OK is this the 87 and later controller like your old 7.3 had if so, you have a bad glow plug or a disconnected one, only thing it could be. They should stay on 7-10 secs. even in hot weather. I have the same thing going on but my truck starts fine anyway it's so damned hot down here, and digging for that glow plug doesn't seem to appealing. If it's a 6.9(83-86 style) controller I really don't understand those symptoms as it should leave them on a long time as long as you have it hooked up right and there's at least one good glow plug. As Mel likes to say the one glow plug doesn't even know the one next to it is there...not so on the later ss systems. If you do have the older style, if you want to keep the factory system you should get with Mel and copy his failsafe for the factory unit. If factory look and functionality isn't important to you, you should use a momentary toggle, my preference in theory would be the 12V ones out of a "slow glow" 5.7 diesel GP system, because in theory 12V to a 12V plug should be sustainable for a long time. Micheal Fowler actually did this, I don't know how it worked for him. The downside is they heat up slower and they plugs are relatively expensive, but you'd probably only be doing it once and you could leave them on for 25 seconds...50 seconds..longer? If you felt the need to, whereas even with high quality beru or some of the widely available dual coil 6.9 spade type glow plugs that are 6V rated, you really don't want to consistently leave them on beyond 8 seconds...having said that that is PLENTY for a healthy IDI in all but the coldest conditions...assuming everything else is right.