Seems like its running lean. scan gives me a misfire on 8..sometimes different cylinder. throws a code lite. when you restart lite goes off.no stored codes except misfire on 8. cyl 8 is burning good. checked for intake leak none found..do these trucks have a nox sensor? could the f. pressure regulator be bad..how do I test that?
Do you have live data on your scan tool? If so, what are the fuel trims and O2 sensor readings? Do NOT start throwing parts at this.
SOURCE: S10 pickup misifirng when light highway load
I had to rebuild my engine at 97000 miles cuz of low engine oil pressure and bad berrings
SOURCE: 1998 Chevy s10 4 cylinder running rough
1- check there is spark come from spark wires
2- check the fire timing by looking for every wire in the right place
3- check there is injector signl
4- check for any vacuom leak
5- try to clean the MAF by brake and parts cleaner
6- try to reset all the code and see what will come after that
SOURCE: 2001 2500hd 6.0l multiple cylinder misfire at higher rpms
If you have someone scan it with a scan tool that gives you data stream they can tell you exactly what cylinder is doing it. You might have a bad coil or wire.
SOURCE: 2003 Chevy Venture misfiring
The injector is a good possibility, although you should check for spark in the wire first, as the electrical is the weakest link in the system. Put a spare plug in the end of the wire with the plug resting on metal, then have someone crank the van to see if you have spark that far, test and replace spark plug, (do not get shocked!) then look into a new injector or a good cleaning.
SOURCE: misfire code, rough idle, full tune up complete,
The black soot on the plugs means that you are running extremely rich. The fuel injectors ma test good electricly, but they are notorious for leaks. The spider-looking thing sandwiched between the plastic intake plenum and the intake manifold gets cracks in it's "legs" and leaks fuel ito the plenum area. The fuel pressure regulator is also housed in this same assembly. if the diaphram in the regulator gets a pinhole in it, it also leaks fuel into the plenum area, causing all kinds of erratic idleing, black smoke from the exhaust, and will cause your fuel trims to drop into the negative 20s.
There is a technical service bulletin covering this subject. This procedure outlines the steps required to replace the existing Central SFI Fuel System with a new MFI (Multiport Flexible Injection) Fuel System.
It is removing the injectors which are all housed in the center body of the injector assembly and replacing them with a new version where the injectors are located where the ols "poppets" used to be in the intake manifold ports. The picture below is for a V-8 engine that is covered by the same TSB.
The poppets and the pressure regulator are available individually to fix your existing fuel injection system, but I gave up on that after replacing several poppets only to have them come back in a few months with another popet line broken. I recommend replacing the whole assembly with the new and improved version. My customers have told me after doing this that the engine never ran that smooth when it was new.
The GM part number for the new injector assembly for your V-6 engine is 12568332 it is also now available at several aftermarket parts outlets.
As for the timing chain solution posted by doodieman200, all I have to say is that I have enough fingers on one hand to count all of the failed timing chains I have replaced on these engines. If you don't have a cam/crank sync code, the timing chain has definitely not slipped a tooth.
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A faulty o2 sensor will not cause a misfire. This is exhaust related, which is more than likely due to a clogged catalytic converter, which was is likely due to your misfire leaving unburned fuel to go to the exhaust, which clogs the catalyst. You likely smell a rotten egg type smell in the car at times. Your misfire is likely another cylinder. The OBDII codes are not great at finding the cylinder of the misfire in this particular car. Your best bet is to replace all 8 coils and plugs at the same time with OEM Motorcraft coils only. One bad coil makes the others work harder, meaning if you replace them one at a time, you will be constantly chasing a misfire.
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