So this has happened to me a few times now. Sometimes when I am driving, the car's RPMs drop drastically and the car putters with little power. I let off the gas and the rpms jump back to normal and the car doesn't die but soon as I hit the gas, the RPMs drop and the car has no power but still doesn't stall. If I turn the car off and on, the car will turnover and drive fine. I have noticed that if I get down to about 2 gallons of gas left or so, when I come to a stop the car will sometimes stall but then start right back up. I thought it might be the fuel pump but it was recently replaced within the last year. When I turn the key just to the "ON" postion (without crank) I can hear the fuel pump prime but sometimes when I do the same thing I won't hear it prime and the car hard starts. So usually I flip the key off and on until I hear the fuel pump and then it starts and drives fine. I thought it might be the fuel pump relay so I bought a new one and it still does it from time to time. Like it did this one day a couple months back and after a few start ups, it drove fine and didn't do this again until like last month....I did the same thing and it drove fine for another month until today it did it again when I came to a stop. I am confused as to why this is happening so sporadically. I thought maybe injectors but if that was the case then shouldn't the car shut off completely and not start again? Sorry so long but any insight is appreciated.

 

The ECM has a decay timer for the fuel pump prime command and it will only prime the fuel pump once or twice in a row before it disables pump prime for several seconds. In your case it sounds as if the pump isn't priming on the first try sometimes. It may be a circuit fault between the ECM and fuel pump relay, or a faulty relay. Or, since you are having drivability symptoms that seem to be fuel related, it may be that the pump is faulty. Just because it's new doesn't mean it isn't failing. One thing to keep in mind on in tank fuel pumps is that they don't like to be run dry. They don't really like to be run with less than a half tank since the pump relies on the fuel for cooling.

The stalling that you've experienced on stopping with little fuel in tank may be due to fuel slosh causing the pump to draw air.

The test for verifying whether it's the pump or the pump control circuit is to monitor fuel pump power using a volt meter and long test leads that allow you to back probe at the relay and carry the meter where you can see it while you drive. Backprobe at the gray fuel pump power/relay output wire. Watch for voltage to drop off when you experience the lag on throttle tip in. At the same time you're watching voltage on the pump power circuit, be watching fuel pressure with a gauge taped to the windshield. Pressure should be about 35-38 lbs at idle and increase to 40-45 when the throttle is depressed. If pressure is dropping off and fuel pump voltage stays constant, you have a pump issue. Be sure when you perform this test that the tank has plenty of fuel so that the pump isn't sucking air. It may not be the pump itself but rather the rubber hose between the pump and the sending unit.

To test the pump prime command, backprobe the pump activation circuit from the ECM at the relay and verify that it gets 12v for 2 seconds the first time the key is turned on. Once you have verified the command signal is okay, check fuel pressure and fuel pump power when the key is first turned on to be sure the relay is working and that the pump is running and producing pressure when the relay is activated. Check it for at least 45 psi with the key on, engine off. Also, watch how quickly the pressure drops off. If the pressure drops to zero quickly, it's a sign that the hose in the tank may be ruptured.

 

Ok thanks ASE I will get a fuel pressure guage and look for my voltmeter. Appreciate the detailed response.