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 ??

Except: you are missing a key factor; you are absolutely correct that when you lose pressure to the engine, example of the pump is pushing 55psi, 45 goes to the engine and only 10psi goes to the siphon jets, the return line to the Jets will not siphon but they will instead go directly back to the surge tank. This means that the surge tank will lose the amount of gas consumed by the engine during that time.

That only happens at wide open throttle. Once you drop off throttle, the pressure will rise both on the output of FPR and the return line. Then the siphon will start to flood the surge tank again.

That is: until the pump is weak enough that it relies on the depth of fuel to help the pressure on the inlet of the siphon jets. When this happens you will run out of gas with fuel on the gauge.

If the pump can't supply enough pressure to even get to 50 psi you won't have a functioning car since as soon as you hit the gas hard the rail pressure will drop like a stone and you will have terrible power performance.

What usually happens is the pump gets weak enough that at normal throttle the engine performs fine but the sucking jet pump stops sucking (which ironically sucks) at this point, the siphon jet stops working and you will leave 20-30L of gas in the tank when the engine is staved of fuel.

After this happened to me twice I did the most in-depth research of exactly how the X5 fuel system works and published it to the net via xoutpost.

What I found in 20 hours of research was that 100% of the posts online describing the X5 fuel system were wrong most were very wrong and some were effectively opposite of correct.

My fuel system description on xoutpost is the most complete most accurate description available online including accurate specs from BMW explaining how the jet pump works and how much psi is required to operate them (15-19psi).

I've discovered and described a fatal engineering flaw in the siphon jet that so far 100% of x5s that I've worked on had and I show the free fix DIY that the dealer will charge over $500.

I appreciate your fuel system thread, but that isn't the syphon jet where you found the flaw with the o-ring getting pushed out. I just had mine apart to check and I was going to ask you what is inside that section with the pushed out o-ring ? I think it is a flow divider for the syphon jets and possibly some kind of valve but I didn't take mine apart because it was a nice tight fit and didn't want to wreck it.

Don't take that apart if I don't need to. If the o-ring hasn't spilled out yet then I personally would drill a 7/64 hole through and put in a stainless 3mm screw (flat head so the cone of the head will make a tight seal). Unless they changed the design it's inevitable to fail, the part that holds it together is opposite the side that pushes apart it's quite literally designed to fail.

Yes that is the connection to the siphon jets. There are two hoses going down to the two siphon jets, and from one side two hoses come back to the surge tank : the extra is the pressure relief valve to protect the siphon jets in case of over pressure.


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That's what I was going to say, mine is a 2004 and obviously improved from yours. I measured everything to do with the O-ring portion and it's all good. I measured about 0.2mm clearance(although there does seem to be a taper) and the o-ring is 2mm thick. I'm curious though about what's in that section? is there a valve or is it just a flow divider for the 2 syphon jets?

Haha, you beat me again! I think the syphon jet relief is at the fuel pump side along with the 2 syphon jet feeds. Feeds, might be wrong term, outlets would be better.

Faster when fuller? What is going on?
 

There is a check valve inside the o-ring connection or the extra unlisted and no seal connection but it's not to hold pressure (no seal) it's just to keep some fuel in the siphon jet lines so they don't have to prime each time the car is used.

There are two connections at the top of the siphon jet intake (where the return line from the FPR is attached). I forget which is above and which is below but I think the check valve is inside the one designed to come apart (with o-ring and design defect)

The defect in the o-ring design is that it's only held together from one side and not from both sides. With 15psi and over an inch of area means 20 pounds of force approximately an inch offset or 20 inch pounds of torque for thousands of hours. It will twist enough eventually that the o-ring will squeeze out of its groove and has will spray out like a garden hose.

I would premptive strike on ANY BMW with this design defect at about 100,000 miles to beat it to self destruct.

It only takes a couple zip ties or a single screw to permanently fix the problem rather than wait until it strands you with 75 miles distance to empty (TWICE in one week for me thanks to the hundreds of erroneous web pages completely void of accurate info on how the BMW X5 fuel system works)

First time was at 6°F on my way to cat sit for a client and the second time was 25° at 11pm with my 74 yr old dad with me. Very uncool (figuratively) both times. Was enough to piss me off enough to actually solve exactly the why to educate myself to avoid repitittion and actually solve the actual problem at hand.

The pressure relief for siphon jets is the "extra" hose going from the port to starboard tank. I believe the pressure relief valve is actually right at the end of the third hose that comes from the port side front jet foot.

In spite of the flaws it's the best fuel delivery system you are ever going to find. It will allow you to drive up or down a 35° grade with less than one gallon of gas these guys were off the charts brilliant to come up with this solution on the very first real SUV.

My previous truck was a Ford explorer of near identical vintage and it wouldn't start with over five gallons of gas in the tank when parked on an 5° driveway. That's moronic. I lost effectively 30% of my range when I went off road. The X5 can drive right down to zero DTE no matter what the terrain!

On flat ground you should be able to drive 15-20 miles past zero range because the gauge reads zero when there is approximately 1 gallon of fuel remaining

I can totally relate, but I think the newer ones and the replacements have been improved. I agree with the screw idea for extra safety but a person would have to be careful not to have the screw push the connection too far in the opposite direction. I know you don't agree, but I still think a back-up ring and maybe a zip tie or something to hold the thing straighter would be the cat's ass. It would be really nice to know for sure if there was an improvement in the newer/replacement ones.

Faster when fuller? What is going on?
 

There is no advantage to a second ring if adds no purpose.. If it held the thing straight it would do the trick but I simply put a zip tie around the other side to act as a lever to push the side with the o-ring tighter and it works.

But putting a screw on that side on the low pressure side of the o-ring it won't have to go through into the high pressure pipe just a "dent" that it can grab so no issue with creating a leak. It will hold it perfectly straight for a million literal miles

I haven't seen newer designs in person but from pictures online of current models for sale it appears the flaw was never fixed

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Another interesting thing, maybe I am in the twilight zone, but what the heck is with the stainless screw in the rounded foot of the syphon jet side fuel sender ? I agree the fuel system is pretty awesome, including the spring loaded reference fuel senders!

Your not understanding what I mean by backup ring. Maybe google it, commonly used in hydraulics. I see your theory on the screw but it would need to be on the opposite side of where the o-ring fails correct ? Or maybe a screw on each side to keep things centered, or maybe a back-up ring! ;)

I just read your comment on the backup ring it has some sound science behind it but it would only help no guarantee.

The screw I'm describing would hold the thing absolutely to prevent the o-ring from getting crooked at all. 100% cure so no need for the positioning ring.

https://uploads.tapatalk-cdn.com/201...a73e581faf.jpg

Red arrow points to where I would add a screw: the left side is held fine by the clips the right side is not held at all.

My quick fix was to add a spacer were the yellow arrow points to act as a lever and push the left side down to hold the right side up it seems to work well but longevity who knows?

The green oval marks the unlisted seal that also leaked on every BMW fuel system I've worked on so also a design flaw. Any leak there will reduce the effectiveness of the siphon jet and therefore reduce the hours the pump will last. This is also the joint where the check valve is located.

The blue oval marks the o-ring connection and that joint is easy to open and work on to repair.

I would need to see the picture of the screw at the bottom but I would guess it's holding the foot to the spring steel rod that holds the foot in place


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I have read this post and the "Fuel supply detail" post from andrewwynn several times. I have learned a lot, but am still confused about a couple things.

First and most importantly, what preemptive maintenance is appropriate for my son's e53 with roughly 150K miles? The in tank fuel pump will eventually wear out, so replacing the pump or the fuel sender unit seems reasonable, but what about on the port/siphon side? If I understand the posts correctly one can have a nice new pump/fuel sender but still run out of gas with roughly 1/4 tank - correct?

So how to prevent this? One option is to replace the siphon assembly, but that's expensive. The only "wear item" I see in the siphon assembly is the o-ring (part #4 of this realoem link).


http://www.realoem.com/bmw/images/diag_3adr.jpg

I have read about backup o-rings, replacement o-rings, drilling holes, stainless flat head screws 3 mm, clamps, zip ties, and teflon tape. I have looked at the picture on post #25 but I'm still confused.

Any more detailed explanation? Perhaps additional pictures from different perspectives? Sorry if I'm being stupid, and thanks for all the work on this issue - it is much appreciated.

In addition, I have also read the "fuel delivery problems" posting that is here. However, I am still unsure of how to prevent problems with the siphon system. I see the folded up o-ring, but it is implied that merely replacing the o-ring is an inadequate fix. There is mention of teflon tape, but I don't see where it is applied. I see the zip tie, but don't understand what its purpose is - to hold something tight, or just to prevent the male/female plastic pieces from overlapping too far and causing a twisting of a different nearby connection? No mention of a screw in this posting. And, by the way, what kind of a screw are you talking about? Machine screw with a nut or a wood screw? 3mm thread, correct? And, again, where would this screw go?

Sorry if the solution is staring me in the face and I can't see it.

It is confusing. The best thing you can do is pull it apart and have a look and maybe take some measurements. It becomes a lot easier to understand when you have it apart and it is an easy job, just be a bit carefull with the plastic especially the hose nipple and have a replacement hose clamp ready. If the o-ring is not pushed out it will be fine to re-use and like andrew said, I wouldn't mess with the clip together portion that doesn't have a seal if it looks good. Again will become obvious when you have it in front of you.

I should say that both clips that hold the stiff wire portions on need to released at the same time.

Found another posting by andrewwynn that addresses the siphon system. The link is here. I'll post on that link since I am taking this thread somewhat off topic.

After some more research I need to verify my findings but I will be recommending re-working the non-seal seam as well as that seam leaked like a sieve before redone in both mine and wife's car.

I used an o-ring on wife's and Teflon tape on mine. I will be examining my Teflon seal after about five months and report back the findings. If as I expect the INERT Teflon is just as I put it in I will update my repair posts to reflect that (of course they will be revised to the FIXED method should the Teflon be an epic fail)

do you have the "fuel saver mode" option selected in your cluster hidden menu?
just kidding.