I have a 2005 silverado with diablo sport predator, k&n intake,and pacesetter shorty headers. Started to misfire after instaling the headers and thought it was just an exhaust leak but fixed the leaks and it still miss fires. Took it to a mechanic and he checked all vacum hoses and intake gasket but still no problem. He says from his experience it may be a broke valve spring, lifter, cam, or injectors. He also says that cylinders 1 and 8 are the ones misfiring and the miss fire together. Any ideas on what it could be would be greatly appreciated.

Being basically coil over plug.. hmmmm 1 is the front left and 8 is the rear right. I got a call on this same type of thing last night except the problem was one whole bank.

I'm wondering how the crank sensor works and or if a case learn might be needed.

Have already had that done

Have you pulled the plugs and inspected them? Misfire can come from a bad coil, bad plug wires, bad spark plug, or mechanical problems such as broken spring. Pulling the plugs will tell you if those cylinders are doing anything different. Post up pictures of the plugs and that may help.

its coil near plug. would go through and reseat all the wires look in the boots and make sure the terminals are still good on those cyl. it just seems unlikely that something else happened all the sudden, most of the time its what ever you just did caused the issue.

You also my need to scan for a long time. Many times you get a P0300 then all of a sudden you get a P030x and that will tell you which cylinder is the problem.

have already looked at the plugs and ran for a week before checking the codes again and it still is only throwing a random miss code. Hooked up a computer to it and most of the time 1 and 8 were the ones missing and missed together

and firebird, the only thing i did right before it started missing was put headders on it and thought it might be the wires but replaced them too and still nothing

Any chance you have a plug wire touching or very close to your O2 sensor wire?

Ok, the other one I got a call on that had one bank misfiring had a power to injector issue. Not sure if wiring issues are common to the trucks as mine has been great so far.. (11 years 140K miles) It'* worth looking into though.