Fuel pump problems. - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #1 of 9 (permalink) Old 05-05-2019, 06:02 PM Thread Starter
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Fuel pump problems.

So i recently picked up a 1986 300zx turbo manual. Its got a couple minor mods done like wheels, exhaust, coilovers, and an air intake. However, my biggest problem is my fuel pump. Something happened electronic wise with the car in the past that affect the fuel pump system. To start out, what the previous owner has done is they have taken one of the plugs that powers the fuel pump while the car is running and basically bypasses the relay and is taking power straight from the plug that powers the wipers and has it grounded in the trunk. So now I have a super ghetto wiring job that isn't the safest, and also am without wipers. I know the wipers work, but the plug is just being used to power my fuel pump. The car runs fine with the way it is wired currently but it is running rich at idle as it isnt connected to the computer and is just getting a constant voltage. Now when I try plugging in the pump to the plugs that it is supposed to be connected to, the car will crank and start for a couple seconds then die after it is out of fuel from being primed. I've checked the relays and fuses that relate to fuel delivery but they seem to be working/fine, but the fuel pump just wont kick in when the car is actually started. Im not sure if there is an inertia switch somewhere that has gone bad and isn't allowing the pump to run, if so, where would i find it? Is there anything else that anyone would recommend I check? I'm trying to check the simple things first to narrow the problem down as much as possible. Also the guy that sold it to me told me that the owner before him had the battery on backwards, Im not sure how someone does this but I guess he was just very oblivious. Im not exactly sure what all this could've damaged. So far everything electric that I know works is the g-meter and compass gauge, all the seat adjustments, and all the lights. Gauge cluster is out, and so is the radio. Any input or advice helps! Thanks.
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post #2 of 9 (permalink) Old 05-09-2019, 02:13 AM
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I would suspect the car starts to die about 30 seconds after it is started.

Check the Factory Service Manual section EFEC. The section on the control of the fuel pump is on page 29 in my year.

From that page: there are two circuits that power the fuel pump. (warning pin number can change with car year, check the FSM for your car year.

  1. The fuel pump relay (ECCS pin 20) - this is turned on for 5 seconds after the ignition is turned on, when the car is starting, and for 30 seconds after the engine starts.
  2. The fuel pump direct circuit from the ECCS (pin 108) This is used to run the fuel pump at a lower voltage at all other times.
It looks like pin 108 from your engine control box is not working. You should check for a wiring problem, but this pin failing is not uncommon. The usual cause is an after market fuel pump that draws more current than the OEM pump.

1984 300zx non-turbo
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post #3 of 9 (permalink) Old 05-16-2019, 08:54 PM Thread Starter
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Originally Posted by thansen View Post
I would suspect the car starts to die about 30 seconds after it is started.

Check the Factory Service Manual section EFEC. The section on the control of the fuel pump is on page 29 in my year.

From that page: there are two circuits that power the fuel pump. (warning pin number can change with car year, check the FSM for your car year.

  1. The fuel pump relay (ECCS pin 20) - this is turned on for 5 seconds after the ignition is turned on, when the car is starting, and for 30 seconds after the engine starts.
  2. The fuel pump direct circuit from the ECCS (pin 108) This is used to run the fuel pump at a lower voltage at all other times.
It looks like pin 108 from your engine control box is not working. You should check for a wiring problem, but this pin failing is not uncommon. The usual cause is an after market fuel pump that draws more current than the OEM pump.
Thank you so much for the feedback! I'll make sure to check on that when I get off of work. Like you said, It'll run a little and then just die after the 30 seconds. Do you have any suggestions on how I might check to see if pin 108 is working correctly? Or rather what I should do if I find that it is not working correctly?

Last edited by gtree06; 05-16-2019 at 09:42 PM. Reason: Add an additional question
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post #4 of 9 (permalink) Old 05-17-2019, 07:33 AM
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Originally Posted by gtree06 View Post
Thank you so much for the feedback! I'll make sure to check on that when I get off of work. Like you said, It'll run a little and then just die after the 30 seconds. Do you have any suggestions on how I might check to see if pin 108 is working correctly? Or rather what I should do if I find that it is not working correctly?

First, I have never had this problem, so I have never done this. I need to think about a good diagnostic approach. Unfortunately, I have never found a schematic of the the Z31 ECCS (ECU).

Get the FSM and look at the ECCS schematic in EFEC section. This will help you with wire colors and positions in connectors.

Do you have a stock fuel pump?

1984 300zx non-turbo

Last edited by thansen; 05-17-2019 at 07:42 AM.
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post #5 of 9 (permalink) Old 05-17-2019, 03:53 PM Thread Starter
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Originally Posted by thansen View Post
First, I have never had this problem, so I have never done this. I need to think about a good diagnostic approach. Unfortunately, I have never found a schematic of the the Z31 ECCS (ECU).

Get the FSM and look at the ECCS schematic in EFEC section. This will help you with wire colors and positions in connectors.

Do you have a stock fuel pump?
I've been reading the fsm and figured out which pins go to which wires on the plug related to the fuel pump, so I will use a voltmeter later to try and see if that specific pin is doing anything or not. I do have a stock fuel pump. If it turns out my ecu isn't working properly, would it be better to just find one online that works?
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post #6 of 9 (permalink) Old 05-19-2019, 12:31 PM Thread Starter
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Further inspection

I've tested pin 108 on my ecu with a voltmeter. It would give me a reading of 2-4 volts when the key is turned to the "ON" position then drop to zero after a second or two. I'm not sure if this is how it is supposed to read or if it should be higher, or read a constant voltage? If my ecu is faulty, because like I mentioned in the beginning one of the previous owners put the battery on backwards and I'm not sure how much damage it all caused other than my cluster will come on sometimes and read random like speed will usually be random and oil pressure and temp stay in one place as well as battery voltage and my radio doesn't have any power, I was wondering if anyone knows where to find a replacement ecu part no. A18-654 M38? Or if there are any aftermarket ecu's that would work with the factory harness? I'm not really looking for anything to make any more power right now because I just want to get it running reliable and clean as since the fuel pump is on a constant voltage being hard wired instead of running through the ecu it runs hot and rich so I'm not driving it till I can figure this out. Any feedback appreciated!
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post #7 of 9 (permalink) Old 05-22-2019, 09:18 PM
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Originally Posted by gtree06 View Post
I've tested pin 108 on my ecu with a voltmeter. It would give me a reading of 2-4 volts when the key is turned to the "ON" position then drop to zero after a second or two. I'm not sure if this is how it is supposed to read or if it should be higher, or read a constant voltage?

I would expect pin 108 to be at 0V (measured to ground) for 5 seconds after the ignition is turned on. You should be able to hear the fuel pump running during this 5 seconds. This voltage is from the fuel pump relay controlled by pin 20.

After that pin 20, and the fuel pump relay, should turn off, and the voltage should rise to 12V. Both 20 and 108 are off.

When starting the car, both 20 and 108 are active and the ground voltage to the fuel pump should be at 0V.

30 seconds after the car starts, pin 20 should turn off and 108 should remain active what is the voltage then?

It would be good to measure the amperage draw of the fuel pump. If you have a clamp meter, put it on the positive wire to the fuel pump.

1984 300zx non-turbo
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post #8 of 9 (permalink) Old 05-22-2019, 09:41 PM
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Originally Posted by gtree06 View Post
If my ecu is faulty, because like I mentioned in the beginning one of the previous owners put the battery on backwards and I'm not sure how much damage it all caused other than my cluster will come on sometimes and read random like speed will usually be random and oil pressure and temp stay in one place as well as battery voltage and my radio doesn't have any power, I was wondering if anyone knows where to find a replacement ecu part no. A18-654 M38? Or if there are any aftermarket ecu's that would work with the factory harness? I'm not really looking for anything to make any more power right now because I just want to get it running reliable and clean as since the fuel pump is on a constant voltage being hard wired instead of running through the ecu it runs hot and rich so I'm not driving it till I can figure this out. Any feedback appreciated!

You can probably get an ECU from a junk yard.
This is a website that show parts from salvage yards everywhere. http://car-part.com

Here is a website the specializes in 300zx parts http://www.300zxpartsforyou.com/

1984 300zx non-turbo
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post #9 of 9 (permalink) Old 06-04-2019, 09:09 AM
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Originally Posted by gtree06 View Post
I'm not really looking for anything to make any more power right now because I just want to get it running reliable and clean as since the fuel pump is on a constant voltage being hard wired instead of running through the ecu it runs hot and rich so I'm not driving it till I can figure this out. Any feedback appreciated!

Having your fuel pump constantly on should not cause your car to run rich. The pressure regulator should maintain the correct pressure as long as there is enough fuel. I don't remember if you have done an ECU self diagnostic.

1984 300zx non-turbo
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