Low fuel pressure (EFI) - bad pump, bad plumbing or ?

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Joined
Aug 8, 2018
Messages
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Location
California
My Car
72 Fastback - 351C-4V, Fitech EFI, T56 Magnum 6 Speed
So a little background. Earlier this summer one of my high pressure lines started leaking so I parked the car to replace the high pressure hoses. This is the setup, I have Walbro pump mounted to a bracket in front of the tank, with a 100 micron filter in front of the pump and a 10 micron on the way to the EFI.

1694156060520.png

While this has worked well, I don't like that I can't practically get filters without getting the car pretty high up in the air since it's right in the middle. So while I was going to do this, I decided to move things around so I could get to the filters easier as plugged filters have left me stranded more than once. Not pictured, but I moved the 100 micron filter over to the side closer to the pickup so I can get to it from the back and I moved the 10 micron filter up under the hood behind the throttle body so I can get to it up there. That all went fine and used a new filter on the high pressure side.

Started the car and it worked fine as before, parked it for a couple weeks while I was finishing up some brake work and finally took it for a ride this weekend got less than a mile from my house down a big hill and lost fuel pressure. Basically did what had happened before when I had a clogged fuel filter where it would idle but nothing totally deal and wouldn't go past 1000RPM.

I went ahead and installed a fuel pressure gauge that I had been planning on adding and I had assumed the 100 micron filter was clogged so replaced that with a new one thinking I'd be back on the road in 15 minutes. Basically getting 40 PSI according to the gauge and again the car starts and idles perfect but won't do anything past idle and fuel pressure drops to about 0 as soon as you touch the throttle. With the engine off if I cycle the key, the you can hear the pump prime and it made some weird whining sounds a few times.

So with all that, I'm wondering anyone has some thoughts on if there's something wrong with my plumbing by having the filter up under the hood that can cause this, or some chance that I have air trapped in the pump and it needs to be primed or something, or has my pump failed which would be a weird coincidence to happen at the same time as I ended up replacing everything else.

Appreciate any feedback on how I probably screwed this up.
 
What is your fuel level? I had issues with an external pump below half a tank.
It's a little over half a tank. I've also had issues when I got really low on fuel where it would suck some air. In those cases I'd get some surging that would usually go away once I got to level ground. I will say that I was able to drive this thing out of the neighborhood where I have to go down a big hill and this happened as soon as I got to the bottom. In the past where I had low fuel issues it would just do that momentarily and settle down after a minute on flat ground. Now I can't even touch the throttle and it won't come off idle at all.

I think might have to try just dumping more gas in it to make sure it's not something like that.

You mentioned the pump making an unusual whine, is that while at 40psi or when it drops to zero psi? Also where is your pressure gauge plumbed in?
That was when turning the key on but not starting, so when it's priming the system. I just turned if off and on a bunch of times to watch the pressure and listen to it and this happened while doing that and the pressure showing 40. I should say since I just installed the gauge I have no idea if it's accurate, but I can certainly see the pressure dropping to zero when I touch the throttle.

The gauge is physically right behind the throttle body plumbed in between the last fuel filter and the throttle body.

Is the tank pickup maybe getting obstructed or clogged?
I don't think so. It's pretty new and I actually stuck an endoscope into the tank for good measure and while I can't see it great, the sock still looks nice and white from what I can see through the gas.
 
Yeah it feels a bit like that, like a kinked garden hose. That's why it seems a little like it's air locked. I've used water pumps (not for cars) that would get air in them and move a bit of water but never full speed till you blow out the bubbles. Not sure how that would have worked in this case as it certainly used up the gas in the lines in the mile or two that I drove before it died.

I guess the there could have been a bunch of crud in the new fuel line that clogged up the new 10 micron filter almost instantly as well. I might just go get a few gallons of gas to get well above half a tank for good measure, but it really does have a decent amount in there.
 
I would disconnect the fuel line at the underhood filter and check the quantity being delivered. If you have any air in the pump this might also flush it out. Also see if it's fuel of air bubbles.

Make sure all the connections before the pump are tight, some fuel pumps have a hard time pumping any air, plus it's a lot easier for a fuel pump to suck air than fuel.
 
Reading the above. Could be that it worked for two years, but was slowly degrading until you are finally seeing issues.

If it is the pump you might look at the in tank options that are available now.

I went with the aeromotive stealth.
Thanks. Could be and hope not. I actually have a spare I can try if needed. I should add that in those couple of years it didn't have a ton of miles but it hasn't had any issues other than a clogged filter or when it gets very low on gas.

I really like the internal pump setup, but I don't love that you can't replace the pump without dropping the tank (as far as I can tell). I know that's a catch 22 as they are usually going to be more reliable than external which is why every new car for decades is setup that way, so shouldn't need to replace it as often, but I just hate dealing with dropping the tank. Still I'll look at that as I am considering replacing the tank anyway.
 
Here are pix from my tank. 340 stealth.

Only need to buy the fill tube gasket. Rest is included.
 

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After finally doing some more testing it does look like the pump is bad. Still odd timing that it would randomly die after replacing everything else in the fuel system, but oh well.

I think I'm going to stick with an external pump for now as I don't want to spend the money on a new tank setup right now and I am still pretty hung up on not being able to service the pump without removing the tank. I get that's a bit of a catch 22 as it should be more reliable in there, but it's also pretty impossible to work on from the side of the road.

So sticking with an external, current pump is a Walbro GSL392BX good for 255l per hour and can just get another one of those. But thinking I might want to go for something like a Bosch 044 / or the newer 200 as it seems like they might be more reliable (assuming I can find a real one) over the long haul. Walbro seems popular with the hot rod crowd, but Bosch is used by OEMs everywhere, so seems like it might be a more reliable choice?

Appreciate any opinions on that
 
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Hmm, wondering about sticking a fuel pump on the end of the stock sending unit. Gets me an in tank pump and serviceability, albeit with having to empty the tank though.

https://www.holley.com/blog/post/ho...el_sending_unit_into_an_efi_fuel_pump_module/
Not sure I have enough room on my sending unit as I have the fuel return on there as well so it's pretty tight already for the additional wires, but an interesting idea I think.
 
Hmm, wondering about sticking a fuel pump on the end of the stock sending unit. Gets me an in tank pump and serviceability, albeit with having to empty the tank though.

https://www.holley.com/blog/post/ho...el_sending_unit_into_an_efi_fuel_pump_module/
Not sure I have enough room on my sending unit as I have the fuel return on there as well so it's pretty tight already for the additional wires, but an interesting idea I think.
Depending on how much originality you want to keep you could make a hole in the trunk to access the pump.
 
Depending on how much originality you want to keep you could make a hole in the trunk to access the pump.
Thanks. That's a very good point. The cars not very original but all the sheet metal is and I know that's more and more rare. So I'm not too keen on putting big holes in it if I don't need to, but maybe I need to get over that and just do what I want and not worry about the next guy.

If you’re considering an intank DIY pump, don’t forget the excellent article on our Tutorial pages on how to do that. I did it on my 73 and it works great. Here is the link:

https://7173mustangs.com/threads/in-tank-fuel-pump.40147/
Thanks much for the link, hadn't seen that article. Great write up. I'm thinking about running the pump from the sending unit rather than from up top. The link in that article mentions spectrapremium.com. They have this that'll work on earlier cars. Wonder if it can be made to work on a 71-73?

https://ecat.spectrapremium.com/en/parts/SP169B1H

Having said that, I don't see it actually available anywhere and the one reference to it is almost the same price a complete tank from Stealth like @rcadd1ct mentioned above.
 
Like everything the price has gone up since I bought it

This is the one I have

https://www.summitracing.com/parts/aei-18469/make/ford/model/mustang/year/1972
You will need the fuel fittings at the tank and the filler donut if I didn’t say it before.

I used neoprene weather strip on the straps and the top of the tank to insulate it from the body. It definitely helped quiet the pump.
 

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