2001 Ford 550
 

Wont start unless plugged in.

thats not much info to go on man, how cold is it, any mods? need more info to dig into problems. :c:

Probably a glowplug issue. You are waiting for the wait to start light to go out correct?

I've replaced the glow plugs, glow plug relay & the valve cover gaskets w/harnesses attached.
It can be pretty warm out and still doesn't want to start unless it has been plugged in. No mods that i know of. We bought it used w/almost 300,000 mi.

Well are the glow plugs functioning?

I would start by checking hpop pressure and cylinder compression

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Also, leaky fuel lines can make it hard to start

Glow plugs are functioning, & also waited on the "wait to start" light to go out.

Try what autostroker said and report back.