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96 LT4 Stumbles Once @ 3500 RPM after warmup

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96 LT4 Stumbles Once @ 3500 RPM after warmup

  #1  
Old 10-25-2018, 03:32 PM
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Default 96 LT4 Stumbles Once @ 3500 RPM after warmup

So, I have an interesting issue that combing this forum, reading the SM and having my local shop scan everything hasn't led to a resolution. When cold, like in the morning and cooling down after setting a while, my 96 LT4, 6-speed, bucks once at 3500 RPM and then is fine the rest of the drive. I had my local shop work on it multiple times, with the last one being front O2 sensor replacement because that was the only thing that wasn't right-on during a check ride scan. In the last year I've installed a new Opti and two ignition wiring harnesses, plugs and wires, coil, fuel pressure regulator, fuel filter, two ICMs, and a battery. I have a rebuilt PCM waiting at home as the next idea. I have 115k miles, runs great except for this issue, 18 MPG combined, no oil or coolant loss, all maintenance up to date.
Any ideas/experience with this?
 
  #2  
Old 10-26-2018, 01:45 PM
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Welcome to the forum.
Sounds strange. Has the fuel pressure been monitored during the glitch?
 
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Old 10-26-2018, 03:12 PM
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Thanks for responding and the welcome . I don't see anything in the charts that looks like fuel pressure stats, and I haven't connected a guage to do it. I started to check the FP relay condition this morning, but got sidetracked. I forgot to add earlier that there's no CEL and it happens during open loop operation.
 
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Old 10-26-2018, 07:36 PM
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Since it only happens during open-loop while the engine is warming up, that pretty much eliminates most of the sensors, if not all of them. I guess you should go ahead and try your rebuilt PCM if the fuel pressure looks okay to see if that has anything to do with it.
 
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Old 10-27-2018, 01:40 PM
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Many thanks. Hopefully I can get to it today .
 
  #6  
Old 10-28-2018, 08:18 AM
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So, I installed the refurbished ECM, and the glich didn't occur during the first check flight, but it did on the next two after letting it cool down a few hours in between. It's less noticable now however, and the single exhast backfire is quieter. Thus, it got better after changing two of the four O2 sensors and the ECM. I misspoke earlier. It happens during closed-loop, but none of the sensors revealed and anomalies. Next, I intend to change the fuel filter and fuel pump assembly, and back-blow the fuel lines. I'll probably replace the other two O2 sensors also. It bugs me to just throw parts at it, especially since it only happens once a drive cycle, but I'm at a loss on what else to do.
 
  #7  
Old 10-28-2018, 11:57 PM
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Good luck. Let us know what you find.
 
  #8  
Old 10-30-2018, 01:50 PM
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I will get back to you in a few days. Thanks!
 

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