i have a 98 gmc suburban k1500 4wd 5.7 vortec. i had a misfire and overheat problem. i replaced the upper and lower intake manifold gasket, replaced old spider injectors with new multiport injectors, i also replaced the distributor cap and rotar, spark plugs and wires. i am still getting a misfire on cylinder 3 and 6. i hooked a fuel pressure gauge to it and it held pressure at about 55 but when the key is turned off it drops fast to zero. anybody have any ideas what else could be the problem?

Maybe head gaskets it not very likely that both went at the same time though.

something is wrong with the fuel, the pressure should be like 58ish and stay there. and go up into the 60'* when running. what brand regulator/mpfi conversion did you get? has the pump been changed? should get delphi for both, makes a big difference on the 96-99'* because they are very picky. also you take the upper intake back off and see if the regulator was leaking when pressurized?

i forget the brand mpi but it has a brand new fuel regulator on it. i was told it might also be a fuel pump check valve?

ive heard of that but its part of the pump module, you want to get the delphi one of those too. it has the fixes for the common fuel sender problems and the new style connector assembly. make sure you throw away the butt splices and solder and heat shrink the new harness