Having just done my glowplugs on my JTD, I have a tip for anyone about to do the same...

USE OEM GLOWPLUGS!

Sounds obvious, but I originally bought a set of NGKs since they are supposedly good quality plugs and I wanted to make sure I had a decent set in so that I wouldn't have to replace them again for a long while.

First thing I did before I even installed them was to test one on a battery (just out of curiosity to compare them against the old plugs.) They were relatively slow to heat up, and I mean slow enough to get me concerned enough to go out to buy an OEM plug to compare them against. The OEM plugs are Bosch.

Connected in parallel to the same battery (as they would be in the car,) the OEM plug heated up substantially faster than the NGK! It also appeared to reach a higher temperature (by glowing more) and after disconnection, retained the heat for much longer. As well as that, the shaft between the head and thread of the plug is thicker on the OEM Bosch ones, meaning they are far less likely to break when you come to replace them!

Hope that helps someone in making the right choice The glowplugs on Shop4Parts are OEM and fairly reasonably priced.

Thinking about it, I'm not sure whether I should have put this in the guides section or not?

I might also write a guide on removing seized plugs as two of mine were really bad... so bad infact that the diesel specialist I took it to originally told me they WILL snap if they try to remove them, so I thought I'd give it a go myself first. Managed to successfully get them out over a period of 3 days and lots of patience!

Guide on removing seized glowplugs uploaded here: https://www.fiatforum.com/stilo-guides/322787-seized-glowplug-removal.html

i thought you wasnt supposed to test modern glow plugs like that?

I'm not sure... first I've heard of that dave :/ you could be right. oh well, at least I've done it so no-one else needs to

Not sure whether it applies to the JTD engine, but most glow plugs are pulsed and never get a constant supply current. Usual way to test them is with a low reading ohmeter but analogue ohmeters are seldom accurate enough to register a low resistance correctly.
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On the JTD, the glowplug module just triggers a relay that sends just over 11v to the plugs for around 15 to 20 seconds on start-up, so I'm guessing its not pulsed. Whether there is additional electronics to limit the current to the plug, I'm not sure. I assume the PCB in the glowplug module is there just to energise the relay coil and check for feed, open circuits and plugs with incorrect resistances and fire a CAN message if it finds any of those... there doesn't seem to be any high-current electronics there at all other than the actual relay itself. Unless perhaps theres further electronics somewhere on the live feed in to the relay...

Interesting point about resistances though... my old ones showed between 0.8 and 0.9 Ohms, the new ones are 1.0 Ohm... not a massive difference which is probably why it didn't fire the Preheat fault. Fingers crossed this is the cold start problem with my JTD though because I'm flat out of ideas now if its not - I've tried so many other things! We will see in the morning if she decides to fire up in a reasonable amount of time or not!