I have a 7.3 Diesel F-350. When it gets below about 45 degrees it hard to start, it takes numerious trys the colder it gets. I have to plug in the block heater all the time now. Before i never had to plug it in unless it was really cold.
I think i need new plugs but the light goes off at about 6 seconds

 

You can pull them out an jump them on a battery. A glow plug that isn't open can still be bad. Hit them for a couple of seconds and see if they heat up evenly or at all. Later

 

Go to the 7.3 forum for discussion about cold starts. From what I have learned it is mostly from the GPR going bad. I replaced mine today and will see if my starts right up in the morning.

 

a 7.3 will start even without the use of glow plugs. sounds like your injectors are to weak to work when its cold.

 

Welcome to FTE Fordtlover.
What year is your truck?
For the moment I am going to assume you have a Powerstroke and recommend checking the glow plug relay on top of the engine. They have a reputation for quitting and then none of the glow plugs get hot, hence the hard start.

 

This is a 2000 F-350 and the motor is a 7.3 powerstroke, is their any way to check the glow plug relay? and thanks for the welcome.

 

Easiest way to do that is to get a test light and see if it lights when you turn the key on. Test for power at the large output post that goes to the glow plugs. One large post will be hot all the time. Of the two smaller posts, one will show power with the key on and the other is grounded through the PCM. That is how the relay is commanded to come on, by completing the path to ground for the relay through the PCM. So it will show no power first thing in the morning and then when the PCM commands the relay off, the test light will come on due to the power going through it from the opposite small post. If you have a big enough light bulb in your test light, it might carry enough current to energize the relay and you will hear (or feel) it click. But that will only happen when the PCM does not want the relay on.
So when it's cold out, first start up attempt of the day, you should see power to all three posts and no power to the small post in the LR position of the relay with the key on. Work quickly or have a helper turn the ignition on while you are over the engine. If the relay is dead, you might hear it click on, but with burned contacts inside and it won't pass any current to the other large post.
Hope this helps.