Air in fuel system

My 91' 940 B230FB+T with LH2.4 is making a weird noise whenever there is demand for a lot of fuel. It sounds like there is air in the system. The main pump also gets loud when the tank is at 50%.

I replaced the o-rings on the fuel rail and the small rubber hose in the tank without any change.

I also swapped in another fpr and it still made the noise.

While you were inside the tank did you verify that the lift pump was indeed pumping fuel? You are describing, especially with the 50% full comment, lack of fuel to the pressure pump.

I only replaced the hose. How could I verify that? I can hear it buzzing inside the tank.

Could it be worn and about to fail?

Disconnect the fuel hose at the input to the pressure pump and run the pump. Might be a good idea to have a container to catch the fuel if the pump is pumping.

Due to the effort required to replace the hose I would have replaced the pump at the same time. Pump is about $25.

Brotkasten said:

I only replaced the hose. How could I verify that? I can hear it buzzing inside the tank.

Could it be worn and about to fail?

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in my opinion such an electric pump either works or it doesn't. Not much in between.

but there could be other issues that prevent the liftpump from performing at 100%
- There is a filter sock connected to to the liftpump. It should be clear of gunk and debris.
- There is also a piece of hose between the liftpump and the appendages outside the tank.
This piece of hose should be leak free also, oherwise the leak would act like a shortcircuit, the result will be a liftpump that doesn't perform.
- the lift pump also needs good electric connections. Pay extra attention to the ground connection.

I already replaced the hose and cleaned the filter sock. Thats why im confused about the "air in system" sound.

The Ground connection is located at the left tail light right? I tried to clean it a bit but the bolt is not in my reach... LOOKS clean tho...

another idea: when you experience these problems, what happens when you screw the tank cap off while the engine is running/idleing?
Do you hear a rush of air going in? The tank needs to have an open tank ventilation to compensate for fuel being used. That space has to be replaced with air.
If ventilation is not working the engine will eventually suck the tank vacume. Both fuel pumps will start to have trouble providing fuel to the engine.

Additionally... did you mess with any of the wiring connections to that pump and cause it to run backwards so the two pumps are just fighting each other?

The noise just "appeared". I changed nothing and it stopped working properly...

I've tried listeling for the hiss when opening the tank while refueling. The car was just shut off. I'll try to listen for it while its running.

I'm almost ready to just change the pre pump. It's had a 27 years of duty...

The feed tube or hose from the tank pump may be leaking, or the pump may not even be attached.

They use less power when they are just laying in the bottom of the tank making whirring noises!

If you crack that sucker open and start swapping crap around. Pay attention to which color wires go where and do what. A bunch of the replacement fuel components back there swapped wiring colors around and it jacks up a whole buncha stuff if you're not paying attention.

Im not at all certain it's air in the fuel system anymore...

I changed out the fpr to original and the main pump noise is gone. The fuel pressure is now 0,5bar lower (bigger inectors) and the sound did not change a bit. I think it should at least sound different - pressure and spray pattern changed).

In this video you can hear the sound near the end.

drive.google.com/file/d/1YDH3y…k-Rn7SCl/view?usp=sharing

Diagnose intank fuel pump issues.

A cavitating pump will be very noisy and have a shortened lifespan in addition to not supplying a consistent fuel pressure.

Fuel pumps don't like to suck they need a flooded or fed inlet.
When a fuel pump has to suck they will often cavitate.

Early setups with an external pump had a 1/2" feed line to try to limit this, but the solution is to have a feed pump in the tank.

Diagnose intank fuel pump issues.

Hearing an intank pump run is only a partial diagnosis, it has to actually pump fuel.
It should also be checked for volume of flow, if it doesn't move enough fuel it's still not doing it's job.

Any Fuel filters including the tank sock should also be checked /replaced if needed.
Also check for any other restriction or defect in the fuel lines.

You have to commit to fixing a problem not throw parts at it from across the garage or try to get your daughters magic wand to work on it.
If your fuel pump is noisy there is a problem, and it needs to be addressed first.
EFI relies on a consistent fuel supply, without that it can't begin to work right.

Dirty Rick said:

If your fuel pump is noisy there is a problem, and it needs to be addressed first.
EFI relies on a consistent fuel supply, without that it can't begin to work right.

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It's not noisy anymore since I replaced the fpr. Thats why I'm beginning to wonder if the sound really is air in the fuel system.

Maybe the fpr was restricting fuel flow.