When you say the #8 plug is black it tells me that you may have a broken piston ring that damaged the #8 cylinder wall to the point where oil is coming up and fouling the plug, along with NO compression, or you may have bad valve seals or a bent valve. A compression test would be the best way to go now, and testers are not expensive, you can go to an auto parts store where they will rent you a compression tester. I am only guessing, and you may have to go into the engine, with a possible rebuild or replacement forthcoming. GOOD LUCK FRIEND.
If the end of the sparkplug is back, indicates carbon build up. Buy a compression tester, way cheaper and easier, although I'm not sure if it is a head gasket. What color is the smoke that comes out of the tailpipe, a greyish blueish or is it black? Does the engine shudder? Where the spark plugs themselves changed and gapped correctly? It may be detonation also.
SOURCE: 99 5.2 durango :pulled codes for multipe missfire cylinder #5-7
Aloha We have just replaced all of what you have replaced, plus water pump, fuel pump, pcv valve and 02 sensor. Took it to mechanic today we had misfire on 1,2,6,7. Mechanic said that the 99 dodge is known for bad computers, he checked ours and works great turns out that it was the coil. Ours was jumping 1/4 inch and had a orange/yellow spark. it is supposed to have a 1 inch spark and be blue. This has solved our problems after 527 dollars later.
Check the coil!
SOURCE: I have a 1997 Chevy Cheyenne, K2500 4x4 350
Recheck the spark plug gap - these so called pre-gapped plugs are not always correct, also check the primary and secondary resistance on your coil.
SOURCE: i own a 96 chevy k1500 5.7 l vortec and have a no 5 cyl misfire
It could be a faulty fuel injector, try running a fuel injector cleaner through your fuel system. At worst the poppet nozzle has gone bad and will need to be replaced, you can buy the poppet nozzle it self individually at Auto Zone or buy upgrade MFI system which is much better and less pron to fuel leaking room a cracked poppet line or nozzle which will cause what you have described.
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SOURCE: 93' chevy 4.3 V6 vortec. engine problem
fuel lines inside the intake plhenum may be cracked and leaking inside the plhenum. same thing happened to my 1992.sonoma. they are made of a plastic material and tend to crack right in the bend of these lines.
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