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I have an '85 300ZX (non-turbo) which has developed a problem with fuel delivery. The car will start & run at idle for about 60-90 seconds, then I hear the fuel pump relay behind the passenger seat click, which shuts down the fuel pump about 5-10 seconds later, killing the engine. The car will immediately start up again, and repeat the above.

I have changed the relay with one which I know is good, didn't solve the problem. After running all tests I could, I took it to the dealer for analysis, and the diagnosis was "bad computer". It was returning a 31 code. I have purchased and installed a new ECU and fuel pump (the pump WAS bad, the new pump tested fine before installation), and after spending $ 700. and lots of effort, I am back where I started, with the EXACT same problem: Code 31(and now, also Code 23), starts up, runs for 60-90 seconds, relay clicks, fuel pump shuts down, engine dies, will fire RIGHT BACK UP and repeat.

I have cleaned all grounds, checked connections, etc. Dealer is ABSOLUTELY no help....can someone PLEASE tell me what's going on?



Post Edited (Aug 8, 8:58am)
 

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The dealer may not have told you a couple of things:

1) Code 31 is not a showstopper, and nothing to do with the fuel pump.

2) The ECU controls the fuel pump relay, and when the relay is not triggered to ground the fuel pump directly, the ECU controls the ground to the fuel pump to reduce the voltage the pump gets. If replacing the ECU does not make a difference then you have to suspect the wiring harness to the ECU.
 

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I know they changed the 31 code from 84 to 85; but in 84 this code was the A/C and operated like the throttle position sensor. When the ECCU received an on/off input, the code display was turned off. Check the sevice manual for an 85 to see what that code is in 85.
 

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87+ is when then Code 31 would have changed to an ECU fault, which coincides with the (then) next generation 16-bit ECUs.
 

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just a hint there is a realy in the fender by the battery
it is a safty reley .my fule pump would not turn off
and it was that realy.
 

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Two suggestions:
1) Code 23 is the Throttle Valve Switch. It may or may not be a problem depending on how correctly you performed the self-diagnosis procedure. TVS adjustment is not hard.
2) Get a factory service manual and learn to fix the car better than the Nissan dealer ever could.
 
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