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  1. #1

    Default PO300 nightmare-2003 Sonoma-4.3L

    I have a 2003 GMC Sonoma, extended cab, 4wd with the 4.3L (vin has an X for indicator of injection system). I love this truck and have spent alot of money and time trying to get rid of the PO300 condition that keeps returning no matter what parts I've had put on. This includes new plugs, wires (replaced twice), cap & rotor to (vent screens removed), new distributor, re-flashed PCM, shimmed crank shaft sensor (that temporarily fixed it). Now my timing chain cover is leaking pretty good. I've burned thru one catalytic converter because its running too rich in fuel again, I'm well on my way to burning up another one. This misfire condition is predictable to occur at certain spots on the highway when shes acting up. There are times when I can run whole tanks of gas and not throw the code. They're other times where shes throwing the code alot within the same commute to work. When it occurs its always in a situation where I've been running at same speed and I have to throttle into it to either get up hill (loses 5-10 MPH), pass a vehicle, anything to do with need for additional shot of power. I also have a strange tinny rattle noise that a TSB was written up for. This rattle only is audible between roughly 1400-1800 rpms. Apparently a teflon timing chain tensioner wallers out allowing timing chain to have slop. I also have had the unfortunate experience to learn that when your transfercase vac switch is starting to go out it allows tranny fluid (my case uses transmission fluid as opposed to grease) and this allows the fluid contaminate all the vac lines, actuators, even the HVAC control unit. I was trying to fix the HVAC cuz all the air started blowing out the vents rather than the setting I wanted like defrost or floor. I've had alot of trouble getting that fixed. Ive literally worked my way from the fire wall drivers side to across the dash to the passenger side. The vac leak kept coming back and the last thing changed out was a check valve that was bleeding off. I finally got fed up with taking truck to the shop and recently swapped the orange vac line and the gray vac line that operate two actuators behind and under the glove box. The gray vac line is now on the actuator closest to right front tire in the passenger area and the orange vac line is now on the actuator kind by where a passengers left leg/foot would be. Please help me. I got to be able to get to work everyday.

  2. #2


    Welcome to GMTC. I have moved your thread to a subforum that should net you more answers. I have a friend with the same truck. He was having the "rattling" issue also. Yes, there is a TSB on the timing belt tensioner. But, his turned ouy to be a bad crank sensor. He too shimmed his and it worked temporarily. Then and it came back and he replaced the sensor. Same issue. When he needed power it would fall on its face and rattle. The sensor was causing major timing issues

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors,
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  3. #3


    I can't help with your issue but wanted to welcome you to GMTC!


    P.S. Paragraphs would help make that wall of text more readable...

  4. #4


    If you have a scanner, Monitor your oxygen sensors to amke sure that both of the upstreams are sqitching when in closed loop. If one is not switching replace it. We have seen a few of these in the past couple of weeks at the shop

  5. #5
    Jr. Engineer
    Join Date
    Aug 2010
    eas' bum fuk, maine


    is vin x the poppet valves or the electrical injectors? not many people here have 4.3L as apposed to the people with v8s so you may or may not have a better chance in lots of good info. if you have the poppet valves, i wunder if you got some sticking? i just swapped to electrical injectors in my 1500 with a 4.3.
    "i dont think you got the picture. i got a beautiful picture. this baby happens to have an extra turn of speed, which is the only thing i care about. you dont understnad what happens do you? they make cars. they make 'em exactly the same way. one or two of 'em turn out to be something special. nobody knows why. i know why. i may be kiddin' myself, but i think i can make somethin' out of that sad little bucket of bolts." ~ Tennessee Steinmetz

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