1996 940 DTIC air in fuel lines

[Superbracey]

In my 12 months of ownership my Volvo 940 dtic has always had an issue where if the fuel level is low, eventually it will lose power at motorway speeds and need the fuel filter bleeding and the fuel topping up. Once bled the problem would not come back for days/weeks. However, this problem now happens all the time on the motorway, regardless of how much fuel is in the tank and how often I bleed it. I've no idea where the air is getting in.

I've replaced the fuel filter to no avail. There does not appear to be any diesel leaking anywhere and all the obvious connections under the bonnet have been checked.

It has what looks like an aftermarket fuel pump (very noisy) wired in just before the fuel filter. Should it have another pump near the tank? When the feed from the tank to the inline filter was disconnected and the engine run, there was no fuel being pumped to the disconnected pipe, so it seems that the pump under the bonnet is the only one.

Cheers