Why do e30s have two fuel pumps? How do they work together?

[Raven Tooth]

I'm trying to diagnose a very tricky problem related to fuel delivery. Can someone explain to me why my car ('89 325i) has two fuel pumps, and anything about how they are used together? Are they both supposed to run all the time? How about at start-up, does only one run? Right now the car won't start and I don't have any power going to the second (axillary) fuel pump when cranking the ignition.

Thanks in advance. I can't seem to find an explanation of this anywhere.

[StereoInstaller1]

'89 does not, AFAIK.

[NC325iC]

'89 does not, AFAIK.

but the early models with two use 1 pump to pull fuel out of the tank and the other to increase the fuel pressure. My guess is the high pressure pump cannot generate enough NPSH to pull fuel out of the tank on its own. (glad one of my engineering terms has gotten some use)

[nando]

the 2 pump systems are actually better from an overall flow/pressure standpoint. But they're obviously more complicated and more expensive. I suspect BMW went with a single pump later due to cost reasons.

[Raven Tooth]

Ok, so what is the thing under the access panel under the backseat on the driver's side in my '89 325i? I thought this was the second fuel pump.

[nando]

it's a fuel level sensor. there are two, one attached to the pump and one on the drivers side.

[Ruffian]

Well, there is fuel level sensors on both the pump, and the stand alone sensor. One is for level indicator, and one(on the pump) is for "low fuel idiot light". The pump is the one under the passenger side of the seat.

The early system that utilized two pumps really had a better design as the in-tank pump was the low pressure and the inline pump was the high pressure. This was better because the high pressure would experience less contaminant and survive longer.

[lennon]

I'm trying to diagnose a very tricky problem related to fuel delivery. Can someone explain to me why my car ('89 325i) has two fuel pumps, and anything about how they are used together? Are they both supposed to run all the time? How about at start-up, does only one run? Right now the car won't start and I don't have any power going to the second (axillary) fuel pump when cranking the ignition.

Thanks in advance. I can't seem to find an explanation of this anywhere.

since you car is an 89, it should only have one fuel pump (under backseat, under access panel).

it is the early cars that have two- one in the same place as yours that just sends the fuel to the high-pressure pump located under the car. these cars will run if the in-tank pump dies because the high-pressure pump can usually do both jobs (though probably not for long).

what makes you say yours has two? and what are you calling the auxiliary pump? sounds like you might have an early car (not 89) with a broken in-tank pump.

[Raven Tooth]

Thanks everyone. Turns out I have a later car with only one pump. What I mistakenly thought was a second pump was just the second fuel level sensor. I appreciate the clarification.