'04 Disco is misfiring in cylinders 4 & 8. Changed plugs, wires & coil packs. What would be my next test? I am getting spark. Wondering if I need a head gasket but don't know a surefire way to tell. Any help is appreciated.

Thanks,
Pat

 

Only 2 other things that can cause a misfire, burned/chipped/carboned valves or head gasket and there are tests for both.

 

You've covered the internal engine, now you're left with the internal engine. The next easiest thing to do is probably a compression test. I would suggest testing all of the cylinders on that side (2 and 4 are both on the passenger side). Low compression in a cylinder can cause a misfire. Compression loss can be caused by either valve or the piston rings. If you are having a problem with a ring, you will usually see oil on the plugs or evidence of oil being burnt by the plug when you pull it.

You can get a compression tester for like $20-40 and takes about 15 mins to do four cylinders.

 

And there is this Buy Tracer Products Coolant/Auto Body Dy TRATP39000008 at Advance Auto Parts

a coolant dye, after running truck and engine cools remove suspect plugs, look for flecks of dye on plugs....

 

Thanks, I will start with the compression test and see what I come up with.

 

Also, if the compression test does not show well for those 2 cylinders, then do a leak down to verify burned valves.
There could also be an issue with a couple cam lobes, push rods or rocker arms having wear.
Let us know what you find.