'91 740 Fuel Pump issues

TommyB · Post by **TommyB** » 29 Sep 2006, 11:06

My 1991 740 (non-turbo) fuel pump has stopped giving that "cycle" of gas when you go to start the car. There is no sound heard of the fuel pump cycleing when you turn on the switch. When the car is cold, it takes considerable cranking to get it started, but it will start and run fine for the day. All starts after the first in the morning are okay. I replaced the fuel pump relay and checked all the fuses and still have the problem. I don't believe this is related to the start injector as I understand that these don't come into play until the outside temp is below 40F. Currently it's in the 60F to 75F here. Does the fuel pump have within it a seperate component that gives that "starting cycle" of gas and is it possible that part of my pump is defective but the pump will continue to operate normally once the car has started? This is driving me crazy! Help!

Post by **billofdurham** » 29 Sep 2006, 12:49

It is my understanding that the start injector is there to provide extra fuel during engine starting. The outside temperature would affect this, but would only stop the fuel supply if it was above 95F. The amount of fuel supplied by the start injector is controlled by the thermal time switch when the engine is cold and an impulse relay when the engine is hot.

As your problem is with a cold engine I would suggest that you check the start injector and the thermal time switch.

Bill.

Kmaniac in California USA · Post by **Kmaniac in California USA** » 30 Sep 2006, 10:02

Here is my two cents on this cycling fuel pump issue.

First, lets review the fuel injection system, so we all have a basic understanding. Gasoline moves through a continuous circuit through the system. The pumps take it from the tank, passes it through the filter and into the fuel rail. You have injectors on the fuel rail that bleed off fuel as the car runs. There is a pressure regulator at the other end of the fuel rail that maintains a preset pressure on the fuel in the rail. Excess fuel moves past the regulator and returns to the tank.

Now, when the car is running, the fuel pressure in the rail is at the preset pressure. When you turn the car off, the pressure is maintained in the fuel rail as long as the injectors and pressure regulator don't leak off fuel.

The running of the fuel pump you hear when you turn the key is the fuel pump pressurizing the fuel rail. The pump stops when it is fully pressurized. This will occur whenever the pressure in the fuel rail leaks off since the last time the car was run. If, however, the fuel pressure remains constant in the fuel rail, you will not hear the pump run when you turn the key.

Having the fuel pump not run when you turn the key is not a bad thing. Having it run each time you turn the key not necessarily a good thing.