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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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