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Thread: 5.3 hesitation, stumble, lean and random misfire codes, worse when cold.

  1. #11
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    Quote Originally Posted by dirtrider73068 View Post
    Fuel pressure should be 60-66 and that should be engine off, turn key on let fuel pump run then stop then see what gauge reads.
    It sets about 60-61ish and holds.

  2. #12
    Senior Moderator chevychase's Avatar
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    Your ok on the pressure... Only thing i can think it to be spider or injectors. You have done a good tune up ect.
    You can get a replacement mpfi/ muti port injection for around 200 bucks will not have to worry about stuck poopets ect.
    Easy to install fit right back where your old stuff goes.

  3. #13
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    I have the same problem with a '96 Chevy w/ the 4.3 v6 that developed a hesitation problem last winter. It only happened on cold days and disappeared as soon as the truck warmed up. Since then the problem has gotten worse. About four months ago it started to happen every time I drove it for the first time that day, but went away as soon as the vehicle warmed up. Now the problem is constant at any speed/rpm and temperature.

    What it does is stumble while driving down the road when not under load. If I accelerate the problem goes away, when I'm cruising down the road under no load conditions, the truck studders until I get on the gas again. So far I've changed the fuel pump, fuel filter, ignition module, ignition coil, spark plugs, plug wires, distributor cap and rotor, cam position sensor and the throttle position sensor the IAC motor and cleaned the MAF with the correct cleaner.

    A friend of my suggested that it could be the poppet valves in the fuel injectors (I have the "W" engine code motor) that have gone bad, but I don't have any way of testing them and don't want to plow about $500 into a truck with 310k miles on it. Another person said it might be a plugged-up catalytic converter?

    There are no stored trouble codes and the "service engine soon" light does not come on when the problem is present. Timing doesn't seem to be the issue, since it is preset and controlled by the ECM. I had also replaced the entire distributor at about 230k miles so its fairly new. I thought that the problem might be the TPS so I ran a wire up to the cab off the return signal and monitored the voltage with a multimeter while driving the truck.

    Voltage would rise with throttle position, plateau when I let off the throttle and then start to fall erratically when the hesitation problem begins. I changed out the TPS and the problem remains. I think that the problem has something to do with the system voltage going awry, but I don't know that for sure.

    I have 55 lbs of fuel pressure at idle 60 when under load and 50 at no-load cruise. This is a brand new Bosch turbine pump. I've never changed three of the four O2 sensors -- is there a good way to test them that won't damage them? Aside of the O2 sensors, the only thing left to try is the crankshaft position sensor and to replace the spider/poppet injectors with the upgrade. Somebody said that the ECU probably isn't the problem because it either works or it doesn't? What to do now with a truck that doesn't run right but has body damage on three different places and a slew of other -- mostly minor -- problems I don't know. I'm hoping gets totaled by a drunken tree that jumps out into the road or something, but as that never seems to happen, I'd like to fix it if possible.

    Thanks,
    Mike

  4. #14
    Senior Moderator chevychase's Avatar
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    Left you a message in your first post.

  5. #15
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    Quote Originally Posted by chevychase View Post
    Your ok on the pressure... Only thing i can think it to be spider or injectors. You have done a good tune up ect.
    You can get a replacement mpfi/ muti port injection for around 200 bucks will not have to worry about stuck poopets ect.
    Easy to install fit right back where your old stuff goes.
    Can you point me the direction of this mpfi unit for $200? I am almost to the point to try that. I do work on many cars, so maybe I will get one in with the same setup as mine that I can use for testing. I hate spending another $200 on it and end up not fixing it. I haven't put the injector cleaner in yet as we are still above 1/4 tank. Should be able to run it through this weekend.

  6. #16
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    The cheveron techron will treat up to 15 gallons on the big bottle. So if you have about that you can add it in now if you wanted.

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