77 280Z Dropping Fuel Pressure once Warm

[Z-Heap77]

Is that gauge from lowes an adequate replacement for the fuel rail? i want to try to find something locally so I don't have to go through the whole process of ordering it online and waiting for it to arrive. Or does anyone know of another gauge from Advanced, Auto Zone or something like that more suitable for a fuel application?

[Zed Head]

The liquid-filled gauges are known to be sensitive to heat. Many have warning labels that say so. But they're cheap and durable. The dry gauges are sensitive to vibration. So really, even though people use them, there is no gauge that is really suitable for long-term use on a fuel rail. It's just too hot and vibrationy.

If it were me, I would get another gauge, that's in the right price range, stick it on the rail, and check pressure cold using just the pump. It if reads 36-37 psi, you'll know that the other gauge was bad and you can look for other issues. If it it still won't hold 36-37 psi, then you need to figure out why your new FPR is isn't R'ing. You have a new FPR and a new pump, you should easily see 36-37 psi immediately by turning the key to start with the solenoid wire disconnected.

You could also borrow a fuel pressure a gauge and just use it temporarily, in-line between the filter and the rail. Then you'll know if you need to replace your rail gauge.

[Z-Heap77]

Ok. So I have some new information to share. I'm still stuck and the car is still acting up, but here's what we've done.

I got some additional hose, the Lowes gauge and some direct nipples to try the gauge in different areas of the system. I pulled the fuel filter and stuck the gauge into the fuel line come straight off the pump. Cranked the engine a few times and it show up over 40 psi. I got the same thing after the fuel filter. We started the car up and it was running fine. We let it sit for a while a warm up. Continually checking the car's ability to rev without any breaking up. It continued to do this without any issues so we decided to take the care for a ride. BTW the gauge on the rail was reading about 26psi when it first started and eventually got down to 20psi before we took off to test drive. I didn't worry about that anymore because I think it's clear that it's not longer accurate.

We drove about a mile up the road and all was good. We decided to drive to the gas station to get som fuel because we were low. That's about 8 miles away. We pull into the gas station, put the car in reverse to back up to the pump and that was the end of it running well. It immediately started breaking up if you gave it any throttle. We couldn't get the car to move under power. We had to push it off to the side. The car continued to idle just fine, but would not take any throttle increase.

Someone happened to stop and asked us if we needed some help and he claimed to work for a mechanic shop that specializes in muscle cars and older cars. He asked some questions and after our description of what had been happening he suggested that the fuel pump relay could be failing when it gets hot. This would explain why it behaves poorly when hot, but it you let it cool off it will run well again and then become poor running when hot again. Could this be possible?

The electrical aspect of things is where I lack experience. I've tried doing some searching online for where the fuel pump relay is located on the 77 and I'm coming up with tons of contradicting information. The EFI Bible states that the fuel injection relay is a two part relay. First part is the Power Relay and the second part is Fuel Pump Relay. I found that part in the driver's foot well above the ECU. There is one mounted in the factory location with nothing connected to it, and there is another one that is connected and stuff up above. So it's been replaced before and it looks like the previous owner couldn't get to the screws to remove the old one. However here's where it gets really confusing. The Black Dragon parts book lists the Relay-Fuel Pump as a single relay, not this much larger double relay. So I'm lost as to what exactly I should be looking at or testing.

Can anyone shed any light on the electrical aspect of this? If the fuel injection relay (larger double relay) is the only relay or if there is another single unit and where that might be?

[Zed Head]

You're getting closer. If you could have measured fuel pressure while it was running well, then when it was not, you would know a lot more. Instead of a direct nipple, a T-fitting might have been better, with the gauge left under the hood in a cool spot while driving. If the problem was the fuel pump relay you would have seen low fuel pressure. But even watching the incorrect number on the bad gauge would help. If it shows 20 psi when it's running well, and 20 psi when it cuts out, then you could reasonably assume that it's not lack of fuel pressure causing the problem.

The 40 psi is on the low end for maximum fuel pressure but it's close to the spec. for the bypass on the factory stock Nissan pump. Did you get a brand new Nissan fuel pump or an aftermarket pump? The aftermarket pumps typically will reach 90 psi. The Nissan pumps have an internal bypass that kicks in at 43 - 64 psi.

I would T that gauge in between the filter and the rail, and take another drive. Take a note of fuel pressure several times while it's running well, then as soon as it cuts out get another. Proper fuel pressure is critical and you just don't have any good data on what it really is when the problem happens. You have to get good numbers to understand what the system is missing.

[Z-Heap77]

Just to add to a little more of my previous update. We just drove up to the gas station where we left the Z several hours ago. It started right up and we drove it home with no issues at all. We didn't leave it running long enough to heat up and start behaving poorly, but do you think that confirms it's an electrical issue with the fuel system and not something mechanical?

One more thing I'd like to note. I pulled the AFM just to check the flap and make sure there isn't any issues with that. All seems well. We also checked to make sure the pump would come on with the key in the ON position and pushing the flap open. The pump fired right up, but... here's the funny part. The gauge on the rail immediately showed 36psi. That's the first time that's every shown that high of fuel pressure. So what does that tell us?

[Z-Heap77]

Zed,

Thanks for the response. Just a quick note of reply to your message. When the car was running poorly at the gas station the pressure went as low as 10-15 on the rail guage. After sitting at only idle for a while it raised back up to 20. I do have the T as well so I can put the alternate gauge in line between the filter and the rail.

Do you have any comments on the rail gauge reaching 36psi when the pump was running by opening the flap on the AFM?

[Zed Head]

The 36 psi with pump only (engine not running) shows that the pump and regulator and relays can work correctly. That is actually normal, and is one of the standard tests of the system. But that's with everything cold.

One thing you could try to see if it's electrical is to just hard wire the pump directly to battery power. If it works, then you know that you have an electrical problem. I had an AFM problem once, on the road, and used the back window defroster circuit to run my pump. They're conveniently close (two connections at the top of the window), can handle the current and have a switch (the defrost switch). You can run the wires out of the hatch. Or you can find a way to wire it in at the relay. You're kind of at that point where you need to break the problem in to pieces.

Edited June 21, 2014 by Zed Head

[Z-Heap77]

When you say to hard wire the pump directly to the battery do you then mean to attempt to drive the car that way and see if I have the same problem? If the pump is hard wired to the battery then it's not using any of the sensors and/or relays throughout the system to get it's power, is that correct?

[Zed Head]

Yes, take all of the relays and switches (except the defrost switch) and the AFM micro-switch out of the picture, to see what the pump and FPR do when they have a solid 12+ volts like they should. If the system fails, you'll know that the pump and/or FPR and/or tank screen or fuel supply, are faulty. If the system works fine, you can focus on the relays and switches.

From there you can break it down even smaller, like FPR versus pump, or AFM switch versus fuel pump relay. The potential sources get reduced until you're left with one.

[Z-Heap77]

Do you have any info regarding the fuel injection relay and the location of the fuel pump relay? Are they one in the same or is there a separate fuel pump relay in the car? I looked under the passenger seat and there is a relay there, but its not even plugged into anything so I have no clue what that is. I didn't see anything in the passenger footwell and all I saw in the driver's footwell is the fuel injector relay.

[Zed Head]

The EFI relay and the fuel pump relay are combined in to a 1x1x2.5 inch silver box, above the hood latch release handle, under the driver's side dash. And yes, the screws are hard to reach, mine only had one installed when I got my car.

[FastWoman]

I don't know where the relays are in a '77, but the '77 is more like a '78 (my car) than a '76 (Zed's). Nissan located the relays in various positions throughout the years. However, on a '78 (and maybe also a '77?), the relays are hidden within a black, box-like housing screwed to the inside of the passenger inner fender (i.e. in the engine compartment). Rossiz is giving away his relay housing on the pay-it-forward thread, so you can see one pictured there. There's one EFI relay and two fuel pump relays. One fuel pump relay enables/disables the fuel pump based on whether there's alternator output and/or oil pressure, and the other relay simply feeds power to the pump.