2001 f-250 I have had the glow plugs tested they where OK I replaced the relay and it still won't start in the cold about 30 degrees or less Any suggestions?

The link below may help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-8.html#post1415281