fuel pressure loss 86 na z31 - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 4 (permalink) Old 03-19-2017, 02:53 PM Thread Starter
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Question fuel pressure loss 86 na z31

ive replaced the fuel pump the, all hoses on rail and regulator and car just wont stay running after prime. thought it was strange because the car sounds mechanically sound when turned over but after fuel pump prime it loses pressure. after i installed the fuel pressure gauge i realized it was only getting 25psi at prime. thats when i replaced the rubbers on the rails because the had a stronge fuel smell. after replaceing those i now get 38 at prime and after prime in falls quickly to about 15. im going to start volt testing and see if useing a jumper for the fuel pump changes anything today. i have a strong idea that its a ecu problem or the fuel module. just posting to see if anyone has ever had this problem. any advice will be greatly apreciated
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post #2 of 4 (permalink) Old 03-19-2017, 06:05 PM
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put a new fpr in there before you do much else.

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post #3 of 4 (permalink) Old 03-20-2017, 04:10 PM Thread Starter
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i did, i have the same issue
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post #4 of 4 (permalink) Old 03-22-2017, 04:33 AM
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I don't know what you mean by "fuel module".

A fairly typical failure mode of the Z31 ECUs is to not supply volts to the pump at low load (including idle).
Under idle and low load conditions, the ECU directly handles fuel pump current and supplies less than 12V by floating the ground for the pump.
At higher loads the ECU switches in the Fuel Pump Relay to give the fuel pump max available volts. The Fuel Pump Relay is the same relay that provides the ~5 second prime when the key is switched on.

The FSM is hazy on where the bounday between low load and high load exists.

When testing the fuel pump by powering it with, for example, another separate battery, the pump should be completely disconnected from the body harness since you do not want to be putting 12V into the ECU where it does not expect it.

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