Firstfives.org

Sparkling clean tank and filter. Tank full. Still intermittent buzzing noise.

Is failure imminent, freeway crap out ? Does it effect the working of the pump over the axle ?

Could the noisy axle pump I replaced be fine, if down stream from good in tank pump ? ( I.e., keep it for spare.)

Do we recommend replacing the whole pump tree assembly or just the pump cylinder at the bottom ?

Thank you,

I don't think I've ever had a working in tank pump in hundreds of thousands of miles on injected M30 powered cars. IMHO the real advantage is it allows you to more fully drain the tank. When I injected my Bav many years ago, I found I would get fuel starvation/cutting out around hard corners at about the 17 gallon mark. With carbs it was over 19. A bad in tank pump could contribute to a noisier main, but not really failure. The main pumps are pretty sturdy.

I'm going to take a flying guess the in tank pump is redundant. If water runs down hill, the fuel in the tank only needs to get over the top of the tank once to begin and hold siphoning principles. If the larger pump over the axle is stout enough to pull/push the fuel from the low point forward, buzzing or crunchy noises only bother perfectionists who don't listen to their sound systems.

Who cares. My car. If it craps out I'll post the adventure.

The tank pump extends the life of the main pump, probably by 40% or so. I think the tank pump adds reliability by making the main not work so hard. On my '85 635, ( same main and tank pump, ) the main lasted till 154K before it crapped out and then I had about five minutes of intermittent working to get it home. I gladly change 'em out well ahead of time, because I hate those $400 tows.

My 81 euro has no in-tank pump; never did.
Problem with in-tank pump is if it fails. You can't really run with it dead in place and use the entire tank of fuel.
I got about 300K out of the OEM pump.