Odd miss at low RPM

[SHOracer14]

I have a weird problem that I can't seem to figure out. On my 91' I've noticed lately that at low RPM's 2k and below, and usually at half throttle or less I have a missfire. This missfire never happens at above 2500 or at full throttle. The car has new plugs and wires (also spray bottled to check for arcing), new valve cover and plugwell gaskets, new fuel filter, I cleaned the MAF and ran some injector cleaner through just to see if it would help. I have noticed I only get around 23-24 mpg on the highway if I am lucky. The only code I pulled was 67 (A/C related according to SHOtimes). I am at a loss... nothing seems to be wrong with the car yet I cannot get this misfire to go away. I've tried searching similar problems and gotten no result. Thanks in advance for any input.

 

[SHOracer14]

Anyone? Anyone? Bueller...

 

[SHOracer14]

Ok! I spent a little more time on the car this weekend and figured out the problem. I eliminated any fuel problem with filter replacement and checking the PSI. All good. Cleaned the MAF and IAB still miss. Sprayed the brand new wires and around the coil with no Arcing. I switched coil packs with my other car... problem still remained. I thought hmmm.... maybe a very slight vacuum leak I can't hear (although the motor runs smooth at idle). Sprayed soapy water everywhere and found nothing. Finally I said screw it. I decided to pull the intake and check all of the brand new plugs just to be sure there were no problems. I also figured I might as well check compression while the intake was off and I was pulling plugs. I found the plug in cylinder 2 had a hairline crack in the porcelin (sp) causing it to arc but was invisable when I sprayed because it sits far enough down in the head. I replaced that plug and BAM! no shit stains in my undies.

 

[shomesomesho]

Just curious, what brand plug was it?

 

[SHOracer14]

The plugs I had in were probably the very last set of Motorcraft double platinums (AGSP32PP) that were on the shelf. That's what I had pulled when I replaced them when I first got the car. I replaced all 6 with the Motorcraft single platinum (AGSP32FM I believe is the part number).