Fuel pump on the way out?

[scourtaud23]

Thanks for the link, My fuel filter and regulator were changed a few months ago. It made a difference but not quite enough.

I have to check how the fuel pressure evolves when I turn it off and after a while. I'll try and do this soon but I've had my time taken up by building a new massive 3mx3.5m steel front gate for my GF's family home.

[lincolnshibuya]

you can buy a $10 check valve and put it outside of your fuel tank, that's what I did with my e39 when it always starts at the 2nd try. Pump was replaced 4yrs ago but the check valve is already kaput..




https://www.facebook.com/diybmwx5/

[scourtaud23]

I've read about putting a check valve inline but I've also read they don't last very long so I wonder if its not worth it to buy a pump for 120€ and change it when it leaks back.

Anyone has long term experience with those check valves? Are we sure the pump isn't sometimes supposed to leak back if there is too much pressure?

[andrewwynn]

I would rather just wait 3 seconds to start rather than add a low-fi check valve But I don't see any real problem other than adding two extra joints.


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[scourtaud23]

The mounting is not the issue as I see it, its more of a "how its designed to work". All this mess is controled by the computer and the pressure regulator is supposed to regulate the pressure under the theory that the pressure is good and constant between the pump and regulator. having the check valve failed in the pump (and non inline) could produce inconsistent fuel pressure as the regulator opens and closes.

I also don't know the age or origin of the pump (not sure its oem) so I'll check but in doubt I'll play it safe and put a new one in.

[andrewwynn]

I would replace the actual pump only vs entire sending unit if my check valve didn't work. Actually the PRs job is exactly the opposite of your description. It's job is to deliver constant output regardless of the input as long as input is above a minimum. Any excess goes back to the fuel tank through the return line.

There is an over pressure release valve on the return line

[scourtaud23]

I must not have been clear in my description. My issue with the check valve not holding fuel is that when fuel is needed, there would be a (slight, I'll admit) delay in the fuel pressure until it is at the requested 50PSI.
Therefore the FPR doesn't only deliver the right pressure, it also has to deal with sub normal pressure and there lies the delay I was talking about.
This would create a lazyness in the fuel supply system.

I will, if I do, change only the pump, having the auxiliary heater, the sending unit is a pain to get and of course more expensive than the normal one.

[andrewwynn]

There will never be "sub normal pressure" if the pump is working. The check valve does nothing when the car is running its purpose is exclusively for power off to hold pressure for a re-start. If there is ever a dip below 50psi the pump or the fpr is at fault.

When there is a delay after a short term engine off condition that of course is likely from a leaky check valve allowing fuel to flow backwards through the pump into the tank. As long as the pressure builds quickly and doesn't dip below 50 at the rail, the check valve situation is "cosmetic" and just an annoyance


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[scourtaud23]

I see... Its still annoying so I'm going to take care of it. I don't like knowing its wrong and having to think about it.

Thanks for the insights, its quite an obscure subject.

[andrewwynn]

As I'm well aware. I "wrote the book on it" when literally 100% of every post online was incoreect when describing how the X5 fuel system works.


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