Fouling Plug #5, Lower Compression

[Little Bandit]

Been looking through this site for a few years now, usually have been able to find answers to my questions through searching. Have looked for this one, but would like some more suggestions.

I have a 1992 4.0 4x4, 385kms (240K miles), still runs quite well.

Recently it has been fouling the #5 cylinder plug on a regular basis, it will foul it within 1-2K. Usually looks like gas/oil deposits, the first time I noticed it, the plug was eroded as well.

I did a compression test, and all cylinders were at ~160psi, except for #5 which was ~125psi.

I had thought that I might have blown a head gasket or cracked the head, but I was expecting a lower reading if that was the case. What would the pressure reading be if it the head gasket was bad?

Other symptoms the truck has.

Tach always reads high, will read 3,000 rpm at idle
Hard cold starts, below -10C/-14F
Oil looks ok and not milky
Doesn't appear to be any bubbles in overflow or radiator while idling
No vacuum leaks

I have replaced plugs, wires, charging system. I haven't touched the injectors or MAF, O2 sensors, but don't want to blindly swap parts.

Thanks in advance

 

[Little Bandit]

I should note that all the other plugs appear to be fine, and as of right now I am assuming it is the valve seal.

 

[Big Jim M]

I am assuming the plug is unable to fire under certain pressures.. I'd be looking at the spark trail. The cap of wire or the plug gap or whatever to see what difference I could find in that single plug that wasn't in the others.
Big Jim

 

[Little Bandit]

The compression of 125 is ok for that cylinder then? I know it is within 75% of the others.

 

[Little Bandit]

I will check out the ignition, but it might take a while to see if the plug will foul if I don't find anything.

Also the fuel pressure always bleeds off when it sits for a few hours, usually I prime it a couple times to prevent having to crank it for a while. I always assumed it was the check valve in the pump, is there a chance it might be a stuck-open injector?

 

[Big Jim M]

That would be highly doubtful.. But a very small leak in a particular injector is more likely.
Big JIm