Fuel pump problem

[lou1990gmc]

I have 1990 GMC Suburban 2500 4 wheel drive with a 350 and 140k miles. I am having problems starting the truck. When I finally get it started, it runs for about 10-15 seconds then just shuts off. The truck won't idle unless I have my foot on the accelerator. So far I have replaced the fuel pump 3 times. The second time I saw that the ground wire was frayed, so it was replaced. The fuel filter is new. I'v checked and do have fuel coming out at the throttle body (I took the fuel line off). The injectors are spraying an even flow. I have replaced the map sensor, coolant temp sensor, O2 sensor, the fuel pressure regulator was rebuilt, new cap, rotor, coil, and plug wires. I can remember that the check engine light comes on. One of the codes was that its running lean. I think at this point that the problem is under the hood and not the fuel pump, (especially after three pumps the last of which had 36 gallons of gas in the tank. I couldn't get the siphon to work.) I am thinking that this may be a relay or electrical problem because it shuts off after the same amount of time, and during those 15 seconds it runs like there is a slight miss. Is it possible that some sort of vaccum leak is preventing the truck from idleing? Is there a way to check the idle air control valve, (exactly what is its job?), and would It prevent the truck from idleing at all? I am pretty sure that this is an easy fix, i just can't figure it out. I tried to take the truck to the dealer, but to no avail their zillion dollar diagnostic machine couldn't find any problem. Apparently the same machine is being used to look for Osama, Saadam, and WMDs. Please help..