Engine light on at 70mph???

Whilst travelling along at 70mph( and if I have less than half a tank of fuel) the engine management light comes on and I sometimes get reduced power. If I then stop and use the manual diesel primer/pump the old girl runs fine for a while.
However, if I have a Full tank of fuel the light sometimes comes on, but no loss of power.
Any ideas?

air leak around the filter maybe?

Clogging fuel filter?...........vacuum line leaks?

I"ve had it happen, at any speed. Not often but very annoying. Changed the fuel filter now happens every blue moon. Should I be using adiitives? Is it from poor quality fuel? HELP!!!!

If you have to use the lift pump you have to getting air in somewhwere. In the case of the under 1/2 full maybe the pickup in the tank is sus..

or fuel lines sucking air from a loose joint.........

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the case of the under 1/2 full maybe the pickup in the tank is sus..

Good point....might be alot of dirt in the tank too perhaps........

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Should I be using adiitives?

additives help keep the injectors clean...burns more efficiently...less smoke when you boot the throttle..........

Thanks for all the suggestions, I think I shall drop the tank out and check for debris and clean the pick up, then I'll gradually work along the fuel lines and check all the joints and filters.....should be fun. Won't be for a couple of weeks though....