I have an 81 320i with the stock k-jetronic. My in-tank pump is toast and I am gathering parts for an e30 L-jetronic conversion but would like a working in-tank pump in the meantime. I have seen kdanielson mention that an rsx pump will fit in place of the factory in-tank unit. This would be ideal because it would give me 2 working pumps in the meantime, and after the conversion I can just rely on the rsx in-tank.

So my question is- will an efi pump feeding the stock cis main pump overload the return system and make the car run like crap?

I don't think anyone's tried that. It should be OK, the stock external fuel pump should only let the normal amount of fuel by-the EFI in tank pump MAY run hot because it's not moving enough fuel to keep it cool-don't really know, just speculation.

ken