Fuel Pump Question for 96

[Mciamp99]

I was wondering if someone could answer this question. Is the fuel pump set vertical or horizontal in the tank? Also is there a overheat circuit in the fuel pump? And last but not least under what conditions would the Fuel pump stop pumping even when it has power and fuel? Neither the relay nor the PCM is shutting it down, yet it quits, resets, runs and quits again. I am on my second pump and both have acted the same way. In my set up with the 75 I currently have it set Horizontal and wonder if that is contributing to my problem. Any insight will be appreciated.

 

[Chick]

Here you go...This is picture of it as it sits in the gas tank.
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Fuel Pump

The fuel pump attaches to the fuel sender assembly inside the fuel tank. The fuel pump is a roller vane pump that produces high pressure. The fuel pump supplies fuel to the fuel rail assembly at a specified flow and pressure. Excess fuel returns to the fuel tank by the return pipe. The fuel pump delivers a constant flow of fuel to the engine even during low fuel conditions and aggressive vehicle maneuvers. The PCM controls the electric fuel pump operation through a fuel pump relay.

Fuel Pump Electrical Circuit

When the ignition switch is in the ON position (before engaging the starter), the PCM energizes the fuel pump relay for two seconds, causing the fuel pump to pressurize the fuel system. If the PCM does not receive ignition reference pulses (engine cranking or running) within two seconds, it shuts OFF the fuel pump relay, causing the fuel pump to stop. As a backup system to the fuel pump relay, the fuel pump switch and engine oil pressure sensor can also energize the fuel pump. The sensor has two internal circuits. One circuit operates the oil pressure indicator or gauge on the instrument cluster. The other circuit is a normally open switch that closes when oil pressure reaches about 28 kPa (4 psi). If the fuel pump relay fails, the fuel pump switch and engine oil pressure sensor runs the fuel pump. An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The fuel pump switch and engine oil pressure sensor energizes the fuel pump when oil pressure reaches about 28 kPa (4 psi).

 

[Mciamp99]

ok, so I hear the relay click, I know I have power at the pump, and the pump still quits, resets itself, runs for several minutes and shuts off again, even though there is power to it all the time. I know as I jumped power to it. It is definetly not being shut off by the PCM or some other source, it is the pump itself shutting down. I replaced the pump and within moments had the same thig happen,I replaced the relay and within momnents had the same thing happen, same for the Fuel Filter. The lines are all clear and not kinked. I do have spark and fuel injectors working during this down time as I checked with Noid light.

I am thinking of switching to external fuel pump as taking the tank in and out of the 75 has become a pain the *** over the last 2 weeks. The LT-4 Runs fine right until the pump quits. It sounds strange but th is problem arose after I finally installed the Interior in the car. I had driven about 60 Miles before that without a problem and figured I was safe to finish my project. Boy was I wrong.

 

[bz1mcr]

My bet is you have a grounding problem related to installation of the interior.
The pump assembly has no way shut down. If it has power and ground it will run. When you "jumped the power" did you also jump the ground? If not that's the next thing to try. All control of the pump is external. Check the ground circuit, some thing you did in the interior could be causing the tank ground to go high. If that is not it, put an amp meter on it and be sure it continues to draw current. It should draw 5-10 amps and be steady after a few seconds. If the current says on the pump should stay on. Try puting an other load like a 12v brake lamp (or two to get about the same amp load) in the circuit in place of the pump (same power and same ground) to see if it will stay on. If it goes out you know the problem is not the pump.

I can't think of any failure in the pump that would cause it to cycle on and off as you have reported. And I am certian you would not find two pumps that do it. A light bulb will definately not cycle if it has power and ground.

 

[Mciamp99]

I tend to agree with you, but it happens without the car moving, and it even happens if I just powere the pump without the car running, no moving parts or anyting to jiggle. I will try to address the ground. I did put an Amp meter on the power side Sunday and it held steady even after pump shut off. Very strange and very frustrating.

The part that kills me is this did not happen after a bump or major shock. I am considering taking the cluster back out. at this point I am just reaching and want to exhuast all possibilities before I go to and IN-Line pump.

The one other thought I have had is that it is loosing pressure in the tank, that is where te pump connects to the fuel line it is not tight enough and thus pushing fuel into the tank instead of into the line. Pressure at the head is only 38 PSI and I know it should be higher. When I put the tank back in I am going to address that as well. I have eliminated the horizontal pump mount as an issue as I ran the pump in a bucket for 20 minutes without issue. I will go after the wiring again and see if I missed something, Thanks.

 

[bz1mcr]

You have a real mystery here! I will be interested to learn the final cause.

You say "I did put an Amp meter on the power side Sunday and it held steady even after pump shut off." Steady current indicates the pump is running steady. If it bound up due to excessive pressure or seizure it would draw high current. If the power or ground was interupted it would draw zero or maybe very low current. If the current stays on when the pump is "off", I think you can rule out an electrical problem.

Is it possible the pump is running steady and you just think it stops? Does the pressure hold steady when the pump is "off"?

The bucket test could help to resolve the plumbing vs electrical. Use some extra hose to connect the pump to the vehicle fuel system. Then use some long wires to power the pump from the vehicle connector. If it only stops when using the vehicle wiring it's electrical. If it only stops when connected to the vehicle fuel system it's plumbing.

 

[Mciamp99]

When the Pump Dies, the pressure drops to zero. Then the motor dies. I hope to have it resolved soon, I am waiting to get my sending unit back as I broke a wld last time I took the tank out. Should be together with more questions I am sure by friday.

 

[Mciamp99]

Also the pump is definately what is quitting, I have blown out the lines with high pressure air and there are no kinks. you can har the relauy click, but you can also not hear the pump anymore, which you can hear otherwise.