GP Mystery.
 

So car has been rough starting lately, GP light has been ok. #3 measures open between fw plug and valve cover. HOWEVER, plug to gp tip reads fine and gp tip to valve cover reads fine. I have checked this 4 times now (including a drive for coffee between #3 and #4 and an am certain this is the setup. Can anyone explain this to me?

I can't figure out what you did, after reading it multiple times. How many cups of coffee did you have before going for coffee again? :D

Please explain, in greater detail, exactly where you put each probe during each of the aforementioned tests. I'm not understanding any tests to the gp tip unless the gp is out of the vehicle.

My take right or wrong on your post. The actual glow plug does have resistance. So you are left with a really bad case of oxidation at the harness connector at the plug or the other end of the wire .. Or the actual wire feeding the plug has gone open. It going open is not that common but still a possibility.

Your ohmmeter will check this. You are looking for less than 1 Ohm. You cannot even have a few ohms resistance or the plug will not be operrational.

I assume 'fw plug' means the fenderwell plug on the GP Relay?

When you measure to the valve cover and you get an open, you need to walk it back - base of the glow plug, tip of the glow plug, back of the connector, until you find continuity again. You should be able to measure from the GP relay fuse to the tips of the glow plugs with no resistance.

Look at voltage with the GP relay energized. What voltage do you see at #3?

I've always had a hard time getting good ohm readings from my plugs. I always pull them out and give them 12v from a bench battery to make sure they are glowing bright orange. Be careful not to touch them after you've glowed them they get to something like 1600*F so they're REALLY hot.

More testing today and open at the tip to ground. Bought a replacement plug and it started raining. Odd that now it acts as a bad GP should.