Fuel or spark issue possibly

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As for this issue, if you have single cylinder misfire on cylinder 8 and you just replaced the head gasket then we need to start with checking the compression on this cylinder.

Here is a guide that will show you how to do this:

https://www.2carpros.com/articles/how-to-test-engine-compression

I would run the compression on all the cylinders on that bank just to get a comparison but it is unlikely to be something that all cylinders are effected by like fuel pressure or even spark. It is going to be something specific to that cylinder like the injector, or spark plug, etc.

What I would do is instead of replacing parts, swap them to another cylinder on the opposite bank and see if the misfire jumps over there. Then you at least know what the issue is and can replace it accordingly.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Misfires are unfortunately sensitive things once you get past the common things like a plug or coil. So at this point we need to cover all the basic items first or we are going to chase our tail.

Compression would be the first thing I do because if that is not correct then nothing else matters.

Thanks