rough engine when fuel tank is low

Hi,
Got a bit of a head-scratcher here! My M20 runs badly when there's less than 20L in the tank (no idea what that is in gallons, but it's 1/3 of the tank). When the fuel is this low, starting takes a bit longer, the idle goes really rough and the initial throttle response sucks (I first suspected it was a TPS problem but I refuelled and the problems instantly disappeared). Has anyone had any similar experiences or got any suggestions?

Some ideas I'm considering are:
- fuel pump (replaced recently, had same problems with old pump) somehow needs the static pressure of the petrol above it to deliver correctly?
- E30 fuel tanks are shaped so fuel can get trapped away from the pickup (but the idle sucks when stationary, so I doubt it)?
- how quickly does petrol "go stale"? I go through maybe a tank every 4 weeks, so maybe it's just that the petrol in the bottom 1/3rd has gone off by this time?

Thanks,
Simon

Dears, I have the same problem with my 325i 1988. When fuel level indicator show 1/4, my horse starts jump, but still runs, after 10 km, yellow light of low fuel goes on. I stop to refuel, only 43 liters and tank is full. ( the tank is 56 or 62 liters).
I know that the tank has 2 parts, in the right side part is fuel pump, switch of low fuel to yellow light, in the left one is fuel level indicator and (its very important but I don't know and have not time to check) the fuel return pipe from fuel pressure regulator. I thinks the return pipe could come to right side where is pump or must be pipe connected 2 parts of tanks.
I must check and give you result, otherwise I must refill when in the tank is 1/4.
Thanks.
Chan

One thing you might try is some gas line anti-freeze. There might be some water in your fuel that is picked up when the tank gets low and sloshes around more.

Bob in Everett

Dear Bob,
In my case, the yellow light is on, that mean in the right side part of tank is nearly empty. I believe that must be more than 10 liters in the left side tank.
I think Engine dies because air gets into fuel pump and goes to injectors.
In our country is always hot and have taken off the tank to weld damage hole and cleaned it carefully. I wonder only if I connected the fuel return line correctly.
Thanks.
Chan.

I think that your fuel pump is going or the intake filter on the pump is partially clogged. Pull the pump and look.

Bob P.

Thanks folks. (it's taken me this long to reply because I go through a tank of fuel so slowly!)

The fuel pump and filter were replaced recently so I don't think that's it. (the new siemens-VDO pump has a much louder hum than the old Bosch one did even when it was dying).

In seemingly unrelated news, my battery was dying so I got it replaced. With the new battery, the low fuel level roughness has gone, or at least reduced heaps. Strange! Maybe the voltage before was borderline and having the extra weight of a full tank in the back somehow made a better electrical connection? (yes, this is just clutching at straws!)

Simon

PS BMW E30 BMW BMW E30 (my message was picked up by the spam detector, just trying to convince it I'm legit!)