Spark Plug not Firing - Looking for Help

[SuperG]

The #7 spark plug is showing up like its been burning all over...it's black...so I replaced the plug and the coil-on-plug and am still getting the same result.

All the plugs are gapped the same so I am at a loss as to the problem...could it be the wiring harness? Could the piston be bad? The head is brand spankin new...

Any help is greatly appreciated!

 

[FormulaFord]

Had the same problem. The piston was coming apart. Do a compression check.

 

[MaximumVelocity]

Had the same problem. The piston was coming apart. Do a compression check.

+1, sounds like you melted a piston and compression is low. Why'd you replace the head? Did they check the pistons and bore while it was off?

 

[wjurls]

Either piston or in my case it was a sunken #7 exhaust valve. Either way a comp test followed by a leakdown would be prudent. Could also possibly be a leaky injector causing a super rich condition in that hole. If it the plug wasn't firing at all you would have a very noticeable miss and the new plug would be clean, not black and sooty.

 

[SuperG]

Ahhh, great guys! I'llk go in to do both = Check compression (although I am under the impression that was done after the rebuild) and the injector, which I hadn't though about!!