Huge misfiring problem

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by siavreg10, Oct 25, 2009.

  1. siavreg10

    siavreg10 New Member

    Hello all,
    I'm new to the forum and i am looking for help so who better to ask then the silverado forum.
    I have a 98 silverado extend cab with 5.0 vortek engine and have a huge misfiring problem that my local garage can not find a fix for.
    Here is what is happening and later i will tell you what they've done.
    Apon start up the engine runs extremely rough, shakes the whole cab. Once put into drive and run up past 1500rpm the engine runs smooth but once its brought back down to idle it runs extremely rough again.
    Now the shop has bench tested and found the cylinder 5 and 7 are misfiring at idle and when reved up cylinders 5 and 7 come back in and number 6 starts misfiring. They have called around to all other shops including 3 dealerships and not one of them have heard of this nor has the local mom and pop shops heard of it either. They did find one article on misfiring P0300 on my year of truck and it told them to clean the injectors and make sure there is not a torn gasket on manifold.
    Heres what i've done,
    Changed all wires, cap and rotor and plugs and verified firing order.
    Heres what they have done,
    They have done a compression test on all cylinders and the are all around 150.Verified my work. Removed manifold and changed one fuel injector and made sure all others were working perfect.Gasket had small defect in it and replaced it. They have verified that all rods move when head manifold was off and everything else they could see while it was removed.
    Now i'm no mechanic so if you have to ask me questions i will answer them to the best of my knowledge as to what they have told me but as of now it has been in the shop for 5 days and sitting at $1000 and they still have no idea whats wrong with it.
    Its not nice to say but i hope someone has had this problem and was able to fix it and to let me know so i can show or tell them. I say that because i would not want this to happen to anyone but if you could help me out that would be great.

    Thanks in advance for any help
  2. vncj96

    vncj96 Epic Member 5+ Years 1000 Posts

    Mine had very similiar problems and I had to have the heads rebuilt, I had a valve go bad. Have they run a bore scope through it??
  3. siavreg10

    siavreg10 New Member

    As of Friday they have not or at least they didn't tell me they did, I will mention this to them tomorrow. Not to tell them how to do there job or anything but they are running out of ideas and at this rate i'm going to shortly run out of money.
    So what cylinder was bad then as i'm having conflicting issues at different times with different cylinders?
    Thanks for the help

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