Still Have the P0304- 6 months and more $$$

Discussion in 'Engine & Drivetrain' started by gbsharon, Apr 18, 2017.

  1. gbsharon

    gbsharon TYF Newbie

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    Jan 17, 2016
    As per previous posts, am still chasing a cylinder 4 misfire. This happens when I am pulling up long hills and is way worse under load i.e. with passengers or a trailer. I wanted to go into a bit more detail and see if anyone could help. Started noticing going uphill- would get a random misfire code. Took the car to mechanic and was told dirty injectors- car was seafoamed (or equivalent) on machine. Ran well for about 2 months after this (towed and drove up hills no worries). After another 2 months, mis came back. Took to another mechanic who told me it was the injector spider. Had the updated MPFI spider put in along with new upper intake gasket at the same time. Although this got rid of the random misfire, I was now dealing with a P0304. I can feel when the miss is getting bad- front of car starts to vibrate with increasing force. Does not trigger a code until I push it further. Backing off the throttle usually calms her down a bit and the vibration eventually goes away. Took to original mechanic who said they would figure it out- performed leakdown test and compression test, both with positive results. I have since replaced the following myself all with AC Delco or OEM equivalent parts:

    Distributor Cap
    Fuel Filter
    Crank Position Sensor
    T-stat (unrelated but done nonetheless)

    Also New:
    MPFI Injector Conversion
    Upper Intake Gaskets
    Fuel Pressure Regulator

    What steps can I take to check from here? I really am tired of guess work.

    From research, I have narrowed it down to the following:
    - Bad ECU
    - Bad valve springs
    - Bad valve

    I was told the fuel pump was good by a mechanic, but I really wanna double check this. Is it possible that it is supplying adequate pressure but not adequate flow?

    Car runs really well on flat ground and starts right up every single time.

  2. parkerpress

    parkerpress TYF Newbie

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    Sep 6, 2018
    Somewhere in Colorado
    I have the same issue. Did you ever pin this down?

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