Today I noticed something I never noticed before. When I was sitting in my car with the engine running and it happened to be real quiet in the neighborhood, I could hear the fuel pump hum, or whir? Kind of cyclicly, where it would do it for say 1 second, then be off for 5 seconds, then on for a 1 second, then off for 5, etc. The car idled and ran like normal. Now tonight, I really tried paying attention to this again and I heard it do this like one time, and that was it. ???

Is the pump supposed to run all the time? Or instead is it supposed to cycle, based on the pressure? Or are there sounds that can be heard if there is some over-pressure that discharges gas back into the tank? Or ?

 

as far as i know, the fuel pump runs constantly, excess pressure is relieved by simply dumping it back into the tank.

Not sure what might have been running cycling like that.

 

I had nothing else running in the car. I turned off the radio to listen. I even went out by the gas tank and undid the cap and put my ear up to it, and it was coming from down there. And as I said the car ran fine. No stumbling or anything. It's kind of disconcerning though, if you hear from people that it should be running all the time and now suddenly develops some mind of it's own. But then, how could the car keep running?

 

is this a newer car . it maybe the evap. ststem it pulls a vacume on the tank to burn off excess fumes in the tank and the vac pump will cycle till it pulls a small vacume and hold,s it.

 

vacuum for the evap system is supplied by engine vacuum. not a pump

 

pump may be making a cyclic noise, but it is not stopping, engine will only idle about 1/2 second with the pump off. The cars injectors must have full pressure to work properly. pressure is controlled by a pressure regulator that varies pressure according to engine vacuum, and the excess is allowed to return to the tank.

 

Could I have been hearing the sound of excess returning to the tank? That 'hum' sound I could hear sounded like that of the whirring sound you hear when you turn the key to on - only fainter.

I did not pay attention yet today to see if I could hear it doing it again. I'll have to do that, later.

 

THE REASION I ASKED THE YEAR IS TO SEE WHAT EVAP SYSTEM AND FROM ABOUT 1999 TO 2000 YEAR TIME FRAME AUTOS HAVE AN ELECTRIC VACUME PUMP. AND THAT WAS APOSABILTY. A FUEL PUMP DOES NOT CYCLE WHEN A CAR IS RUNING ONLY WHEN CAR KEY IS TURNED TO RUN AND NOT STARTED. IF YOU HAVE A PLUGED RETURN LINE IT MAY BUILD UP PRESSURE AND CYCLE OFF AND ON. IF YOU LISTEN IN THE TANK WITH A HOSE IN THE TANK WHEN IT IS LOW ON FUEL YOU CAN HEAR WHATS GOING ON WITH THE PUMP

 

HOW DO YOU KNOW THIS WITH OUT THE YEAR AND MAKE OF CAR

 

[/B]Fuel pump does NOT cycle when engine is running, if return is plugged engine will run excessively rich but pump will continue to run, there is no cycling control in the fuel pump circuit.

 

SORRY, I have been in the business professionally for thirty-five years and I have not seen a vacuum pump for an evap system mounted on a frame,Why would a manufacturer have this design when engine vacuum is readily available. I have seen secondary electric air pumps on some models,but not evap pumps.

 

Let's keep the discussion civil and please turn cap locks off.

 

Someone else just said that the fuel pump does not cycle when it is "running", in rebuttal. But if you notice, shadetreeall, who livened up this discussion, never SAID the car is actually running. He meant when the key is turned to "on". He was refering to the key being turned to the run position before it is started. Carefully read what he said again, and you will see. Therefore, the argument against him was unwarranted.

I forgot yet again about this subject til just now, and need to go out and run the car and hear if I can hear that sound. I filled up with gas yesterday, and maybe now I won't be able to hear it do that.

All this being said - when the key is turned to "on", you hear it whir. What happens after that, once the engine is running? If the thing keeps running, then why does that whir sound only happen for a few seconds when you turn the key to on? Why doesn't it KEEP whirring, when in "on" position (without starting it)? Does it shut off for safety reasons, but then know when the car is actually running, that then it starts back up pumping again?, and THEN pumps continously without cycling? And the only reason you do not hear it is the sound of the engine drowns it out?

If you were in doubt about your fuel pump spinning, could you go under the car and place your fingertips on the fuel pump housing and feel if it is?

And then excess fuel not required by the fuel pressure regulator sends the fuel back to the tank to be recirculated all over again?

Understanding EXACTLY how electric fuel pump systems work will enable one to diagnose possible problems if one understands all the fundamentals.

 

ecman, all of the systems i am familiar with the pump only runs for about 10-15 seconds to pressurize the system, unless it either senses oil pressure or engine fireing impulses, then and only then will the pump be turned back on. you would have to have a lot more sensitive fingers than I have to feel a pump running inside a fuel tank, most of the time you can hear them running, but you must know what to listen for to pick it out of all the back ground noise. no electric fuel pump that i am familiar with cycles when the vehicle is running. if you are hearing a cycling sound it is either a pump that is about to fail or something other than the fuel pump.

Murphy was an optimist
And then excess fuel not required by the fuel pressure regulator sends the fuel back to the tank to be recirculated all over again?

 

I suppose then it be a good idea to say use that hose put down the fuel fill hole trick or a stethoscope, to hear what the pump sounds like running, while it works, so one has something to go up against if anything fails.