The pental of the IAC valve may be sticking or dirty. Also, your MAF sensor may need to be cleaned.
The misfiring cylinders are a common problem with these vortec engines. There are several reasons your cylinders may be misfiring. One is--a clogged or dirty fuel injector--try running the gas tank to below 1/4 tank and put in a large bottle of Cheveron Tectron fuel system cleaner. Run the tank to almost empty and put in another bottlle. This time fill the tank and see what happens. These injectors also have a design flaw and an upgraded version is avalable. The OEM injectors have the nozzle far away from the injector body, -- the new version has the nozzle and injector body connected as one at the inlet of the port---THE WAY IT'S SUPPOSED TO BE!
Also, the vortec head has awsome power potential, however, right around 125000miles, carbon builds up on the inside of the exaust valve guide because of the design of the valve oil seals. As the carbon builds up and hardens, the valve sticks. I discovered this little known fact on my personal 98 Tahoe.
If the fuel system cleaner doesn't work, try swapping out the spider injectors for the up graded version. If that doesn't work then the misfiring is most likely a sticking valve--in which case just put new heads on. New GM performance vortec heads are cheaper than having your heads rebuilt. Part 12558060.
Another thing about your engine. These engines are famous for leaking lower intake gaskets. If yours haven't already leaked, they will--it's only a matter of time. They may be already--this could be the reason for the misfires also. In any case, in order to change the lower intake gasket, the lower manifold must be removed and you may want to consider changing the heads now because your 3/4's of the way there.
Just one more thing--I have also seen the distributer on these engines wear out. This will absolutley drive you crazy trying to find and it will also cause a random misfire. Let me know what you find.
Wednesday, January 16th, 2008 AT 2:19 AM
