Faster when fuller? What is going on?
 

This may sound crazy, but its true 90% of the time. My X5,3.0 has 240K miles, most everything is original. I am noticing that when I have a full tank of premium only gas I get amazing quick peddle/acceleration response. Even the slightest touch and the X takes off smoothly. No issues. However, as I get closer to half tank it starts to become sluggish. At 1/4 tank or less its a very noticeable sluggish acceleration. It's so strange. Is this an indication that the fuel pump system is going? I don't have any SES light on either. Although, the other day, the tranny gear light came on, but it went off after starting it back up. What's going on with this amazing X5? :dunno:

Sounds like fuel pump going is definitely likely. Check your fuel pressure at the rail while hitting the throttle. It should not change from about 50psi

Mine has been doing that since I bought it 2 1/2 years ago. It wasn't that pronounced and didn't do it all the time but there was a "better when the gas tank was full" for sure. I just changed the fuel pump and the throttle response is now better than it ever has been even with low tank. I think you are going to have a hard time diagnosing it with a pressure guage though. I think the problem is that the worn pump doesn't pump enough volume to come up against the pressure relief valve for the syphon jets which results in a slightly lower rail pressure since the syphon jets are fed from the low side of the pressure regulator. It would be nice to find a good diagram of the fuel system to verify this though.

You are half right half wrong.

It's exactly that the pump can't supply pressure @ volume when you will lose power. The injectors base the formula for how long to stay on based on an exact pressure supply. If the pressure is low they spit out less gas meaning less power and less efficient as it will also be lean until the O2 tries to compensate. Zero percent chance that weak pump induced lack of power won't be visible on rail pressure gauge since THE cause of this symptom is low fuel rail pressure.

(There can be other mimic situations but if you have a weak pump and have weak power under load it will be measurable from fuel rail pressure).

Eg if you measure fuel rail pressure you will see it drop when you hit the throttle.

The siphon jet is after the return from the FPR (fuel pressure regulator) and that will also have problems when the pump gets weak but by the time that happens you will likely have had weak acceleration for a while.

Once the pump is worn enough that siphon jet doesn't work properly the engine will start getting fuel starved with gas left in the tank up to 1/4 tank or more.

No, there are no issues under load or highway speeds because the pump has no problem supplying enough pressure to open the pressure regulator and operate the syphon jets. If it couldn't then there would be no syphon jet operation. It's that the pressure ends up being lower on an empty tank because there is not enough volume bypassing the pressure regulator to open the syphon jet relief valve which normally makes the back-pressure to the fuel pressure regulator constant. A full tank increases the back pressure on the syphon jets (back side of the rail pressure regulator) and increases the rail pressure. Would be easier to understand if we had a nice diagram.

I know what you're saying I'm saying you aren't correct. FPR is affected negligibly from back pressure from siphon jet when working properly.

The siphon jets take about 15psi to operate properly and they get the "leftovers" from the fuel pressure regulator. It's not like a+b=c math eg 65psi from the pump to the FPR then 50 to the engine and 15 back to the tank but it's something like that.

The FPR is defective if the backpressure from the jet pumps is affecting the set output pressure.

The output pressure is nearly instantly adjusted you should not see more than 1-2psi difference at the rail and almost always if you see a dip in pressure it is the fault of the pump which must supply the excess pressure to supply the set 50psi forward and 15-19 psi return to the tank to operate the jet pumps.

When the pump can't supply enough PSI to supply enough volume at 50psi going to the engine it is remotely possible the back pressure from the jets could help supply a fraction of a PSI but the difference in back pressure from full to empty is only 0.5 psi where the jet pumps operate at 15-19 so only about 2-3% difference full to empty. You won't "feel that" you will feel when the electric pump drops to say 60psi and gets only 45psi to the rail because that is a TEN PERCENT drop in fuel delivery at WOT which means 225hp is suddenly 180hp and pushing 5000# you will definitely feel that every time.

All that said, the 0.5 psi difference at the foot of the electric pump could affect the suction side of the pump enough to change the output pressure.

Think of this way: from 1/3 of a tank and below there is probably 0.5 psi at the input of the pump and with a full tank there is maybe 1.2psi that is over double the pressure at the suction of the pump and is very likey the exact reason explaining the faster when Fuller rather than the 2% difference in the return back pressure.

If you have the old pump it would be very easy to test the theory: run the pump in a five gallon pail of water and spray the output against a wall. If I'm correct the water will spray lower and lower on the wall as the water gets lower in the pail.

You are making my point for me. The difference is only small which is why it is effected by the tank level and will be hard to see on a fuel pressure test especially if you don't take engine vaccuum into account. Also, why I only notice a lag when first stepping on the gas pedal normally and not on the highway or under load. And why I have driven over 60,000 kms without it getting worse. The syphon jets operate on volume not pressure. The window between the syphon jet relief not opening up (maybe that is 15 psi ?) and the syphon jets not working is fairly big for the fuel pump and only has a small effect on fuel pressure. Either way the answer is a new pump, but I just find it interesting to theorize why the symptoms happened the way they did.

BMW fpr not the type affected by vacuum. The return line isn't going to affect pressure out of the FPR so I'm the opposite of making your point I'm saying exactly the opposite. The intake into the fuel pump may change enough to affect fuel pressure also cold fuel pump may give you more pressure but the return line to the jet pumps is not going to change the output pressure it's exactly not what FPR does.

If you aren't getting stable pressure out there is 95% chance that the fuel pump is shot, 5.9% chance the FPR is the problem and 0.1% chance it's something else like backpressure from the jet pumps (which as I mentioned earlier, if that is changing the output pressure the FPR is defective).

Consider that if the pump is sending 65psi and 50 goes to the engine and 15 returns to the tank: the 0.5psi difference full to empty at the end of the jet pump is not going to make any difference.. that 0.5 is not even supplying backpressure, the Venturi nozzle is supplying the backpressure, I doubt there is any significant change at all to the back pressure since the venturi creates a vacuum it will only change how much fuel will get sucked into the surge tank but the physics don't align with affecting back pressure since the return line doesn't feed into the tank but into the venturi nozzle.

Haha, you beat on that last post, I'm a really slow typer! :) Again, if the problem was that the pump could not supply enough pressure to open the regulator then the syphon jets would not be working especially with the fuel tank getting low and the engine under load. :stickpoke Is there not a vacuum line to the FPR on yours ??