Faster when fuller? What is going on?

[andrewwynn]

Originally Posted by 80stech

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.

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.

[andrewwynn]

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.

[andrewwynn]

https://xoutpost.com/bmw-sav-forums/...stic-help.html

[80stech]

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.

[andrewwynn]

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.

[andrewwynn]

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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[80stech]

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?

[80stech]

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.

[andrewwynn]

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.

[andrewwynn]

Originally Posted by 80stech

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.

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