98 Sierra 5.7 Misfires

[pbellone]

I have a tough one. I have research many forums for answers and now I will test the techs here.

 

1998 GMC K1500 5.7l Truck-Back in Feb I set a P0300/5 codes. Truck ran OK so I cleared the code and it seemed fine for a month. It set again. I ran a bottle of top end cleaner down the intake and left it soak. Started it up and burned it off. Changed the cap, rotor, and plugs. Plug 5 was caked up a little. Couple of hundred miles the next week, back again and a distinct rough accelleration. Mileage dropped and lousy feel. Ran reader looking for fuel trims compensating for something and other bad things. Except for the misses, all appeared ok. Replaced all wires, plugs, PVC, cap, and rotor again. Plug 3 & 5 were caked up with gray crap with only 400 miles since new. Still runs a little rough and I suspect it will set the misfire code again. I am being advised I will need to do the dreaded intake manifold gasket repair. I don't mind doing it as long as it has a problem. The fuel injection setup is a mystery to me along with testing and repair.

 

Any suggestions here besides reading dry TSBs or letting a deal change parts until it runs good? This truck has been well cared for over 105,000 miles. The only item I introduced prior to the problem (3 months) was Lucas fuel additive. Ten ounces per 26 gallons of fuel.

 

IDEAS???