Hard start Plugged in ok

[RUNS]

I am new to this site and have read through a bunch of threads on hard start problems. Here is my issue maybe some one has experienced the same.

I run a fleet of buses 18 of which are 7.3's in E-450's ranging from 1998-2004, mileage between 140,000 and 295,000. I have two or three that just don't want to start when cold unless they are plugged in, Glow plugs, relays and batteries have all been checked and are in proper working order. If they are not plugged in they will smoke and clack and when they finally start they feel like about 3 injectors are firing until they get warm, once warm smooth as silk. If they are plugged in they start fine and run perfect also. I just did all 8 injectors on a 311,000 mile van that was giving me the same problem. I started it this morning after sitting 4 days and it fired up like a champ. I know I just answered my own question but my real question is.

Is there a way to narrow down which injectors are giving me the problem on a cold start?

You can't do a contribution test until operating temperature achieved so that kinda knocks that out.

I am also wondering what is the cause. One thread said something about sludge in the injector oil passage. I thought that may be it has something to do with partially clogged injector tips and the spray, when cold, is not fine enough to fire correctly and when it is hot it is a non issue. I have always run additive in our fuel but it seems like the problem has compounded since the switch to Ultra Low Sulfur diesel.

I was wondering if anyone has run into this problem and if there is a way to narrow it down or correct it. Is there a super strong injector cleaner that anyone knows of that I can give a try. If not I guess I will be stuck with changing a whole boat load of injectors.

Thanks for any help or suggestions you may have.

[SCUFFY828]

if glow plugs and relay are working good and cranks o.k. pluged in ,the injectors are bad,if runs ok.. after warm ,just about would gaurantee it with the mileage on the vehicle ,injectors are clogged and worn bad , seen it before