Which fuel pump?
 

I have a 2012 35i, 3.0 with the turbo, if my memory is correct. It's got a little miss on mild acceleration and it's got the tell tale sign of cranking for a couple seconds before starting. The previous owner handed over all the maintenance records and one of the reports during some service was that the fuel pump might be failing and should be replaced.

I'm to the point now where I'm looking into that but I'm a little confused. Seems like there's one pump under the rear seat, low pressure, I assume, and then there's the high pressure one that's mounted on the lower part of the engine, driver's side, front area.

I know that high pressure one has it's known issues. When I look at the manual on ISTA, I don't see a way to check that high pressure one and even checking the pressure seems to take a rig of tubing and connectors, along with a pressure gauge.

Is there a way to check both of them, or should I only be looking at one or the other? Of course it would be great if the one I need to replace is the one under the back seat. That one's a lot easier and a lot cheaper to replace :)

Any advice to which one of those normally causes the slow to start symptom?

My scanner shows realtime pressure from the high pressure pump on wife's 50i and I think they use similar pump hardware.

My in tank pump on e53 just went kaput and of course the same basic pump supplies the high pressure pump so it is definitely tricky to figure out. I do not recall finding similar pressure for low pressure pump when scanning the 50i.

You can tap in under the seat how you describe to get a low pressure reading or depending on the age you can preemptively replace (do a little math they last 5-6000 hours on average).

If you say you do 30 mph on average, 5-6000 hours says...


150,000 to 180,000 miles.


:confused:

That is correct. Mine and wife's lasted between 130-150,000 miles right in line with average.

I found a very detailed long term study on fuel pumps with different pumps and different fully types.

Ethanol added 1000 hours. Pure gas lasted 4-5000.

Do you recall if it's the high pressure or low pressure one that causes the slow to start issue? Meaning, it cranks for 2-3 seconds before starting.

I'm showing 5mPa on the high pressure and 101 psi on the low pressure, from my scanner.

Get a reading after the car sits a while and before you start it.

I find that long starts often linked to FPR not holding pressure when not running.


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While sitting, not running, the door open, it's showing 101 psi on the rear and I think very little on the front rail. When I start it, the front rail pressure goes to 12 MPa. After about 60 seconds, that 12 MPa drops down to 5 MPa and holds in that range. The rear pump seems steady at 101 psi no matter what.

Does that mean anything to anyone?

I don't know what normal is yet on e70 models. E53 is 50 from FPR.

Unless 101 is out of spec, I'm betting on high pressure pump. I can look at wife's when I'm home but that won't be for a few days.

I did notice that the high pressure changes by demand which seems incredibly complicated but I don't know what normal is, also likely different normal on 50i

I was seeing numbers double that when revving engine in neutral