Fuel pressure/priming issues on a '91

[fastpakr]

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!

[VStar650CL]

Pump priming is only about 1.5 seconds on most Nissans. Sounds like the fuel pump motor has bad brushes, causing it to spin up slowly or not at all when given a short pulse.

[fastpakr]

Installed a new pump yesterday. Haven't had a chance to put a gauge on it afterwards, but it's definitely not priming every time afterwards.

For what it's worth, when it -does- prime, the pulse is a lot longer than 1.5 seconds. I'll have to measure it, but 5 seconds still seems about right.

[VStar650CL]

Pump relay, maybe. Check the voltage when it doesn't prime, if it's low then the relay probably has worn-out contacts.

[fastpakr]

Thanks. I've got some notes on the Nissan relay. Can you sanity check this?

FPR.png (174.2 KiB) Viewed 442 times

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'm assuming that I can pull the relay and confirm with a meter that it's receiving voltage on pin 1 for a few seconds when the key is cycled on. If so, the ECM is at least attempting to command a priming pulse and I need to look at the relay (or further downstream) for the issue. If not, there's some reason upstream from the relay that it's not getting voltage to fire up the pump.

[VStar650CL]

One of the terminals 3~5 should be hot all the time, the other will be the output to the fuel pump. That's the voltage you want to check. With the relay on, if there's a substantial voltage difference between the always-hot terminal and the output terminal, the relay contacts are crappy. Most Nissan relays are driven from the ground side of the coil, so one of the terminals 1~2 should either have keyed power or power all the time. The ECU will pull the other terminal to ground to activate the coil. It's very rare (although not unheard of) for a relay coil to cause intermittent operation, so check the contacts before worrying about the coil.