a Split Second PSC1-003 on it to accomplish fueling changes. In this day and age a vehicle, especially a GM Vortec can be tuned using an 0411 and HP Tuners so the piggy back method is not needed.

 

[suspect racing]

What are your O2 mV? Really sounds like a couple O2 sensors that I've had take a dump over the years. One leaned a bank out, the other caused the truck to roll coal. Both caused it to fall flat on its face. Have you ohmed the O2 wiring end to end? If you want to keep it in open loop just unplug an O2 sensor.

Both are new and according to my scanner they are working fine. It does not stay in open loop when O2's are unplugged. Every sensor on this truck test fine and look good while driving. It is a random misfire.

 

[suspect racing]

I don't give a crap about "nice and tidy". That was not what I asked you. I asked if the plug wires were routed EXACTLY like the factory service manual illustrates.

"No crossing wires"? That may be part of the problem. Running wires parallel to each other invites cross-fire, and that's especially dangerous on #5--#7.


You aren't data-logging.

Any decent scan tool can record a "movie" on a vehicle, going back to the beginning of computer control in 1980 1/2.


Yes, I understand that you haven't found the problem yet, and you're frustrated.

The PCM in 1996 to 97 7.4 trucks will not allow for data to to recorded to a file. It is called a black box PCM. I can drive the truck and watch what is happening but it will not save the data.

 

[suspect racing]

The PCM in 1996 to 97 7.4 trucks will not allow for data to to recorded to a file. It is called a black box PCM. I can drive the truck and watch what is happening but it will not save the data.

The PCM in 1996 to 97 7.4 trucks will not allow for data to to recorded to a file. It is called a black box PCM. I can drive the truck and watch what is happening but it will not save the data.

The wires are exactly like the factory set-up and use the stock clamps Because I like to keep things tidy.

 

[suspect racing]

The 97 PCM can definitely be datalogged. Around 2006 I had an ancient DB9 serial cable OBDLink interface and GM Enhanced software.

The first tuning on my L31 was a MAF T+ translator designed for a LT1. It connected between the MAF sensor and PCM as well as the Ignition Control Module and PCM. Thus it could alter the fueling and timing without having to flash the PCM. Later on I found someone local with TunerCatsOBD2 that could write to the 97 PCM and did a rough base tune based off the changes the MAF T+ had programmed into it. I already knew how much the fuel/timing was being manipulated and where. Even after tuning the Black Box, I left the MAT T+ on the thing and used it for small adjustments in fuel and timing until I 0411 swapped it In 2013. Similarly my 03 Dakota had a Superchips flash and I ran a Split Second PSC1-003 on it to accomplish fueling changes. In this day and age a vehicle, especially a GM Vortec can be tuned using an 0411 and HP Tuners so the piggy back method is not needed.

I was told by a number of tuners that you cannot datalog the computer in 7.4 powered trucks in 96 and 97. Because the truck is so old I cannot find any tuner that knows anything about them.