Glow Plugs or what?!

[timpug]

Good morning guys,

I am pretty sure this has been asked 100 times but I dont understand fully.

Ok so this is the situation:
- Changed the glow plug relay
- Changed the glow plugs themselves

This is what I tested so far:
- All glow plugs have the same resistance or so (when crossing the negative and positive terminal of the glow plug
- Voltage when the key is turned escalated from 11v to 12.*v.
- After 12 secs or whatever, the voltage on the glow plugs is zero (when the relay kicks in)

Issue experienced:
When trying to start the car initially, you cant just turn the key once and turn the engine over. I have to turn the key once, leave it for 10 secs, turn the key off, and turn the key on again and let it heat up for a further 6-10 secs and then when I turn the key over it works perfectly everytime!

What I have read:
1. It could be air in the pump/fuel system
2. Issue with plugs themselves
3. Issue with the plug relay

I dont see how it can be #1, if its this, no matter how many times I heat the glow plugs it should remain constant.
No 2. and 3. I checked with the voltmeter.

Please can someone give me an explanation. Is my battery not outputting enough amps or something? What should the amps be when the key is turned on?

Thank you experts