If its the 4.3 litre vortec with the CMFI (Central Multiport Fuel Injection) most likely its the CFI assembly (Central Fuel Injector sometimes referred to as the spider assembly), they are notorious for the plastic lines going from the single fuel injector, to the poppets, cracking and leaking, in turn spilling raw fuel directly into the intake. Also theres a Fuel Pressure Regulator attatched to the side of the CFI, it too can leak, causing extra gas in the intake. And lastly there are 2 fuel return lines that supply the CFI assembly, they are also both plastic, and can crack sending yet more raw fuel into the intake. If your lower end and upper end speeds are good, and it seems to start and stumble around 1800 - 2500 RPM, s thats usually what it is. The computer is trying to compensate for the random fuel anomoly, hence the stumbleing and studdering. This unit is underneath the upper intake manifold plenum. The top piece of the engine, under the nifty little black vortec emblem on top of the engine. Theres a simple test to see if you have a leak somewhere in those components. Get yourself a Fuel Pressure Guage, Hook it up to the schrader valve, on the fuel line, found behind the intake plenum, between it and the firewall (usually a little cap on it that you have to screw off) making sure its nice and tight so it dont leak thats important. Now with that all hooked up have an assistant go turn your ignition to ON, (NOT CRANKING THE ENGINE) that will charge the fuel line. You should have around 60 +/- PSI initially. Watch the guage, if the pressure starts to slowly drop after about 3-5 seconds (the fuel pump turns on initially with your ignition for about that long to charge the fuel line) then most likely you have a leak somewhere under the plenum in the CFI, Regulator, or fuel return lines. Before you start the process of pulling off the upper intake plenum, you can take a quick look inside fairly easily. If you loosen the 2 screws on the vortec emblem and take it off, you will see a round black unit, with a 2 wire connector hooked to it, thats the manifold tuning valve. You can take out the 2 torx screws that hold it on, then gently pry it up a bit with a screwdriver (it is sealed in with an O-ring and sometimes its sticky after time) pull that out and look directly down into the plenum, with a small flashlight. You can see the fuel regulator (passenger side of vehicle) and the one injector of the CFI (drivers side), and some of the lines "spidering" from it. Sometmes you can see if the regulator is spitting fuel this way. Have an assistant turn the key forward to ON (DO NOT START OR CRANK) to prime the fuel line, you should hear the injector, and see if any fuel is spitting from around the regulator. Sometimes you will see clean spots inside where the gas has been pooring out, washing away the carbon. If you cant see anything from there, you will have to pull off the upper intake manifold (which you will have to do anyway to make the repair). Usually around a 45 minute job or so, and not too terrible, then you will be able to see all of the lines. Inspect the regulator, all 6 of the lines coming from the injector to the poppets, and the two fuel return lines for leaks. Look for puddles of raw gas initially, if you have tried the pressure test immediately before this, Or for clean spots in the intake where gas has washed away carbon deposits. That will lead you in the area where the leak usually is. If you dont see anything initially Have your assistant prime the lines again (may take a few times of turning the ignition to On, then Off again at 5 second or so intervals). Look at each line as the pump is charging the system, usually its fairly noticable, but sometimes its close to the intake wall, and you might not notice it readily. But after enough priming and gas coming out you will see where its coming out at. The regulator can be replaced indipendantly from the CFI, however the CFI can only be replaced as a unit, and will come with a new regulator. The fuel return liines usually come as a set and can be replaced indipendantly. Most parts stores have them all available, although internet prices are usually cheeper. I have seen the CFI assembly as cheap as $180 and as much as $350, part alone no labor. Fuel regulator is usually about $40 +/-. And the fuel return lines are usually about $60 +/-. I hope this will get you somewhere, Good luck
Wednesday, July 17th, 2019 AT 6:10 PM
(Merged)
