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Car more powerful when full -tank?What does it indicate? 700 1990

The car has two fuel pumps. The pre-pump is in the tank and is a low pressure pump. The main pump (high pressure) is under the car just under the driver's seat (LHD). When the pre-pump quits, the main pump will have to suck the fuel out of the tank - something that electric fuel pumps are not very good at. The lower the fuel level the worse the problem.
With the car running, remove the gas cap and you should be able to hear the pre- pump running at the filler opening. A piece of rubber hose held up to the ear with the other end in the filler opening helps. If you can hear the whir of the pre-pump, and the problem seems to manifest itself at half tank, the problem is likely a short (1 1/2 in.) piece of fuel line that is used to connect the input of the pre-pump to the fuel pickup at the bottom of the tank. These hoses commonly fail and cause the main pump to suck some air when the fuel level is below 1/2 tank. The pre-pump/fuel sender will have to be removed to fix either problem. They are accessed from the top via a small cover in the trunk.
Tips - have a new O ring for the tank seal - you will have a terrible time re-using the original. Lightly lubricate the O-ring with some dish detergent when installing. If you replace the filter sock on the fuel pickup, inspect the inside of the sock for hair balls. I had a brand new pre-pump jam up with a nylon hair ball that was in the new sock and had to do the job twice.






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