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In-tank fuel pump story

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In-tank fuel pump story

Post by whiskeydry » Tue May 14, 2019 9:20 pm

I needed to find an in-tank fuel pump last week to get my M635 through the MOT. New ones seem either very scarce and/or very pricey, if you can find one. There don't seem to be many new ones around in the UK. Having contacted some e24/e28 breakers who had sold out of second hand ones, I was told that people are beginning to get inventive at fixing them, as a lot seem to be failing currently. However, I felt reticent to try to fix an electrical item that I was going to immerse in petrol (gas), although I have subsequently been told it is relatively straight-forward to just replace the pump motor (if anyone has tried this it would make for an interesting and useful article). So I tried a number of online searches and found this thread on the MYE28 board: ... cbb0504d65

I decided to take a punt on this as there was only one Airtex E8140H left on Amazon in the US, none in the UK. It arrived in less than 4 days from Baltimore and was an exact fit. As described in the above article the photo on Amazon is not the right one. It shows a screw plate as opposed to the twist fit I needed. Happy days. Unfortunately for anyone needing one now they are on back-order but it might be worth keeping tabs on when they might come available again.

Symptoms of the failure of this pump might be of interest as I suspect it might have failed a number of years ago. Car seemed to start fine but, when the tank was low, it started to cough and splutter a bit under acceleration and became difficult to restart, once stopped. I put this down to possible dirt in the fuel or just old fuel as the car is not used in UK winters. If left for a while it would start fine.
Once I had a chance to fill the tank, the car would run fine, but would hesitate occasionally when accelerating hard. Subsequently I used to keep the tank fairly well topped up.

Having researched these symptoms on the forums I suspected the pump might have been faulty and, sure enough when I tested it, it would not run at all.

My take on this now is that the in-tank pump just keeps the in-line pump primed. If the tank is low then the in-line pump becomes starved of fuel hence the above faults. Once the tank is filled, gravity keeps the in-line pump pretty much primed except when accelerating hard, hence the hesitation. I suspect that the ECU had compensated for this to some degree by adjusting the mixture to be a bit leaner. The reason I say this is that after I fitted the new pump, the car started and ran fine. However, it would not immediately restart and backfired once (something it has never done before). I put this down to over-fuelling. Once I had an opportunity to take the car for a run I think the ECU readjusted the mix. I asked the MOT tester to scrutinise the emissions and they were spot on. Car now runs like a dream.

1985 RHD M635csi #128
1999 E36 M3 Evo Convertible
2010 E91 320d Touring

E32 735i
E28 525E
E24 635csi
E28 525i

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Re: In-tank fuel pump story

Post by johnn635 » Sun May 26, 2019 2:42 pm

Thought I could add something useful to this thread. Same situation, the in tank pump not running. I separated the plastic halves and cleaned it all up and it runs so well that it sucks up every bit of crud in the tank against the foot filter, which then causes fuel starvation. Presently removingthe crud, but would also like to get the low fuel level switch working. The sending unit has swaged outer tube, so will resist inspection, so does anyone know if this is a reed switch, like they use on the coolant sensor, or just a contact. If the latter, then perhaps some vigorous/chemical cleaning will resurrrect it. Mind you, should never need to see the low level light - keep it full - right?

FWIW - the in tank pump is still available about £330, as is the sender, as is the tank, but £1000 to remove crud seems a bit wasteful.

Also, if it proves useful, the in tank takes less than 1A, and the main pump about 4A. Return line flow rate is good, unless the filter blocks. Car then idles perfectly but dies on WOT.


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