Re: Cylinder 6 Misfire Help
Ok, P0300 is a random misfire code, the P0301 and P0306 respective cylinder #1 and #6 misfire.
Misfires can be caused by several things and i presume that the coils changed where for cylinders 1 and 6?
That the misfire only happens on the highway, makes me think that it could have to do with sticking exhaust valves. (a valve which is sticking due to carbon buildup on the valve stem, will normally show up at higher rpm. The carbon makes the valves closing operation slower, so at higher rpm it can still be slightly open and reduce significantly the pressure in the cylinder during compression, causing a misfire)
I would try a seafoam treatment through the brake booster line. Seafoam is a strong de-carbonisation product. You will have to let half of the bottle be sucked slowly through the brake booster line on an idling engine, without letting it stall. As soon as half of the bottle is sucked in, shut down the engine and let it soak for an hour or so. Then take it for a ride on the highway, to clean it up well. Be prepared to see lots of white smoke coming from the exhaust, from the moment you feed the seafoam through the brake booster line until quite some time after starting to drive. It can be necessary to repeat this procedure a second time.