GM Inside - Muscle Cars - Jeep

Support FSC and see no ads! - Click Here
Results 1 to 7 of 7

Misfires and codes P0300 and P0302

This is a discussion on Misfires and codes P0300 and P0302 within the Technical / Maintenance forums, part of the General Discussion category; My truck is a 4.3L Wspeed, 235K miles. The truck is a Throttle body with central sequential fuel injection. The ...

  1. #1
    Registered User
    Join Date
    Jul 2004
    Location
    Texas
    Posts
    90

    Misfires and codes P0300 and P0302


    Support FSC and see no ads! - Click Here
    My truck is a 4.3L W\5speed, 235K miles. The truck is a Throttle body with central sequential fuel injection. The truck runs fine and normally starts easily. I was at a light and the MIL light came on. Cked the codes and found P0300 engine misfires and P0302 misfires on the #2 cylinder (only). This was the first humid day in a long time. That night I listened to the motor and it sounded like arching was occurring. I changed the plugs and wires (approx ~90K on them).

    In a few days the codes came back. Cked and changed the cap and rotor.
    Mil light \ codes returned again.

    I noticed that the misfires only happen when the truck is at idle. If you normal and spirited driving no misfires occur.

    Questions:
    1. How does the computer determine a misfire has occurred? Is it a delta in the cam and crank position?
    2. The O2 voltages are between .2 and .8V.
    3. Could the idle air intake valve stall the motor and make the computer think a misfire has occurred?
    4. Bad coil or ignition module?

    Any suggestions are welcome to hunt this down.

  2. #2
    96 chevy sub LT
    Join Date
    Feb 2006
    Location
    west babylon ny long island
    Age
    33
    Posts
    48

    Red face Re: Misfires and codes P0300 and P0302

    Quote Originally Posted by Fast4drss
    My truck is a 4.3L W\5speed, 235K miles. The truck is a Throttle body with central sequential fuel injection. The truck runs fine and normally starts easily. I was at a light and the MIL light came on. Cked the codes and found P0300 engine misfires and P0302 misfires on the #2 cylinder (only). This was the first humid day in a long time. That night I listened to the motor and it sounded like arching was occurring. I changed the plugs and wires (approx ~90K on them).

    In a few days the codes came back. Cked and changed the cap and rotor.
    Mil light \ codes returned again.

    I noticed that the misfires only happen when the truck is at idle. If you normal and spirited driving no misfires occur.

    Questions:
    1. How does the computer determine a misfire has occurred? Is it a delta in the cam and crank position?
    2. The O2 voltages are between .2 and .8V.
    3. Could the idle air intake valve stall the motor and make the computer think a misfire has occurred?
    4. Bad coil or ignition module?

    Any suggestions are welcome to hunt this down.
    hi well i have the same problem to . i have a 96 sub tryed everything . mechechanices tell me that it can be the injectors.to me i am not sure about that. i have the same code come up for about 6 months now i get those coed and p0175 p0172 wich is fule system too rich bnank 1 and 2 . like you i do not know what to do . is there anything else with happening with your turk ? like the way she start or the way she runs while stoped at a traffic like ?hit me back if you get any info .. the guys on here are realy helpfull you just have to give it some time . jay

  3. #3
    Registered User
    Join Date
    Jul 2004
    Location
    Texas
    Posts
    90

    Re: Misfires and codes P0300 and P0302

    Jay

    My truck had a hard starting problem at first but, that is no longer an issue. I think that was the plugs.

    Currently it runs fine but sometimes stumbles at idle with the ac on or off.
    Checked the truck with an AutoTap and found the wierd misfire problem.

    Otherwise no issue. BTW the SES light went off tonight.
    Last edited by Fast4drss; 03-09-2006 at 06:40 PM.

  4. #4
    ASEP Grad
    Join Date
    Jul 2002
    Location
    New Orleans, Louisiana, USA
    Age
    32
    Posts
    416

    Re: Misfires and codes P0300 and P0302

    You wanted to know how your truck knows you have a misfire so here we go.

    Every time your truck fires a cylinder the crank speeds up momentarily. The PCM (computer) looks at the crank sensor and expects to see the cranks speed to increase suddenly. If it doesnít see the increase in speed, it thinks the cylinder is misfiring. It's can see it really good on an oscilloscope. It's like looking at a heart monitor. Youíll see a strait line then a big spike when a tang on the reluctor wheel passes by the crank sensor. The signal should be fairly consistent. Some engines have a odd notch on the reluctor wheel (the 5.3does). This is to determine when the #1 piston is on top dead center. Now, with the odd notch, the PCM doesn't know if the #1 piston is on compression or exhaust. This is where the cam sensor comes into play. The only thing the cam sensor does is tells the computer when the #1 cylinder is on top dead center on compression stroke. That's all it does. Once the PCM gets it's signal from this sensor the PCM knows which cylinder should be the next to fire. If it doesn't see the speed increase on the crank sensor it assumes that the cylinder that was expected to fire is at fault. That's HOW IT WORKS.
    "American by birth, Southern by God's grace!"

    "Decide quickly, make your pitch, nail down the details, donít worry about the money." - Billy Durant (Founder of General Motors)

  5. #5
    96 chevy sub LT
    Join Date
    Feb 2006
    Location
    west babylon ny long island
    Age
    33
    Posts
    48

    Talking Re: Misfires and codes P0300 and P0302

    Quote Originally Posted by Fast4drss
    Jay

    My truck had a hard starting problem at first but, that is no longer an issue. I think that was the plugs.

    Currently it runs fine but sometimes stumbles at idle with the ac on or off.
    Checked the truck with an AutoTap and found the wierd misfire problem.

    Otherwise no issue. BTW the SES light went off tonight.
    now the the hard time starting, is that a cold start like first thing in the morning or is it after the truck runs for a while then try to start a few hours later . how does your truck run with new pluges ? what kind of pulges did you use ? i might do the same thing this weekend.
    Last edited by thej; 03-10-2006 at 04:26 AM.

  6. #6
    Registered User
    Join Date
    Dec 2005
    Location
    wa
    Posts
    106

    Re: Misfires and codes P0300 and P0302

    Both Of You With Misfires Should Find A Shop That Has A Motorvac Fuel Injection Machine And Let Them Run It For A While. At The Dealer I Used To Work At We Fixed Lots Of Misfires By On-car Injector Cleaning Machines.
    P0300 Is A Multiple Cylinder Misfire By The Way.

  7. #7
    NOOB
    Join Date
    Aug 2005
    Location
    Superior, WI
    Age
    28
    Posts
    96

    Re: Misfires and codes P0300 and P0302

    Try changing your fuel filter. If that doesnt help have a dealer do an injector cleaning like someone said above. It does wonders no little fuel injector cleaner bottle could ever think about doing
    97 Chevy Silverado 5.7L engine - Intake, Wait4me tune
    96 Chevy Monte Carlo - L67 engine swap, Intercooler, 42lb injectors, 2.8 s/c pulley, headers, cam, 3 inch exhaust

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •