Nissan Repair: 86 nissan multi/stanza wagon noisy fuel pump, fuel pressure regulator, fuel pump relay

Question
QUESTION: Hi my name is Wayne I have an 86 Nissan 2wd multi/stanza wagon,  the fuel pump stopped working the other day so I replaced it with a new one, it has an external pump.  After about 20-30 mins of idling after the install the pump started making a very loud buzzing/growling noise.  The fuel tank is full I installed a new filter with the pump, the engine runs fine with no lack of power.  the pump sounds like it is cavitating  or starving for fuel but i checked the inlet line to the pump from the tank and there is lots of fuel flow.  I shut off the engine and took the key out of the ign and after about 5mins there was a strange vibrating sound that seemed to have come from the tank, I took off the fuel cap and there was some pressure that vented Is this normal?  I thought the vapor canister vented the pressure in the tank and the cap let in air.  At this point I decided to check on line to see if anyone had posted a similar problem, I did find a post from a couple of years ago on this site and in it there was mention that the volts at the pump should be aprox 9Vdc.  I checked mine and there was 13.8Vdc and the pump was very loud.  I kept the volt meter on the pump while it was running, every once and a while the pump would go quiet at this time the voltage would drop to about 12.5 Vdc.  I had a helper with a second meter checking the voltage at the battery at the same time and it stayed a steady 14.4Vdc while the voltage at the pump would fluctuate from 12.5Vdc to 13.8Vdc any volts over 13 would cause the pump to make noise.  If there was a restriction in the fuel return line or the fuel pressure regulator  was faulty would this cause the voltage to jump around like that?  Is there a resister in the fuel pump relay or the egi relay?  At this point I am very lost PLEASE HELP.

thank you

Wayne

ANSWER: Wayne,

Nissan uses a variable voltage to control the fuel pressure.  I would say the new pump is bad and you need to take it back and replace it.

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QUESTION: Ill give that a try, I wont be able to have a new pump here until a couple of days.  So it is normal to get a voltage range of this much or up to this high?  Could a faulty pressure sending unit or what ever tells the fuel circuit to vari voltage be faulty?  I am also going to check the fuel pressure on the first replacement pump before I install the second new one, what should the pressure be at idle 750rpm and at mid to high idle 2500-4000rpm?


thank you so much for your quick responce today

Wayne

Answer
Wayne,

The fuel pressure/voltage variation is normal for a Nissan.  It is just the way they control pressure and enhance pump life.  You are looking for about 35 PSI for the fuel pressure.  Also, check what the pressure is with the fuel pressure is with the vacuum line disconnected.  The pressure should increase with the line disconnected.