86 eta fuel pump noise question -

[LateFan]

86 325es, 175k miles.

Does noise from the in-line pressure pump mean that pump is going?

Or that the in-tank pump is making the pressure pump strain?

The pressure pump (in front of the rear wheel) is doing the intermittent bzz..bzzz........bz.........bzzzz.... thing. Not steady.

I don't want to replace the wrong one.
Thanks -

[jlevie]

Could be either, both, or from a rusty tank. Further diagnostics will be necessary to determine the cause. Although in the case of a rust tank with sediment in the bottom after dealing with the tank (replacement or acid cleaning and sealing) both pumps will need to be replaced.

[LateFan]

We pulled the access cover tonight and started the car. The random buzzing is coming from the under-car pump. I don't think the in-tank pump is making any noise at all. I should be able to hear it running, no? I used a stethoscope.

I unplugged the electrical connector and it stayed running with no change. I've read the engine can run on the pressure pump alone.

Restarted the engine with it unplugged and it started right up.

Is this pump doing anything at all? Has it shut down and the other one can still pull from a 2/3 full tank?

[jlevie]

If the tank is about 2/3 or more full the high pressure pump is able to suck fuel from the tank. When doing that the pump usually won't make any noise unless it is worn.

[LateFan]

Should I be able to hear any hum from the in-tank pump?

[jlevie]

It should faintly hum if you jumper the fuel pump relay.

[michael325e]

LateFan - I had the same issue with a buzz from the inline pump. I tested the in-tank pump and it wasn't working. I replaced it yesterday, unfortunately with a cheap stand in as the local BMW mechanic will be taking the bulk of my repair budget tomorrow. In any case, since then I haven't heard any buzz from the inline pump. Too soon to tell for sure if all is OK, but fingers are crossed.

My hypothesis is that since the inline is designed as a pressure pump and not a suction pump the impeller is optimized for the purpose. This could mean that it causes cavitation when not supplied with a pressurized feed. Since cavitation is very bad for any pump the buzz is a self fulfilling prophesy.

Michael

[LateFan]

Interesting idea that the noise may actually be cavitation. And a fuel pump spinning without fuel to cool it is eating itself pretty quickly - I've had that happen. Thanks -

[poopin04]

I just swapped them both out with new ones, no more buzz buzz.

[LateFan]

I want no more buzz buzz! And I want it to start every time.

[evoeater88]

Same here with an 89. But in-tank pump only. Started buzzing so I replaced it now its all good. Hard start could be cps not communicating.

[poopin04]

If I were you, I would start with switching out the in-tank, since it's cheaper and then going at the in-line. Oh and make sure you change your fuel filter, always good to rule that out.

[LateFan]

Sounds good.

Filter is new, CPS's (I have 2) are new. We may have a rusty tank and clogged sock as well.

[michael325e]

I've driven my car a couple more times now and still no buzz from the inline pump. And I made a point to remember to get out of the car and listen for it after I had driven awhile. And I filled up with gas and the tank seals aren't leaking so I guess I can button up this job and get the back seat back into the car. When I was at the mechanic's shop this morning getting my driveshaft replaced I mentioned the bad in-tank pump and the buzz from the inline. He said "It's probably cavitating" before I could even mention my hypothesis. So I'm leaving it at that.

[LateFan]

Replaced the in-tank pump with the Airtex #E2485 (Vega pump trick). You can hear it in the floor now, so the old one was not working.

On to the next thing - Thanks.