We spoke several months ago on a thread I started when I was trying to work out the smoking issue on my 7.3 and I found that injectors 7 & 8 were bad. (By the way, I believe the reason 8 is different - beyond Ford's need to reduce noice - is that it actually fires after cylinder 6 (not 7) - the next door neighbor to the front on the same head - and that can "starve" fuel from the fuel line to cylinder 8 when it tries to fire.)
Anyway, having replaced injectors on 7 & 8, I have still noticed some smoke when I first start on a cold day and it goes away in 200 or so feet if I slowly drive away. If I let it sit and idle, the smoke simply gets too embarassing to put up with for very long. In any case, your 70 degree-F temp idea sounds plausible.
Because I am still not happy with the amount of smoke I'm still getting, I am going to pursue three more things.
First, I need to change my fuel filter. I bought the truck a few months ago and this has been on my "to-do" list. There is a chance that the guy that owned it before me may have used farm gas and one of my "experts" said that he has seen the dye from farm gas plate out on the filter and reduce its efficiency. In other words that could cause more smoke than usual.
Second, another guy told me that the water dump valve on the fuel filter could be leaking by the o-rings and allowing the system to actually suck in air during certain times of acceleration/deceleration. I've had another guy tell me that I shouldn't even use the valve because the more they are used the more they leak. I could be sucking in air from the suction side of the fuel pump too. Because I don't know what was done before I bought my truck, I will probably replace the water drain valve when I change the filter. I'll leave the pump possibilities for another time. See number three below.
Finally, three, I have looked into the whole air issue in the fuel system and believe I am going to install the fuel cross-over from Riff-Raff. It's a cheaper way to go than a full blown system and I can reverse the whole process (if needed) without having spent a fortune. The cross-over is designed to stop the "dead head" issues that threaten the 7.3's, it gives another path for the air to go so it can be dumped back into the tank, and it gives me the increased ability to "play" with the fuel pressure at the injectors to see if that changes anything. I have read that air can be a reason for the excessive smoke. Ford even states in their literature that air in the fuel lines can be a cause for excessive white smoke -- particularly when the engine is first started.
I think these things will always smoke a little bit when you first start them up and it is cold. I think the smoke is an inherent part of the design and that is simply what they do. I believe, in part, that is why I smell raw diesel and it all goes away after she warms up and begins firing more completely across the entire cylinder on the compression stroke.
My thoughts above are to simply try some things that 1) need to be done as part of routine maintenance, 2) aren't really things that could cause me any issues by radically changing the design, and 3) all can be reversed if I find a performance fall off or issue (neither of which is anticipated).
I've been too busy lately to chase any of the above changes and, to be honest with you, I can't get too motivated when the temperature outside is in the single digits. I will, however, work the changes soon and let you know if any of these things seem to help.
Hang in there...Sagemonkey