Hey all this is my first time asking a question on this forum so hopefully I'm putting this in the right section...

About a month ago I had a problem with my truck, the check engine light came on and I was getting crazy backfires. The backfires got worse the more I drove (had to get it to a shop that I could work on it in). I plugged the truck into my buddies diagnostics computer and found I was having a cylinder 4 misfire. So I replaced all the plugs and plug wires, but the problem persisted so I put a new distributor cap in and the truck ran fine.
(I did the electrical because it sounded like unburnt fuel was making it into the exhaust system)

Most of the plugs had black crud all the way up the threads, and most all of them smelled like fuel.
My buddy suggested I run a compression test to see if the head gasket maybe needs to be replaced.

So what is a good Psi reading for one of these trucks?
Thanks

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white 1998 Ranger, 4.0L V6, automatic transmission,
4X4 on the fly, stock XLT truck.