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  1. #1

    Default 5.3 Vortec random misfire code

    I have a 2000 Sierra 1500 with the 5.3. Ext. cab, 4x4. The problem started when I got into stop and go traffic following my dad. I felt it missing when i would let off the throttle and when idleing. I drove around 6 miles before stopping. Got a few parts in a hardware store and left. In the next 2 miles it started getting worse. Then the Check engine light started flashing. Got it home, hooked it up to my brothers scanner. P0300 code was thrown twice.

    It was idleing rough, so i wanted to see which cylinder(s) wasn't hitting. Pulled the wires off the top of the coils one by one. only found 1 that didn't affect the idle. it's on the passengers side 1st one back from the front. not sure which number it is.

    Swapped sparkplugs from that cylinder to another didn't help, hooked to another sparkplug to the cylinder that wasn't hitting, and grounded it to the dipstick tube. started it and it had spark.

    Also I can smell unburnt fuel in the exhaust.

    Mods done to the truck are muffler delete, k&n intake, ported throttlebody, screen removed and ported maf. All done over 6000 miles ago. and took the egr out of the intake, plugged hole with 1-3/8 drain plug that seals perfectly. done about 2500 miles ago.

    Last thing that was changed was the fuel pressure regulator, my pressure was around 35 at an idle 39 under accel. new reg. it's 49 idle 56ish under accel. it was done around 60 miles before the problem started.

    I know that was really long, just wanted to give you as much information as i could.

    Any help is greatly appreciated.
    Last edited by dwill3015; 03-19-2009 at 03:24 PM.
    2000 GMC Sierra 5.3. ext cab short bed. 4x4.

    Custom exhaust "y" pipe back.
    K&N Intake.
    DIY Ported throttle body.
    MAF Screen removed.
    EGR... "modified"

  2. #2
    Jr. Engineer
    Join Date
    Jan 2009
    Rochester, MN


    I would start by looking for any type of vacuum leak such as vacuum hoses or intake gaskets, that would be my guess. Otherwise it could be clogged exhaust or low fuel pressure.
    99 Silverado Z71 4.8
    2" Suspension lift (Add-a-leafs and leveling keys)
    Long tube headers with 3" offroad Y-Pipe
    Volant Cold Air Intake with Ram Air Intake Tube
    Jet Mass Air Flow Sensor
    Diablosport Predator Tuner

    Previous Truck:
    97 Suburban K1500
    3" Body lift
    2" Suspension Lift (Add-a-leafs and cranked up torsion bars)
    35" Yokohama M/T's
    K&N Intake
    Airraid Throttle Body Spacer
    Hypertech Programmer
    Dual Flowmasters

  3. #3


    My buddies truck was doing the same thing. Replace the coil pack and it should fix your problem.

  4. #4
    Jr. Engineer
    Join Date
    Jan 2009
    Rochester, MN


    Quote Originally Posted by heavy chevy View Post
    My buddies truck was doing the same thing. Replace the coil pack and it should fix your problem.
    The newer trucks have a coil pack for each cylinder

  5. #5


    I know it isn't plug, wire, or coil. I checked all of them. and when I pluged a new plug into the wire and grounded it, started the engine. it had spark.

    Could a vacuum leak cause just one cylinder to not fire?

    My brother is bringing his compression tester over tomorrow. I'm really hoping it's not a burnt valve. I have 225xxx. but it doesn't smoke at all.

    My fuel pressure is good. 49idle, 56+ WOT.

    How likely is it that an injector is causing this?

  6. #6


    P0300 Code - Random Misfire
    Technical Description

    Random/Multiple Cylinder Misfire Detected
    What does that mean?

    Basically this means that the the car's computer has detected that not all of the engine's cylinders are firing properly.

    A P0300 OBD DTC code indicates a random or multiple misfire. If the last digit is a number other than zero, it corresponds to the cylinder number that is misfiring. A P0302 code, for example, would tell you cylinder number two is misfiring. Unfortunately, a P0300 doesn't tell you specifically which cylinder(s) is/are mis-firing, nor why.

    Symptoms may include:

    * the engine may be harder to start
    * the engine may stumble / stumble, and/or hesitate
    * other symptoms may also be present


    A code P0300 may mean that one or more of the following has happened:

    * Faulty spark plugs or wires
    * Faulty coil (pack)
    * Faulty oxygen sensor(s)
    * Faulty fuel injector(s)
    * Burned exhaust valve
    * Faulty catalytic converter(s)
    * Stuck/blocked EGR valve
    / passages
    * Faulty camshaft position sensor
    * Defective computer

    Possible Solutions

    If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

    If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

    Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

    or, like they say in g'ol Tennessee, don't fix it if it ain't brok. Sounds like your messing with MAF and EGR haunted you down.


  7. #7

    dwill3015's Avatar
    Join Date
    Jan 2007
    Yacolt, Washington
    Blog Entries


    I had the same exact problem on my 2001 Suburban w/5.3 and it was a bad coil pack.
    Washington State
    2006 Silverado 2500HD LT3 4X4 CC SB Duramax LBZ
    Tuff Country 6" lift, 35" Toyo M/T's on 20" Ultra Peacemaker wheels, Quadzilla Stealth2 programmer, Diamond Eye 5" cat-back exhaust, factory Special order color Yellow.

  8. #8


    Could be. I have a spare MAF that hasn't been modified. I just don't see why the problems would start now. The MAF was done around 8000 miles ago. around 5 months. and the EGR was about 2000 miles round the first of the year.

    I know I probably mess with stuff i shouldn't. I just like beating random people at stoplights. mostly ricers.
    0-60 in 6.4, it's see your moms old civic hatch with a 4" tailpipe do that? haha.
    Last edited by 00sierra53; 02-22-2009 at 09:29 PM.

  9. #9


    Quote Originally Posted by mjalstad View Post
    The newer trucks have a coil pack for each cylinder

    If the coil pack is bad it still may spark. It still may need replaced. It could only occure when the truck is under a load. Go to the cylinder you think it is and replace the coil pack. Im telling you 9 out of 10 times it will be the coil pack. Chevy has had there problems in this area since switching.

  10. #10


    I took a coil off a different cylinder and put it on the one that wasn't hitting. no change.

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