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Discussion Starter · #1 ·
So I just bought my Silverado, and when driving it around town, it runs and drives perfectly. Even full throttle off the line, drag race style. However, I took it on the freeway, and between 75-85 MPH, it develops a miss that goes away as soon as you slow down slightly. It's right around 2600-3000 RPM when it occurs. Hooked up a fuel pressure gauge, fuel pressure is 58 PSI steady, so that's not the issue. Don't think it's a valve spring either, as it runs up to redline without an issue at low speeds. I'm familiar with LS engines ( 4 out of 5 vehicles I own have them ) and I'm pretty skilled with mechanical work, so feel free to chime in with any advanced diagnostic ideas... My biggest issue is I only have a cheap scanner and the only thing I've determined so far is my LTFTs are +10% or so, which only slightly narrows the scope of what to look for. I'm sure this problem has happened to others, so getting some feedback would be great. Thanks!
 

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Check the plugs and wires yet? Maybe the gap got a little wide on one or two?

As I was reading your other post, I wondered if you had a Tech-2 to keep up on the diagnostics for your fleet. Such a tool makes fixing them so much easier and faster and less expensive.
 

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Discussion Starter · #3 ·
Nope, I only have a cheap Harbor Freight scanner. I keep hearing about cloned Tech 2s, are they fairly inexpensive? I already bought plugs, it's a cheap upgrade. If this still has factory platinums, the platinum disc could have popped off and made the gap bigger. I figure I'll start with plugs and go from there.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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You can get an MDI interface (it plugs into the OBDII port, and then USB to a windows laptop), and then find a Tech2Win w GM NAO PCMCIA card on the internet for a bit less than a tech ii, or something like this:

I got an MDI unit from that shop a couple weeks ago, and it works great for me. It did take some figuring out, but I'm definitely ahead of what it would have cost to tow my '12 Silverado to a dealer or mechanic to get the theft deterrent module talking to the ecm, program keys to the tdm, and just today, to replace/reprogram the airbag computer (you need a separate subscription to GM's/ACDelco's SPS service to do programming).
 

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Discussion Starter · #5 ·
I'll definitely have to look into that. I deal with GM vehicles enough that it seems like a worthwhile investment. I checked out the link you posted, sort of confused me as to what package I would actually need. I already have a Windows 11 laptop, not interested in having to get an older laptop just to hook it up, which it appears I might. Maybe I'm just mistaken. It's been a LONG time since I've used a Tech 2 in a shop ( 2001 ), so I don't really know specifics. Could you explain a little more about how the MDI interface works? I'm not a computer whiz by any means, lol.
 

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The MDI is an adapter that goes between your computer and laptop. Think of it as a translator. The MDI2 can be had as a wireless option so no cable direct to the laptop.

I like the handiness of the Tech-2 for use around and when driving the car, I can mount it to my steering wheel while driving and fool with it. Laptop is too cumbersome for that. The Tech-2 only goes to about model year 2013 so consider that.

I've had a "clone" for over 5 years in near continuous daily use and the main unit has held up just fine. Just about all the cables, the Candi and VCI have been replaced. This thing paid for itself after the first repair.
 
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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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Yeah that china diags company has a setup with a used Win 7 laptop & MDI clone (it's MDI1 in an MDI2 enclosure), with hacked copies of Tech2Win and GDS2 installed on it. I actually bought that setup for working on my '12 recently.

In a nutshell:
-laptop was configured in chinese, and there were a bunch of other apps on the laptop besides Tech2Win and GDS2, like TeamViewer and some other apps w chinese names that I'm still not sure what they do.
-MDI Manager installed version was really old, but functioned and worked with the MDI
-GDS2 install seemed to work, but the installed configurations are for newer vehicles than my '12 (as GM was switching from Tech2 to GDS around that time)
-Tech2Win install didn't work. It was on a weekend, so their tech support didn't get back to me until the next workday

I found a hacked copy of Tech2Win and a GM NAO pcmcia bin file on the internet, got it installed on a laptop with Win 8.1 (that's just what I have on my laptop for use with Autogenuity w GM Enhancement), installed Bosch's free to download MDI manager and the tech2win, and it worked. At this point, I used MDI Manager to update the firmware in the MDI device, it had to be updated to work with the installed version of MDI Manager.

Then I got a subscription to Acdelco's/GM's SPS service for the truck. As part of the subscription, they have a big installer that includes everything (Tech2Win, GDS2, SPS, wrapper app that I forget the name), the installer didn't like having Tech2Win and MDI Manager already installed, so uninstalled them, re-ran GM's installer, and got GM's SPS app/service working. You MUST launch the app from Acdelco's/GM's website launch button, launching the wrapper app directly from the desktop icon does not work (requests login credentials you do not have).

At this point, the installed Tech2Win doesn't work, as it's coded also be a subscription (that, IMHO, is stupid expensive for an obsolete app), but I found replacing the emulator executable with the hacked copy from the install I found, and the GM NAO pcmcia bin file, tech2win then runs and works fine. You do need to swap the emulator executable between the original and hacked version, when switching between Tech2Win and SPS, as running SPS, the wrapper app checks all the installed files for updated versions, and notices the emulator executable isn't right, and stops the launch.

GM/ACDelco specifies using Win10 or 11, so it should work fine on your setup.
 

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Even w buying the package including the laptop which I probably won't use much anymore (that laptop has Win7Ultimate installed), I'm already ahead vs having to haul the truck to a mechanic/dealership to reprogram various computers that I've had to do.
 

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Discussion Starter · #10 ·
Well, the dealer has the truck again for an unrelated oil leak ( they replaced the oil pan gasket and oil cooler gasket before they sold it, and apparently didn't do it correctly ), so I asked them to diagnose the misfire ( doubtful they will ), so we'll see if I can get it figured out. I have enough extra LS parts around that I'll get it taken care of one way or another.
 

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Well, the dealer has the truck again for an unrelated oil leak ( they replaced the oil pan gasket and oil cooler gasket before they sold it, and apparently didn't do it correctly ), so I asked them to diagnose the misfire ( doubtful they will ), so we'll see if I can get it figured out. I have enough extra LS parts around that I'll get it taken care of one way or another.
if there's a chance for a repair in the future, they'll diagnose it. i go in for oil changes and they come back at me for weeks with e-mails, snail mails, and texts for "suggested services".
 

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Discussion Starter · #12 ·
They did diagnose it, actually. Misfire on #1 and 6, and when I'm finally off work after tomorrow (13 hour shifts are rough), I'm going to do plugs and wires. It definitely still has the factory wires on it, so that's a likely culprit.
 

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Discussion Starter · #13 ·
Updating this with a final fix report: I went to change the plugs out first, and found it had brand new platinums installed, so I didn't need to replace plugs. I did remove #1 and #6 just to be sure nothing was going on in the cylinder, and both looked perfect. The wires passed a resistance test, and must have been a factory replacement set, as I can't imagine 16 year old wires could still be good. So I drove out to the LKQ U-pull-it and grabbed a couple coils and swapped them with the coils on #1 and #6, took it out on the freeway and the misfire is gone. The original coils checked out okay, too... I'm guessing they were just weak but hadn't fully failed. It was worth the $25 for a couple of used coils.
 

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Discussion Starter · #14 ·
If only had been so easy... The miss went away, but I decided to take her on a 250 mile road trip and it came back. I even stopped and put a brand new set of plug wires on it in the middle of the drive, didn't help. I'm now thinking there's a problem with the fuel injectors, because I've eliminated nearly every other possibility. Here's the funny thing: On that drive, averaging around 70 MPH, I got 13.68 MPG. I didn't think it would be nearly that much. Back to the drawing board...
 
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