99 E-350 HELP NO START

[littlebigrig6.0]

My dad has the 99 7.3, A few months ago he started having problems with starting it, he has to start it on ether, don't ask why he waited so long to fix it, So any how, this started when it was still warm out side. Even when the truck was hot he would have to give it a few shots of ether, once the truck is running its fine. We just replaced the glow plug relay, thinking that would fix it but nope. Any ideas would be great, also the truck has approx 400K on it, got to love the old 7.3's

[JSchuricht]

Test the glow plugs and check ICP while cranking.

[littlebigrig6.0]

Thanks for the reply, what would be the best way to test these. Thanks