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I have a 1990 mustang 5.0 auto Lx with gt40p heads, gt40 upper and lower intake. I was wondering when I replace the fuel pump, does it matter if I install a FLOW: 255L/H pump? Would I need to change anything else or would it be better to just keep the same FLOW: 95L/H pump in it? It still has the 19lb fuel injectors if that helps any. Thanks for any help or advice you can give.
 

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With a very mild build like yours, the stock fuel pump is probably fine as long as it isn't worn out. I'd check the pressure and make sure it can keep up.
If the stock pump does need to be replaced, going with the 255L/hr pump wouldn't be a bad idea. They aren't that much more than a standard pump from autozone, and it will keep up with any mods the future may hold. Our cars have a return fuel system, so pumping a greater volume won't hurt. If the pump is pumping more than the motor needs, it will just get sent back to the tank.

This is a good deal for a 255L/hr pump kit. I installed it in my car last year:
Summit Racing SUM-250103 - Summit Racing® Mustang In-Tank Fuel Pumps - Overview - SummitRacing.com
 

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You'll need to drop the tank and replace the fuel pump.

  1. Disconnect battery
  2. Siphon fuel if needed.
  3. Jack up car from rear on center of axle housing. Follow normal jacking precautions.
  4. Support car with two jack stands under the rear axle and near the tires. I had the rear wheels about 5" off the ground. Remove hydraulic floor jack (this is used to help lower the fuel tank.
  5. At this point I removed the fuel filter. It is located in front of the tank centered between the rear wheels. Use flat tip screwdriver to loosen hose clamp that holds filter. Next pry on the white plastic (hairpin) clips that hold the fuel lines to the filter (one on each end). There may be some gas leakage when the lines are pulled, ESPECIALLY IF THE LINES ARE STILL PRESSURIZED. Use goggles, rag and bucket to minimize drippings. The clips come out perpendicular to the lines and there is a triangular tab to pry on. Don't break them unless you bought a new filter (it comes w/ two replacement clips). Pull hose off each end of filter.
  6. While under car it is a good time to use some penetrating oil on the two 1/2" bolts that hold the tank brackets to the chassis. They can be difficult. They are to each side of the filter.
  7. Using 5/16" socket w/extension remove 3 screws inside fuel fill door. I also removed 4 screws on the inside of the panel that hold a rubber cover to the backside of the fill door cavity. There is also a smaller size screw that attaches a bracket to the fill side of the tank. It has a loop that goes around the fill pipe.
  8. This whole assembly needs to be loose to drop the tank. You should be able to pull on this assembly, out of the fill door area, and it should start to slide out of the tank. There is a rubber grommet around the fuel pipe at the tank. You won't be able to pull it all the way out until the tank drops a little.
  9. Try loosening the tank bracket bolts now. If still tight try more penetrating oil. (DO NOT REMOVE THEM UNTIL YOU HAVE SUPPORTED THE TANK).
  10. If the bolts are loose put floor jack under the tank center. Use a piece of wood to lift with to distribute the weight. Once the tank is supported you can remove the bracket bolts. I was able to swing the brackets vertical at the height I had lifted my car.
  11. Disconnect: At the rear of the car at center there is an electrical connector that goes to the pump. Disconnect it.
  12. There is a plastic loop at the front of the tank that the fuel lines run through. I cut this but I guess it could be pulled out of the chassis.
  13. While one guy slowly lowers the tank the FRIEND can work the fill pipe the rest of the way out of the tank. The gas tank has to be emptied to below this level or it will run out. Always look for lines or electrical connections that might be binding. The tank may want to drop one side more than the other but it was manageable with two guys. We were able at this point to set the tank on the ground without stretching any lines.
  14. The pump assembly goes in the tank on the right hand side (from back of car) on a high spot on the tank. Clean this area as well as possible to prevent dirt from getting into the tank upon removal. I wiped the area and used an air gun to blow dirt away (good idea!). My assembly was oriented with a red electrical plug towards the rear of the car and the two metal lines facing the front of the car. You can take off the electrical plug by using a screwdriver to pry on the catch clip between the plug and the metal lines. The clips on the metal lines are two different types. One is identical to the fuel filter clips (hairpin). The other has two tabs that need to be depressed. The clip comes out the end of the holder and stays around the metal line. You should be able to separate the flexible line from the metal lines.
  15. The pump assembly is held in with a large locking ring. It has 4 tabs sticking up. You should be able to see the three bent over flaps that hold it on and the notches you have to rotate it to in order to remove it. I used some penetrating oil here also. Using a hammer and 3/8" extension tap on one of the tabs to rotate the ring counterclockwise. Do not use a screwdriver, it just bent the tab. Once the ring is off the pump assembly should be free. In my case I had to rotate the assembly 180 DEGREES to remove it. It has a "z" shape to it and takes some wiggling to remove.
  16. At this point you should be all set. Install pump in assembly and reverse.
 

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With the mods you got an upgrade in the fuel pump department is needed. A 255 is the way you should go.
 
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