need help

[julezzz28]

i changed the spark plug last week. it had ngk tr5 and i changed to the iridium plug now the runs bad at hwy speeds. it has a 396 with a little bit of a cam msd ignition,headers. should i stay with the low end plugs or use a better plug, and does anyone know what is a good plug to use and gap for it. thanks for the help.

[JaycenK]

Shouldn't be a differance unless you are way up on compression. Are you 100% sure you got all of the boots seated right on the plugs and the opti? Make sure you don't have any wires on the wrong plugs. Swaping 2 would cause that. also burned wire on a header will cause rough running under load. Did you use at antiseeze on all the plugs? if not zinced metals tend to seeze on aluminum if thats what the heads are made out of. It can cause you to think you have the plug seated when it realy isn't.

[julezzz28]

thanks for the help. i did use antiseeze on the plugs. yesterday i bought new tr5 plugs to put back in it and checked to make sure i didnt have a burnt wire or crossed wire. the wires look like they are in good shape and i might have had # 6 & 8 crossed but made sure that they were right when i put the new plugs in yesterday. now i have a little miss or sputter at about 2700 rpms and when you hit the gas and it get to 3000 3100 rpms it hauls ass like nothing is wrong. i didnt built this motor it was already built when i bought it so i am not sure about the compression but i am sure that is higher that stock. just not sure what it is. i was told i could have a fuel pump or fuel injector problem but changing from one plug to another has it running bad at different rpms. i dont have a clue what to do.

thanks again for your help
julian

[5.0THIS]

scan it, look for misfires and what the fuel trims are doing.