I got a mystery p0300 random and I mean random misfire. I have done the basics spark plugs, boots, cylinder balance test could tell a difference after shutting every injector off, compression test was 180 across the board at 185 degrees. The car runs good at cold idle and than around exactly 200 degrees it starts misfiring maybe 10 misfires per cylinder I guess out of 255 counters it doesn’t misfire when cold and the misfires will jump to different cylinders never the same cylinder but it effects all 4. I am using a ms906ts scan tool for diagnosis. The one thing I can tell you is that at cold idle I am pulling 15Hg vacuum and it will decrease and fluctuates between 5-10hg when it starts to run rough but after you take it off idle runs perfect. I don’t have any hissing noise and I have sprayed b12 carb cleaner looking for a leak but haven’t found any thing I have alot of information like fuel trims and o2 sensor readings but I don’t think that’s my problem. I am thinking but not sure maybe the valve lifters are losing there pressure to holding the valves open or close when it gets warm. Any help would be greatly appreciated. I am about to check oil pressure and see what that does since I only have an oil pressure switch.
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