Wont start unless plugged in

I have an 01 Excursion that has been great until lately. I live in Oregon and it doesn't get that cold in the winter. About 3 weeks ago, I noticed it having a hard time starting when the block heater was not plugged in. We had a 2 week cold period (low 30's at night) so I made sure to keep it plugged in when I was able to. (hard to do at stores) The cold weather has now passed and it is a bare to get the beast going when it has not been plugged in. (40's at night) If I leave for lunch (only sits for a couple hours) it starts up but runs really rough. It will die a couple times and it restarts okay just runs rough until the temp. gauge has reached the middle point on the gauge. It seems like a fuel issue but the dealership says they cant find anything.
When I do have it plugged in, it will start with out any problems but I still notice the roughness at stop lights.
I know there is something wrong but I am stumped and not a mechanic.
Has anyone had an experince or have any ideas???

I'm guessing your Excursion has the 7.3 Powerstroke diesel?

What kind of place looked at it? Was it a small town Ford dealership or private shop? The best thing to do if you need somebody to look at it is to bring it to either a large Ford dealership with actual diesel mechanics or bring it to a diesel shop that is reputable and has a mechanic willing to work on your vehicle.

I live in MN and have had the 99 diesel for five winters and the 00 X for it's third winter. With good charged batteries -10 unplugged the diesels start.

Here are things that can go wrong,
#1 batteries, bad batteries can make any diesel hard to start. My 99's dual batteries lasted until Feb. of 2005, long life I thought. The X's lasted until Jan of 2004. If you've got original batteries get them tested for cca.
#2 glow plug relay, about $70-100 at Ford, International does not carry the part.
#3 fuel filter heater element, had this go recently on the 210K mile pick up, only place to buy the parts was Ford $275!!!
#4 which could be the problem but rare in 99+ Powerstrokes but glow plugs. If the truck is not always started after the wts light turns off you can do premature damage to the glow plugs.

I had it checked at a large ford dealer. I live in the suburbs of Portland so we are not a small city. Thank you for your suggestions. I will try those.

I'm guessing your Excursion has the 7.3 Powerstroke diesel?

What kind of place looked at it? Was it a small town Ford dealership or private shop? The best thing to do if you need somebody to look at it is to bring it to either a large Ford dealership with actual diesel mechanics or bring it to a diesel shop that is reputable and has a mechanic willing to work on your vehicle.

I live in MN and have had the 99 diesel for five winters and the 00 X for it's third winter. With good charged batteries -10 unplugged the diesels start.

Here are things that can go wrong,
#1 batteries, bad batteries can make any diesel hard to start. My 99's dual batteries lasted until Feb. of 2005, long life I thought. The X's lasted until Jan of 2004. If you've got original batteries get them tested for cca.
#2 glow plug relay, about $70-100 at Ford, International does not carry the part.
#3 fuel filter heater element, had this go recently on the 210K mile pick up, only place to buy the parts was Ford $275!!!
#4 which could be the problem but rare in 99+ Powerstrokes but glow plugs. If the truck is not always started after the wts light turns off you can do premature damage to the glow plugs.

A couple things come to mind when I hear your symptoms. First is batteries, I notice a lot of very competent mechanics forget to test the batteries on these diesels before they begin a diagnosis. With glow plugs drawing sixty five amps and the IDM juicing the injectors up to 110 volts, there's a lot of amperage drawing on that battery before you throw the starter motor into the mix. Second thing that comes to mind is the glow plug controller relay. These are extremely common and would not set any DTCs at all, glow plugs are a possibility but as was pointed out earlier, kinda rare. Now that the obvious are pointed out, here's some obscure ones I've seen. I once had one that wouldn't start unless plugged in and it ended up being worn intensifier pistions in the injectors, causing them not to seal until they warmed up and expanded. That truck had over 500k on it, and was an exception to the rule but I wouldn't entirely discard an injector issue. Another one had a cam sensor that I guess contracted in the cold but if it wasn't plugged in it wouldn't start, and would die but it never threw any codes, the only way I found it was watching RPM on the scan tool and cranking it. Anyhow, good luck with it and let us know what you find.