About a week ago I started getting a check engine light with codes P0301 (misfire on Cyl. 1) and P1316 (excessive misfires). I was getting some stumbling whilst driving, nothing too major.
I cleaned the plugs (only 3 months old, and I use the factory Champion RNY11YCC gapped to .9mm-1mm), replaced the wires with some 7mm Silicon wires from my local O'Reiley's. I also performed a Seafoam cleaner treatment in the gas tank and straight into the intake manifold. I also swapped out the single coil that powers cyl 1 with a known working replacement.
I am still getting a check engine light, but the only code I am getting is a P1316. The idle is USUALLY great, but recently its been shuddering at idle and low speeds. If I hit the accelerator, it smoothes out, but as soon as I let off, I get the shudder. If I come to a stop the engine sounds like it wants to die. Low speeds and idle are not very smooth.
So I run a compression test. I get 150 PSI on all cyl's but #6 where I get 138 PSI. I am pretty sure that is in check. I would like to go ahead and rule out a head gasket because the oil has no signs of coolant, nor am I using any coolant. I also hooked up a vacuum gauge to my manifold where the fuel pressure regulator normally supplies to. I did get some needle movement at idle. I drove around and when I hit the gas the needle dropped to 5"Hg and set there like a rock then when I let off, it hit back to 23"Hg and started to shake (it literally looked like the needle was trying to dance).
Does this 100% mean a sticking valve? Anything else I can do to either rule out the valve, or specifically test that theory?
TIA