Low MPG after fuel pumps installed?

If the pumps are not putting out the correct pressure, it will cause too much fuel to be delivered. Can I ask if you used a Name brand pump? If not, this can be the issue. Also, I would remove the rear seat again to double check and make sure the O rings or not leaking. The fact that when you fill the tank and it leaks is not good, can you see where the leak is coming from, this can be the problem as well in hot weather fuel evaporates quickly. I have included images of where it can be leaking from. Check out the images (below). Please upload pictures or videos in your response.

Hi Ken, thanks for your answer!

I used aftermarket fuel pumps (the 2008 BMW 328i has two pumps) both were Not OEM...can the mechanics do a fuel pressure test? and someone said it should be around 70 psi, is that correct??? I think I can try to pull that up on my Creator 310 scanner. at any rate, I hope these pumps are okay. the mechanic spent a lot of time putting them in.

In regard to gas leaking. I think I overfilled it and it was really hot out here in Florida like 100 degrees. So
me say the gas miles should change if i drive it more and keep refilling the tanks after a couple months. I don't see any leaks under the rear seat. I can't turn my head due to vertigo, but when i filled it way up to the tippy top, it was gushing out from the tank top i believe.. i haven't had it leak but I'm afraid to fill it way up to the top...

Also ...I do not see ANY leaks at all from under the seat or where you showed me in the pics.
anyway, I can correct this poor MPG?

Also, next time i hope to enclose a video. BMW recall said i most likely had a leaky oil pan, ugh. $$$$/ hope you can help more. thank you

Yes, they can test the pressure which should be

5.0 bar x 14.5038 psi/bar = 72.519 psi

Here is how the job is done, please go over this guide and check out the images below.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Check out the images (below). Let us know what you find.

HI KEN~ I did not do a fuel pressure test yet as it looks complicated but I'm still doing only 13 MPG instead of 19 mpg...Im not sure as there are TWO fuel pumps on my 2008 BMW 328i sedan..one behind drivers side and one behind passenger side (both pumps beneath rear seat.) .
.SO WHICH FUEL PUMP creates the 75 lbs PRESSURE I need? I only know that one pump is a sender... someone has an OEM for sale for half price..but waiting...now i have to replace my PURGE FUEL VALVE OR FUEL LEAK VALVE ATTACHED TO CHARCOAL CANNISTER AS IT WAS THROWING CODES..my gas tank cap is brand new..but dash keeps showing pics of a gas cap detached from car, then that display goes off,,then sets the purge or fuel leak valve code..please help

This is a clue, if you have a large evap leak it can cause the engine to get less milage. Also, a weak fuel pump can do the same, I like the OEM pump best. The pressure pump is on the right side of the tank. Here is the location of the valve which you should not be able to blow through once off the car. Check out the images (below). Please let us know what happens.

the code says I have a small leak ...one shop said it was the fuel leak detector valve located on REAR of car attached to charcoal cannister..other shop said it was the fuel PURGE valve near throttle body..hard to get at,,
WHEN you say the pump that delivers PRESSURE..that ONE is BEHIND PASSENGER SEAT=RIGHT SIDE??? what does the other pump do behind driver seat..will that pump affect low MPG?? both pumps brand new that were replaced..both aftermarket... :)

There is only one fuel pump, and it is on the right side of the fuel tank under the rear seat. I would use an OEM pump.

okay now i know i have the wrong pressure pump in i think. do u have to rest anything after putting in both sender and new pump? bmw calls bot of them fuel pumps but i agree. the main pump is behind passenger seat (right side?)

i hope u can help me on this: my display shows a gas cap off the car.a picture of it.a few minutes after the gas cap pic shows, the car throws a code. one shop said it was for purge fuel valve. other shop said it was for leak pump near rear attached to charcoal canister. would this rear leak pump come on only if gas leaked?

Yes the pump should be on the right side of the tank, (Passenger side).

thanks the oem was continental

do u know why my gas cap light displays on dash then shuts of and throws a CODE (CHECK ENGINE LIGHT) i intsalled a brand new cap still doing same thing

Sorry, we only handle one problem per thread, please post your new question here, you must be logged in.

https://www.2carpros.com/questions/new

Cheers, Ken