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

[Dave WM]

Indeed that I odd. get a battery reading with the engine running vs not running. should be about 12v with just the pump running (engine off) with it idling I would expect to see 14v MAX.

typical behavior would be about 36 psi engine off pump on. Start the engine idle,  and drop to about 30 psi as the vacuum is applied to the FPR thru the small vacuum line.

I like ZH more likely guess of low voltage from a weak battery OR bad battery cables.  Make sure you have a quality set of battery cables. also agree that pressure could cause higher FPR reading but can not see how you could get a positive pressure in manifold in a NA engine.

Edited March 27, 2019 by Dave WM

[mbz]

ZH & DaveWM, thank you for responding to this, I appreciate your help.

I just checked the battery, it's giving me a reading of 12.13 volts.

Tomorrow I should be able to test the voltage with just the fuel pump, and then also with the engine running.

Will report results.

Agreed this seems very odd, the fuel pressure should drop when the engine starts, but it's increasing.

I'm trying to understand how it is that if the battery has low voltage, the fuel pressure increases.... wouldn't it stand to reason that if the fuel pump was not getting enough voltage from the battery, it would be under-performing and thus delivering LESS pressure, and not MORE pressure like it is?

Edited March 28, 2019 by mbz
added photo

[Dave WM]

that's a weak battery, get a good healthy new one, make sure you have good quality cables (not the kind that clamp on the wire) Make sure the ground connection from the batter cable to the chassis is good. Little things like this can add up and make chasing odd problems an issue. I was able to find a nice positive lead with TWO extra aux leads all molded into the cable at a local car parts store.

One problem at a time. you clearly have a weak battery, so the engine off voltage will drop even more with the load of the pump. Next when the alt is working (engine on) the voltage jumps up and the pump will have power running thru it, increasing the fuel pressure.

This is why its important not to jump ahead like I already did. get one thing resolved at a time. 1st now is it get a proper working voltage out of the battery.

a fully charged battery should be something like 12.8 min  (no load), is does not sound like a lot but 12.13 is not healthy. It will be interesting to see the voltages pump on engine off and pump on engine on.

Ultimately you need to get the system working as described 36 psi engine off pump on, 30psi engine on. If that is not happening its pointless to move on to other test. the efi is a open loop system with no ability to compensate for out of spec conditions. the pressure must be correct, the sensors must be correct for the entire system to work (think ZH has already mentioned this). Modern cars with O2 sensors and real computers and adapt and keep a car running with all kinds of system failures (but then you get that dreaded "check engine").

You can still check the return line while working the battery issue out, it only takes a min to disconnect the line in the engine bay, remove the fuel tank filler cap, and make sure you can blow air back thru it.

Worst case the line is blocked, not easy to fix. If its ok and the battery supply is ok, then the FPR maybe not regulating properly. this would require more test with a controlled  vacuum source to make sure the problem is with the FPR. again no point in any other test till you get the 36/30 engine off/engine on with the pump running. 38PSI at idle is more fuel pressure than with the engine at wide open was designed for. no wonder the plugs foul.

 

 

 

 

 

Edited March 28, 2019 by Dave WM

[mbz]

OK so I had a chance to test the voltage with the engine running, it jumps between 12.95 - 13.07 volts. (see video)

Then I tested the battery voltage with just the fuel pump on and it reads 12.34 volts. (see photo)

I did a visual inspection of the cables and ground connection, they appear to be OK.

Will pick up a new battery this weekend.

[mbz]

Not sure if this helps with the diagnosis, but another quick test I did while I was at it, was to disconnect the air hose from the intake manifold to the master vac.

Fuel pressure dropped (although still high) and engine idle went way up. Also, was able to adjust idle with the idle screw.

Not sure if the helps with the diagnosis. (see video)

[Zed Head]

15 minutes ago, mbz said:

OK so I had a chance to test the voltage with the engine running, it jumps between 12.95 - 13.07 volts. (see video)

Then I tested the battery voltage with just the fuel pump on and it reads 12.34 volts. (see photo)

So close, so close.  I can even see the pressure gauge in the back of one of the pictures.

We're really more interested in the fuel pressure at those voltages.  We don't really care about the voltage itself.

 

Looks like that latest video might have it.. let's watch.

[mbz]

Just now, Zed Head said:

Looks like that latest video might have it.. let's watch.

The latest video is with the air hose from the intake manifold to the master vac disconnected.

[Zed Head]

2 minutes ago, mbz said:

Not sure if this helps with the diagnosis, but another quick test I did while I was at it, was to disconnect the air hose from the intake manifold to the master vac.

Fuel pressure dropped (although still high) and engine idle went way up. Also, was able to adjust idle with the idle screw.

Not sure if the helps with the diagnosis. (see video)

Do that same exact test but disconnect the hose from the FPR to the intake manifold.  That's the test!!!.  So close.

[Dave WM]

12.34 with the fuel pump running is not that bad. But it still is a good idea to have a new battery if its even suspect. I replace my battery every 3 years or so even if it test ok. I use a napa legend series fyi.

Now back to the original problem. Just to be sure do the pressure checks again, this time with the running, then turn it off and get a engine off (pull the solenoid key to start) this way the batt will have a fresh charge on it. I just want to see those odd readings again that you had previously shown. If that is still the case then you need to check that return line and if ok it almost has to be something wrong with the FPR. You can test I have a test showing compressed air to trigger the 36 psi bleed off. In your case you would need to supply a vacuum as well to confirm the FPR operation.

 

took a long time to type this, did not see the videos yet

 

 

Edited March 28, 2019 by Dave WM

[Zed Head]

1 minute ago, mbz said:

The latest video is with the air hose from the intake manifold to the master vac disconnected.

The FPR, disconnect the vacuum hose to the FPR with the engine running.

[Zed Head]

Disconnect the hose to the FPR with the engine running and read the pressure on the gauge.  That's the test.

[mbz]

OK, so to be clear.... you want to see a video of what happens to the fuel pressure gauge when the engine is running with the vacuum hose disconnected from the FPR to the Intake Manifold....? Is that right?

Also, is there another place I could disconnect the vacuum hose from that is easier to access that would give the same results you are looking for...? (ie; the vacuum line going to the carbon filter canister).