Glow plugs fault only when engine is hot 99 E300TD
 

On my 1999 E300TD:
My Glow Plugs check out just fine with an ohm meter when the engine is cold, but #5 and #6 test out to have a very high resistance only when the engine is hot which makes the glow plug light come on and subsequently the check engine light. Funny thing is, the car starts and runs perfectly. We have had recent days with 0 degree F and the engine fires right up perfectly smooth so I know all 6 GPs are functioning properly. My only guess is that there seems to be a problem with the wiring harness. The one thing I am wodering is how and why. The thing is, I am new to Diesels so I have not done much work myself though I am learning fast. This problem cropped up immediately after getting the car back from my mechanic ("ex" mechanic now) after he removed the intake manifold to change a few leaky fuel lines. I can't put my finger on what he did, but I am relatively sure he did something to cause this strange connection mystery.

Are there any terminations that might be loose somewhere between the Glow Plug Relay and the glow plug itself?

By the way, in the infinite wisdom of my former machanic, I was advised that "the problem had to be the relay since the glow plugs check out fine" He checked the when the engine was cold. So an extra $130 later, I have a spare perfectly good glow plug relay for a 1999 E300TD. Anyone intereted is a good deal on one?

Looks like I found it - bad ground at the glog plug of all things
 

Well, I just posted this earlier today after racking my brain for a few days and figured out what the problem was sort of by accident. I knew the problem was the back two glow plugs by testing when the engine was warm and since there is just a bit of access to the back two plugs if you can contort your arm just the right way, I tried to remove the #6 plug. They are only in there a few months but it was tight as he** so I pushed pretty hard with a socket wrench but decided to stop trying since I did not have the perfect angle and did not want to risk breaking anything since the car is running just fine and simply has an annoying check engine light on all the time. turns out that little bit of forcing with the wrench was just what I needed to fix the problem. Seems that the glow plugs were wroking just fine when you put curent to them, but when the engine is hot, I would get a bad ground from the glow plug to the cylinder head which sends a fault to the daignostics after the car has started when it does its resistance test. This budging of the last two glow plugs was just enought to fix the bad ground.:)