I just wanted to say that the partshop fastlane at this website lists a Bosch cold start valve for a 1992 300TE for $89.00, so you must have one.
I won't offer any advice, but if you have this valve, did the shop ever test it for resistance, spray pattern, or whether the temp. sensor was activating the valve?
I ask this because on my '88 Land Cruiser I had the exact same problem, and it turned out that the wiring connection to the cold start injector was corroded, or loose.
I'm sure someone else here will be able to give some advice.