Surging idle and stalling? about 5 to 8 seconds it starts to cough and sputter?

1997 CHEVROLET S-10
175,000 MILES • 4.3L • V6 • 2WD • AUTOMATIC
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HITTN8SWITCHES
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Truck starts just fine, after about 5 to 8 seconds it starts to cough and sputter then catches itself and revs up to about 2,000+ RPMs then back down and coughs and sputters. Cycles like this for a minute or so until it eventually stalls or evens out and idles. When it idles it's not smooth and won't accelerate? If the accelerator is pressed or it's put in gear it stalls. On occasion it will start and idle, not a good smooth idle but stays running, can rev it and drive it, doesn't run well, but will drive. Once it's turned off and restarted the coughing cycle starts over.
Started one day leaving work ran fine to work but wouldn't run in the afternoon. Towed it home found the fuel pressure a little low 46-47 PSI. Changed fuel pump, no improvement to the coughing. Eventually I found a sizeable vacuum leak in the PCV line. Repair the line no improvement. Vacuum at manifold holds steady at 20in when it stays running. No DTC set no pending DTC. Not sure what else to look for with no DTC. Forgot to mention fuel filter was changed with new pump install.
Jul 20, 2022 at 4:43 AM
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STRAILER
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What comes to mind is the exhaust system being clogged and more specifically the catalytic converter. Let's try to remove the exhaust at the cat to see if it runs better. Also, we should run the codes to see what comes up.

Here are two guides to help us fix the problem:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

and

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Let us know what happens and please upload pictures or videos of the problem.
Jul 21, 2022 at 3:03 AM
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HITTN8SWITCHES
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Okay, separated the exhaust flange in front of the cat (engine side). No change. There are no trouble codes being set or pending. Upload a short video of what it's doing. Starts no problem then goes into the coughing/ surging then stalls.
Jul 21, 2022 at 10:17 AM
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STRAILER
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Thanks for the video it helps a lot. These engines had a problem with the fuel pressure regulator inside the intake manifold to leak raw fuel inside the intake. I would get an AC Delco unit; the regulator sits at the rear of the fuel injector (spider). Check out the diagrams (below). Please let us know what happens.
Jul 22, 2022 at 10:46 AM
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HITTN8SWITCHES
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Okay, replaced the spider today. No change. Thought this might be it as i found the top of #3 injector suspiciously clean. I thought it could be leaking at the injector, so I switched it out. Still goes through the same cycle. Video from today after putting back together. Still no codes.
Jul 23, 2022 at 10:35 AM
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HITTN8SWITCHES
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Okay. So, if I disconnect the IAC it runs steady no surging just at a high idle. Plug it back in and the issue comes back. What would make the IAC cycle open and closed?
Jul 23, 2022 at 11:05 AM
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STRAILER
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The IAC cycles because the PCM is "hunting". When you drive the truck does it have good power? You might have a bad PCM. Also have you checked for a vacuum leak? This guide can help check when the engine is cold:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please run down this guide and report back.
Jul 24, 2022 at 5:01 AM
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HITTN8SWITCHES
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Last time the truck was drivable there was no noticeable loss of power. Vacuum leaks are one of the first things i looked for. Found one on the PCV tube. Haven't found any others. How can i check the PCM?
Jul 24, 2022 at 10:16 AM
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STRAILER
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A CAN scan maybe able to detect a problem you can get a CAN scanner from Amazon for about $30.00 here is a video to show you how.

https://youtu.be/u-4syLc-ifQ

here is a scanner link

https://amzn.to/3o41KcZ

Or you can just swap it out for another unit here are the directions. Check out the diagrams (Below). Please let us know what you find.
Jul 25, 2022 at 5:38 AM
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HITTN8SWITCHES
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Solved, turns out it was the mass airflow sensor.
Jul 26, 2022 at 2:25 PM
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STRAILER
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Nice work, I have not seen that problem before, thanks for the information. I am surprised it didn't though a trouble code.
Jul 27, 2022 at 5:46 AM
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HITTN8SWITCHES
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That was the most frustrating part, it would not set code.
Jul 27, 2022 at 8:06 AM
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