Random Misfire

Discussion in 'Engine Performance + Maintenance' started by Zachary Cox, May 13, 2019.

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  1. Zachary Cox

    Zachary Cox Newbie

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    I know this is a common issue people have. I've done quite a bit of research but I can't seem to figure it out. I've got a 1998 Chevy K1500 with a 5.7 Vortec tuned by Blackbear Performance. I've had a misfire issue for quite a while now. I finally thought a had it figured out last weekend when I replaced the crankshaft position censor. It went away for a while but it came back today. It doesn't misfire until it gets up to temperature but it also doesn't do it all the time. I replaced the engine in late September of last year and have only put about 12,000 miles on it so far. Since the swap, I've replaced the maf sensor, crankshaft/camshaft position sensor, knock sensor, distributor cap/rotor, spark plug wires, and spark plugs. All of those parts were either AC Delco or Delphi. The fuel pump also got replaced last July with a Delphi unit. I upgraded the spider injectors about two years ago but they were BWD. I'm kinda stuck and not sure where to go at this point. I did just order a bluetooth OBD 2 scanner and purchased TorquePro to help me out and see if I can make any progress. Anyone have an idea as to what it could be or point me in the direction of what I should be looking for diagnostic wise? Thanks in advance.
     
  2. upper_tanker

    upper_tanker I'm Awesome

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    Was the misfiring happening before you replaced all of those parts?

    My advice to you would be to get a scanner that can read live data, and see what cylinder(s) is/are misfiring.


    Just a little side note, and possibly food-for-thought, after we reinstalled my engine after the build and got everything hooked back up, I had one hell of a misfire when the engine would get warm. I think I checked the valve lash about 4 times, and it just kept misfiring. Come to find out, the BRAND NEW MSD cap that I bought had a misfire on cylinders 2 and 4, IIRC. They blew out my header gasket on those two cylinders. I put the old Delco cap back on, and it ran like a champ. I was pissed. Not saying this is your issue, but it may help somebody else that's looking for help. Don't ever assume that just because a part is new, that it is in good working order. LOL

    Also, I'm wondering if a worn out distributor gear would cause your symptoms. I've seen them worn before in a few of my dad's gmt400s, but I don't remember what the symptoms were.
     
  3. Zachary Cox

    Zachary Cox Newbie

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    The computer threw a code for a long time for a random misfire but it never really seemed to have a misfire until about a month or so ago. I thought it may of been a fueling issue but the fuel pressure checked out okay (I haven't checked it recently so its still a possibility). Then I did a tune up, replaced the maf, and the knock sensor and I still had the misfire code afterwards. I also forgot to mention I changed the intake gaskets at the same time I did the tune up because I thought I had a leak. None of that made the code go away. Then a couple weeks ago I changed the crank and cam sensor. The misfire code went away and it was running like it should until yesterday when it started misfiring again. I going to change out the distributor cap with a known good one I have and see if that fixes it. I'm also thinking it may be the spider injectors only because I know they're not a reputable brand that I installed (I didn't learn until later to only use AC Delco and Delphi parts on these trucks lol)

    And as far as the distributor, the distributor gear is in good shape. I replaced the distributor last January and also looked at it when I had it pulled recently.

    Also, I'm not sure if this has anything to do with it, but I did swap in a TH400 back in February due to blowing up my third 4L60E. It throws a few transmission related codes now. One of these days I need to send the ECM back to Blackbear to have anything transmission related deleted.
     
    Last edited: May 14, 2019
  4. Zachary Cox

    Zachary Cox Newbie

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    I changed the crankshaft position sensor again to see what it would do. It got somewhat better but still far from great. So I an some diagnostic checks today to see if I could find an issue. From research I've done in the past, it looks like the fuel trim is out of limits at idle but gets better at 2000 RPM. Fuel trim is also better at 55 MPH but not so much around 25 MPH. Here is a list of what I found:

    Idle-
    LTFT 1-25%
    LTFT 2-25%
    STFT 1-16.4%-20.3%
    STFT 2-16.4%-20.3%
    Fuel Flow-1.4 L/HR
    MAF-3.7-3.9 g/s

    2000 RPM-
    LTFT 1-12.5%
    LTFT 2-10.9%
    STFT 1-1.6%
    STFT 2-0.8%
    Fuel Flow-7 L/HR
    MAF-18 g/s



    I also found the camshaft offset was at -1.5. I bumped it up to +2 which is where I ran the old engine and had no issues. I drove it around for a while around town at varying speeds and for now it seems to have helped but I'm not sure what it will do once I get it up to highway speeds for a long period of time.

    As far as the fuel trim levels, what could cause them to be out of limits at idle and low speeds but be better at higher RPM and higher speed? Possible fuel pressure issue?
     
    Last edited: May 15, 2019
  5. Zachary Cox

    Zachary Cox Newbie

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    Well, I checked the fuel pressure today. It's at about 47 psi at idle, at cruising speed it around 52-55 psi, and on hard acceleration it's about 60 psi. So fuel pressure doesn't seem to be an issue. I'm still getting a misfire while cruising but not during acceleration, normal or hard.

    Something else I forgot to mention is on start up after it's already been warmed up, the idle fluctuates up and down around 400 rpm for about 10-15 seconds then levels out.

    I'm starting to wonder if it's an issue with the O2 sensors. Because when the system is running open loop, I don't seem to have an issue. But when it switches to closed loop while cruising, that's when I have a problem.

    I have no vacuum leaks and it seems the fuel system is working properly. But I can't monitor each injector so I'm not positive about that (unless the TorquePro app can do it?).
     
  6. 454cid

    454cid Sooper Pooper

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    Shouldn't that fuel pressure be at about 60 at idle?
     
  7. Zachary Cox

    Zachary Cox Newbie

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    I read posts from a few other people that have the same fuel pressure at idle and don't have an issue. But I guess that still could be an issue. If I remember correctly, the fuel pressure should be between 55-60 psi. Which it is at higher speed.
     
  8. Schurkey

    Schurkey I'm Awesome

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    How old are the O2 sensors? What do you get for O2 crosscounts?
     
  9. 94burbk1500

    94burbk1500 I'm Awesome

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    Do the Vortec engines have multiple O2 sensors? My tbi only has one.
     
  10. df2x4

    df2x4 Domestics only.

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    Yes. The Vortec trucks have pre and post cat O2 sensors.
     
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