|
In Reply to: why 2 fuel pumps? posted by Aaron on September 14, 2000 at 17:18:41:
This is an explaination as I uderstand it. Fuel injectors demand a much higher fuel pressure than the old carburated engines. It is easier to produce with a pump that higher pressure by placing the pump in the tank or very close to the tank. In the early days (1978-1985) the in tank pump was not up to the higher pressure so a second pump was used outside the tank with the boost from the in tank pump. All the cars today have one pump in the tank as the pumps are capable of producing the pressure needed for the injectors. In your case the engine will run with the outside pump only but the pressure at the injectors may not be high enough for good engine operation. Some of that depends on the amount of fuel in the tank. If the tank is full of fuel the "head pressure" on the outside pump is good and the pressure may be enough for proper injector operation. With a tank almost empty the outside pump may not be able to produce enough pressure at the injectors. As to hard starting this could be a problem if the in-tank pump is not working. I know that on my 85 735i it will start and run with the in-tank pump out of operation. Fuel pressure falls from 36 psi to about 32 psi with out the in-tank pump running. You should be able to hear the pump come on and off if you remove the plug at the top of the tank. Or install a pressure gauge at the fuel rail, start the engine and the pull the plug on the in tank pump and the pressure should fall.
© 1997 -
2009
Jump Internet Inc. All rights reserved.
