Fuel intake pump works when wired up but stopped once put back in tank? 1988 740 GLE

[humphrey677]

So recently I started to suspect my intake prepump was dead leading to my main pump being on its last leg. Main pump was extra loud and would make an intermittent whining cutting out kinda sound. I heard nothing from my tank so I assume prepump is dead. First I did my main pump, ended up getting a messed up replacement and put my original one back in for now as it still runs. The next day I put a different intake pump in. I made sure that everything was working with the tank unit just sitting in the trunk, both pumps were whirring when cranking as well as just jumping the relay. I put everything back, I realized I forgot to put a new fuel sock strainer on the end of the intake pump. The next couple days before I got around to that, I was getting a real hard time starting the car, pretty much as if I wasn't getting any fuel from the intake pump and had to hit the gas a bit until the main pump caught it and started up. Got a new fuel sock, took everything out, and what do I find but that the intake pump I put in seemed to be shorted within itself (showing full continuity from its + and - terminals). also It wasn't working at all now, not even when jumping it straight to the battery. I then tested my old pump straight to the battery and it totally worked?? So since that new pump obviously doesn't work any more I decided lets put the old one back in and see what happens. I wire it up and test it while the sender unit is still out just in the trunk, both pumps audibly whir. Put it back in and its just back to the hard starting and once i get it going, no noise other than the fuel itself moving from the tank (sucked from main pump), since then its gotten worse, even stalled on me a couple of times. This is strange because even if my intake pump was dead before all this, my car was starting fine everytime and just getting bad MPG and running a bit rough. Now it's worse and I still have a dead intake pump and possibly failing main pump too. What reason could it be that it works fine with it all installed but stops once it's actually submerged/in the tank? It's also weird that the first one I replaced was internally shorted when I took it back out? Something in the tank is damaging them? Or both pumps are just dud? The replacement one was used by the way. Anyone experienced anything like this?

 

[burco]

So recently I started to suspect my intake prepump was dead leading to my main pump being on its last leg. Main pump was extra loud and would make an intermittent whining cutting out kinda sound. I heard nothing from my tank so I assume prepump is dead. First I did my main pump, ended up getting a messed up replacement and put my original one back in for now as it still runs. The next day I put a different intake pump in. I made sure that everything was working with the tank unit just sitting in the trunk, both pumps were whirring when cranking as well as just jumping the relay. I put everything back, I realized I forgot to put a new fuel sock strainer on the end of the intake pump. The next couple days before I got around to that, I was getting a real hard time starting the car, pretty much as if I wasn't getting any fuel from the intake pump and had to hit the gas a bit until the main pump caught it and started up. Got a new fuel sock, took everything out, and what do I find but that the intake pump I put in seemed to be shorted within itself (showing full continuity from its + and - terminals). also It wasn't working at all now, not even when jumping it straight to the battery. I then tested my old pump straight to the battery and it totally worked?? So since that new pump obviously doesn't work any more I decided lets put the old one back in and see what happens. I wire it up and test it while the sender unit is still out just in the trunk, both pumps audibly whir. Put it back in and its just back to the hard starting and once i get it going, no noise other than the fuel itself moving from the tank (sucked from main pump), since then its gotten worse, even stalled on me a couple of times. This is strange because even if my intake pump was dead before all this, my car was starting fine everytime and just getting bad MPG and running a bit rough. Now it's worse and I still have a dead intake pump and possibly failing main pump too. What reason could it be that it works fine with it all installed but stops once it's actually submerged/in the tank? It's also weird that the first one I replaced was internally shorted when I took it back out? Something in the tank is damaging them? Or both pumps are just dud? The replacement one was used by the way. Anyone experienced anything like this?

That is a tough one. Usually, when I am in that confusung situation there is something I have missed. ( Like forgetting the filter sock ) You could always set up a pump in a bucket of water and let it run for an hour or so. Sometimes things work for a while, then quit. Quality of replacement parts is at an all time low. No telling the quality of your new pump.