need some help from the experts, We just installed a reuild long bolck in a 06 f150. its the 5.4L. used new exhaust manifold hardware and gaskets, torqued the studs and nuts per the specs. runs good, no codes but we have a slight surge, about 25-30 rpm, most noticeable at about 1700 rpm. with the code scancer in it we see a variation in the rpm, the short term fuel trim and the o2 sensor values. we noticed a crack in the right side exhaust manifold, near the rear, on the top. would this cause the surge problem. how can this be fixed. i have a mig welded but dont think it will work on cast. can it be brazed or is there some other way to fix.
We are open to any ideas.
thansk
gary

 

The only way to weld cast and get a really good bond is to stick weld it with nickel rod. You need to clean the area to be welded thoroughly first and pre heat it with a torch so that you get a good solid weld.