Possible FI problem

Let me pick your brains. Got a fuel injected 4 cylinder, about 2001 model.. (obviously not an A body). At normal temp, runs fantastic. Starts easy enough, but until it gets warm, you can hardly do anything without it wanting to die. Computer shows no codes as a clue to what's amiss. The engine is in Caterpillar forklift. Any basic stuff i could check out? Our Caterpillar guy can't seem to make it right. They've put in a new fuel pump.. new spark plugs (cuz the last genius 'knew' that was the problem. Thay looked great to me).. ran a couple cans of Seafoam though it with the last 2 tanks of gas, no noticeable difference. Is there a sensor that may impact this? I'm afraid they're gonna want to throw injectors in it and I'm not 100% convinced that's the problem. Any ideas would be appreciated..