Main fuel pump whiny hum
 

I've been hearing this strange, medium-high pitched hum on my 86 wagon for a couple weeks. Originally couldn't tell if it was coming from the dash or under the car. It's a constant pitch, dies a second after shutting off ignition. Lately it's gotten significantly louder. Car runs normal regardless.

Sticking my ear under the car and I'm pretty sure it's the main fuel pump. But is it whining because it is about to die, or because the in-tank pump is already dead? Today, I pulled fuse 11 while in idle and the sound got louder with a slightly higher pitch. I assume this means the in-tank pump is working properly and the main pump is about to go, but perhaps someone here might have a better suggestion.

It could also be a clogged up fuel filter, I'd try a filter first if it hasn't been done for a while and then if that doesn't improve things try a new pump, my 1990 760 turbo has the same thing, I've got a filter to fit to it but that's been in the shelf for over 12 months :shocked: I'm really hoping it is the filter.

Could be, but if I'm replacing the filter, I might as well do the pump since they are both interconnected. I don't want to do all that and the real problem ending up being the in-tank pump or some other issue. I'm just seeing if anyone here can check my logic or suggest a better way to verify the issue.

I would normally say a high pitch whine is expected. The fault condition I have seen is intermittent no start of the pump, while it gets more "grindy". But this was with the single intank pump. Basically these are unserviceable brushed motors and only last 100-200k Miles until the brushes wear out.

Whine getting louder seems unexpected to me. If its still working leave it alone, or if it done enough miles and you can get bargain replacements then do both.

Maybe pull the pipes off and check you are getting flow from the tank pump. It might have already failed.

My 940 with a single in-tank pump used to get noisier as the tank emptied, not sure if that is just the fuel damping the sound or it sucking air sometimes. It went for thousands of miles being noisy though.