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.