Andy,
I can imagine your frustration. Here is an excerpt from your original
posting:
"I have also checked for an internal vacuum leak by blocking off either the
PCV hole or the oil filler neck on the valve covers and checking the other
to see if there is any sucking of air. There is none. I've also pulled the
valve covers off again and checked to see if there was anything wrong in the
valve train. Everything appears to be okay and all rocker arms appear to
have the same range of movement as the engine is cranked over."
It does sound suspicious if I understand correctly that the PCV vacuum hole
is on the #4 runner. However, you appear to have run the engine with that
hole plugged, so the current suggestions in this regard would appear to have
been rendered moot. Also, you are obviously way too savvy to have made most
of the other mistakes that have been suggested. I would think a bad vacuum
leak on #4 would adversely affect your manifold vacuum reading and since
this is good, I tend to doubt this type of problem. Which puts us back to
the topic of ignition, which you haven't mentioned checking yet, beyond
pulling the wire. If you haven't done so yet, I second John Clark's
suggestion for checking the spark on #4. Even if it is firing, check the
timing; it should be firing at 10-20 degrees BDC. If that's good, then check
the plug itself, as I previously suggested.
What ever the problem is, I'm sure it's trivial and you're going to kick
yourself when you find out what it is.
Good luck,
Bob
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