So I understand that leaky return lines can cause hard starting bc of air intrusion and the need to reprime the lift pump every time you start, but how would that effect the fuel system after its running ? Is it possible to have air coming into the system on the return side ? My 93 F250 is stalling often at a low idle. About to install the return line kit.

 

One place air can get to the ip, is from the suction side of the system before the lift pump, between pump and tank, cracked line's may not leak, or bad fuel pickup in the tank, could suck air and carry it to the filter head, then on to the ip, however in the factory filter head there is a small orifice in the return line system, and a check valve (in later model's) that when the lift pump push's fuel to the filter head the orifice is in the top of the filter head, so air bubbles are directed to this return line and carried back to the tank, so symptoms of air intrusion when the engine is running would be a gross leak of some type, possibly a failed lift pump, cracked fuel line, or fuel tank selector valve, plugged fuel filter, or fuel pickup in tank, however if most of those this were the case, it most likely would not start at all, if it starts and about 50 second latter it dies, than you have a leak, at filter head heater, O rings or possibly return line caps. if it just stumbles a little, and run's better warmed up it might just be a worn or damaged injector, take some time to inspect the fuel system, b4 spending to much $$$

 

Here's my return line experience. There were leaks all over the return line system but I didn't have any trouble with starting or stalling or running at all for that matter. The only problem was the loss of fuel from a financial point and the constant smell of diesel and the drips under the truck. Soon after I put in a new return line kit, my truck would stall when I would let off the throttle at low speeds, like pulling up to a stop sign. It would always start right up when this happened and would run fine until the next stop. It was suggested to me to run a heavy dose of Diesel Kleen through the system. Within a few miles of driving, the problem went away and has never came back. I use it quite often, no less than every other fill up. The diesel running through the pumps these days is so low on sulfer, it doesn't have the lubricating properties as the fuel did when these engines were new on the market. Not sure if this will help your problem but it's an affordable and easy fix to try first. Good luck.

 

I never had any run issues when my return line system was leaking either. But, Like Crew-It, I got kinda tired of the diesel stink. so, I replaced all the lines and the O-rings while I was at it. Ran the same before as it did after.
Now, I am having a surging issue at the moment, but I am pretty sure my IP is going out. I have an appt with the Diesel shop for next week to find out.