1990 econoline starting runningprobs

[90plate]

Hi there can anyone help
i have just aquired a 1990 econoline and when you start it up it will run ok for about twenty-thirty seconds then start to run rough , seems as there might be a misfire but not sure. the van had sat for some time but apparently only started running bad when the previous owner had to splice the fuel line around the gearbox area . the van struggles to drive and you need to feather the throttle sometimes to keep it going .I have'nt thrown in a set of new plugs or any thing yet as i just don't have the money to be chasing the problem and was hoping someone might just say it's this or that to try first. it does seem to start ok as it is fuel injection but i'm just not sure where to start. Thanks for any help given

[EricsVanWorks]

Assuming the injectors are still good and the timing is correct and there are no bad plugs or wires you should try the following.
-Run the van long enough to keep the battery up then shut it down.
_Inspect the fuel line from the fuel rail all the way to the sending unit which contains the metal lines coming form the tank and holds the in tank fuel pump. There should be no leaks or dampness from fuel. When you turn the key on there should be a hum coming from the fuel tank pump and the main fuel pump located along the fuel line under the drivers seat. Test for voltage to both these pumps during start up.
-Test for pressure in the fuel line with a gauge at the fuel rail for consistent pressure. If you don't have a gauge remove the fuel line delivering fuel to the fuel rail with a fuel line remover or bent needlenose pliers working the spring back while pulling out the line. (do not pull to hard or bend the rail.) There should be a solid 10 inch stream of fuel with no air bubbles gaps or sputtering when you turn the key on and off. If it sputters there is there is an air leak in the line or sending unit.
If all this works use a length of hose and put it against every injector listening from the other end. There should be a happy clicking sound ever time the injector is activated.
Try replacing the fuel pump relay switch. It is one of two small relay boxes located behind the battery with a brown connector.