My GMC 1500 Sierra

Vortec "spyder" injection system is not the best, you may finally have to replace the system, Have you tried run some SeaFoam in the fuel? See pic.

Doesn't have a Spyder

96, 97, 98, 99 That is what it is suppose to have. Has it been modified?

It's a 98

It's throttle body injected

So that would make it an 88 they were TBI. So then have you got the codes out of yet?

Yes it came up missfire

Random multiple, or did it give you a cylinder number?

I just bought a cam shafts positioning sensor I'm going to see if that works if not I don't know what to do LOL

Did you check for a bad intake manifold gasket?

No I will but would that cause misfiring? I mean I'm going to pull my hair out I've done all I can think of

Yes especially if it is leaking internally in the engine valley.

The common problem is anti freeze is leak on the inside of the engine onto the spark plug causing a misfire and needs either a head gasket replaced or the gasket and grommets inside of the valve cover under the lifters , either way it needs to be repaired internally.