1976 280z engine runs rough then dies, will not idle.

[mbz]

OK I think I found the problem..... clogged fuel return line (the metal one under the car - I tested the rubber fuel return line 2 weekends back and it was fine.).

Follow this to see if I am correct.

1) Removed the FPR.

2) Clamped off one of the fuel input lines on the fuel rail. (the right side closest to driver)

3) Connected a 50 psi fuel hose to the return line where the FPR fuel return would go.

4) I blew into the return fuel line hose that I hooked up.

5) After about 50 seconds it got more difficult to blow into the fuel return line and I noticed air coming back from the fuel return line

6) About 10 seconds later, gas started flowing back from the fuel return line (almost like a siphon).

I clamped it off so it would not keep gushing fuel out. I am guessing it would only be as much fuel as was in the return line to the tank where the clog is. If that's the case I would venture a guess that the clog would be about 1/3 to 2/3 down the metal line based on how long I blew into the return hose.

let me know if this is the correct diagnosis.

 

Edited April 14, 2019 by mbz
removed videos to save server space.

[mbz]

Also, I just went and took the gas cap off and it sounded like a vacuum being released.

Not sure if this helps confirm or debunk diagnosis.

- - - ALSO - - -

After depressurizing the fuel tank, I took the clamp off the fuel return hose that was draining to the bucket. No more back pressure.

So I connected the other end to the fuel rail, and checked the pressure with just the fuel pump cycling fuel through the fuel rail and back to the fuel return without the FPR inline.

The fuel pressure was 8 psi.... consistently. (I tried it 5 times)

So maybe it's not a clogged fuel return line after all.....?

Edited April 13, 2019 by mbz

[Dave WM]

I don't know if you hooked it up the way I described, it takes about 3" total hose to bypass the FPR and then block off one of the rail ends.

That stream of fuel looked pretty weak if it was from the bypassed line.

[mbz]

4 minutes ago, Dave WM said:

it takes about 3" total hose to bypass the FPR

I used a long length of hose (like 3 feet)

[Dave WM]

the test you did by blowing into the return line only pressurized the tank since nothing was coming out. you need to hook up as I described, with the return line to a bucket, and then check pressure with the pump on engine off.

in operation the pump is taking out and putting back into the tank (slightly less going back) so a vacuum happens in the tank. the gas cap has a one way valve that allows outside air to replace the gas as its consumed.

Edited April 13, 2019 by Dave WM

[Dave WM]

I am posting up a video.

 

[mbz]

OK, I'm going to use a short length of hose to loop the fuel rail to the fuel return.

Then run the fuel return hose line to a bucket where the fuel return hose connects to the metal return line under the car. (return will not be going to tank).

Then check fuel pressure with JUST THE PUMP.

Stand by while I rig it up and get results....

[Dave WM]

 

[mbz]

OK I did the test and got 5 psi.

To be sure I got the test set up and performed correctly.....

See video #1 for test results.

See video #2 for test set up & configuration.

Edited April 14, 2019 by mbz

[Dave WM]

5 psi does not sound restricted, I presume you had a very solid steam of fuel being pumped.

Now one step  at a time

hook up the FPR ONLY to where you attached the line from the gauge to the fuel rail (taking the rail out of the equation completely) hook up the return line from the top of the FPR to the bucket, block off the other side of the FPR. Testing JUST the FPR now to see where is hold pressure with no vacuum.

[Dave WM]

For fun Tomorrow I will check the PSI on my setup with just the rail and no FPR to see if I get the same 5 psi.

[Zed Head]

Looking for causes for the variety of different readings.... I notice that gauge is upside down in the 5 psi reading.

If you can get the gauge and FPR free on the end of the hose shake it around, turn it upside down, sideways, etc. and see if the reading stays constant.

Just for reference, most of us would connect up a gauge, get the pump running, and the needle would jump to 36 - 38 psi and just sit there.  Nice and steady, within seconds.  Maybe you have something loose in your gauge.  You really should not be having this much trouble getting a good consistent reading.  Even if it's high, it should be 44 psi every time you disconnect the vacuum hose, or just on battery power.  Even with a weak battery, really, you should get 36 - 38 psi.  A weak battery will just take longer to get there, but probably not even a noticeable delay.  The FPR does not open until the spring pressure inside is overcome.  It's a pressure relief valve.