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'91 Pathfinder fuel system issues

fastpakr

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I've been helping a friend look at his '91 Pathfinder that frequently takes 10-15 seconds of cranking to start. We put a fuel pressure gauge on it and confirmed that it's not holding any pressure after shutdown. If you're watching a gauge, it drops from ~35-40psi to 0 instantly after turning off the key. We checked the fuel pressure regulator vacuum line and there's no fuel there, so I don't think the diaphragm is ruptured. By holding a long screwdriver to each injector, we confirmed that you can hear each injector cycling - none of them appear to be stuck open.

Based on that, my assumption was that the fuel pump itself might have a bad check valve (we can't see any evidence of external fuel leaks around the lines at all).

However, there's a second issue which is throwing me a little bit... You can only hear the pump prime maybe 10% of the time that you cycle the key to the run position. Every now and then you get a 5 second priming pulse. Most of the time there's none at all. If the priming pulse happens, you can watch pressure build on the gauge to 30 or so and then fall instantly when the pump stops. I can't wrap my head around what would cause the pulse to only happen infrequently like that. There are no obvious issues with the pump relay, and it the pump runs when there are ignition events to keep it turned on.

So... short version:
1) Fuel pressure drops instantly when the pump isn't running. Best guess - bad check valve in the pump.
2) Priming pulse only happens occasionally. When it does, you do see pressure build properly (and return to zero immediately after the pulse).
3) During sustained cranking, pressure slowly builds from 0 up to 35 or 40 over several seconds of cranking.

Thanks in advance for any suggestions!
 


dvdswan

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Dirty ground. Famous on Nissans. I would check the grounds at the tank for the pump, ground near the relays, and the ground for the ecm inside the cab under the seat.
 

fastpakr

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Interesting thought. Is there a ground that is somehow specific to the ECM commanded priming pulse but not required for the pump to stay running when ignition events happen? Because it doesn't seem to have any trouble continuing to run while cranking the engine or after it's started.
 

RonD

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Not familiar with the 1991 Nissan fuel system

If there is a fuel pump RELAY then it could be at fault or if it has a Fuel pump DRIVER module it could be at fault

Not sure what the spec is on fuel pressure with key off, 0psi could be correct
But it certainly should NOT be 0psi with key on just before starting or when cranking engine over with starter

It could be the computer/driver needs to see RPMs(starter motor turning engine) before it turns on fuel pump, not just with key on, its just a software thing

Most engine computers start spark first and then injectors after 1 or 2 seconds once RPMs are detected
 

fastpakr

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Not familiar with the 1991 Nissan fuel system

If there is a fuel pump RELAY then it could be at fault or if it has a Fuel pump DRIVER module it could be at fault

Not sure what the spec is on fuel pressure with key off, 0psi could be correct
But it certainly should NOT be 0psi with key on just before starting or when cranking engine over with starter

It could be the computer/driver needs to see RPMs(starter motor turning engine) before it turns on fuel pump, not just with key on, its just a software thing

Most engine computers start spark first and then injectors after 1 or 2 seconds once RPMs are detected
I don't have the specs available, but it definitely -does- do a priming pulse sometimes. But when that happens, the pressure bleeds off immediately (as in, if you weren't looking straight at the gauge, it's gone before you look back).
 

dvdswan

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Fuel pump relay is on the pass fender wall. You could swap with another relay (sorry dont remember exactly which one probably AC though) and swap it with the fuel pump relay. They are identical.

I suggested grounds as the grounds can cause some weird crap on Nissans and this is sort of intermittent.

edit: on my 96 IIRC it was 43 psi needed to start. Not sure on the 91. BTW, you can find help here www.nicoclub.com/fsm/Hardbody
 

fastpakr

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Fuel pump relay is on the pass fender wall. You could swap with another relay (sorry dont remember exactly which one probably AC though) and swap it with the fuel pump relay. They are identical.

I suggested grounds as the grounds can cause some weird crap on Nissans and this is sort of intermittent.

edit: on my 96 IIRC it was 43 psi needed to start. Not sure on the 91. BTW, you can find help here www.nicoclub.com/fsm/Hardbody
Your ground suggestion makes a lot of sense. I'm just trying to narrow things down a bit since clearly the ground -does- work when the engine is turning (it builds pressure steadily once you start cranking and never seems to drop while the engine is running). If there's a separate ground path in play when the priming pulse is triggered, a wiring fault there could explain a lot here.

For what it's worth, the fuel pump relay was replaced previously when he was troubleshooting this problem.
 

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I would have someone with finger on the fuel pump relay to see if its working, "clicking" closed reliably with key on
If so then fuel pump or it's ground may be the issue
If not then relay activation would be the issue
 

fastpakr

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Thanks. As far as the pressure bleeding down instantly - would you suspect the pump for that as well?


Edit... I'm guessing I could use a meter to probe for signal to the relay when cycling the key on to see if the relay is even being triggered consistently for the pulse?

Edit 2...
FPR.png
His FPR should look like the image above.

On a normal 4 pin relay:
Pin 30 - 12v constant
Pin 85 - trigger wire for pump
Pin 86 - should have continuity to ground
Pin 87 - output to pump

On this one – I assume 3 is 12v input (pin 30 traditionally), and 5 is ‘87’ sending power to the pump. Is 1 the trigger wire (85) and 2 the ground (86)?

So I can grab a meter and confirm if I get a 5 second pulse of 12v on the trigger lead (terminal 1) every time the ignition is cycled on?
 
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Ranger850

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fastpakr

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dvdswan

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I'm on there everyday...
 

fastpakr

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Maybe it's just the SUV section that's dead. '99 path dash lights' was the only thread in there with posts since mid June. I did put a thread up at nicoclub and was going to at nissanpathfinders.net but haven't gotten an email confirmation back from there yet.
 

Ranger850

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LOL, aside from the part where that forum has had two posts in the last 60 days?
Yeah, kinda like the F-150 section here. BUT the info might be there.
 

fastpakr

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We swapped the pump yesterday. It did seem to start slightly faster, but definitely isn't priming every time yet. I'll try to do a bit of testing at the relay location the next time we can stare at it (and check with a gauge to see if it's holding pressure at all).
 

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