Truck Repair: ford diesiel trouble starting, diesiel, test light

Question
i have a 94 f250 with a 7.3 liter diesiel. we bought it used and it ran and started great for a few months. then it became harder and harder to start. it would take 20 minutes or so. we replaced the fuel filter and glowplugs. it did not help at all now it will not start at all when it is cold unless we spray starting fluid into the air intake. after the first start in the morning it runs fine and starts great until it sits overnight or a long period of time.do you have ny suggestion to what theproblem may be? maybe something with the glowplugs or fuel system. I have know idea please help.

Answer
Check the glow plug relay.
It is probably right on top of the engine.
Even if you feel and hear it click, check both large studs with a test light.
One should be hot all the time, and the other should be hot after the key is turned to run.
I have had them where you could hear and feel them work, but they didn't make contact inside.

Van