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

[mbz]

3 minutes ago, Zed Head said:

If you cut the line where I marked and have some sort of connector it would work.

You mean like the clear plastic connector in this photo?

[mbz]

I also took the lid off the AFM, it was silicone sealed shut.

It looks like some of the silicone sealer stuck to the metal coil on the upper right side. (see photo #2)

Anything look wrong with the AFM?

[jonathanrussell]

The silicone theoretically indicates that the flapper door tension hasn't been adjusted from spec. Adjusting the tension changes mixture. Of course, someone could have adjusted and re-applied silicone.

So, does the AFM flapper door move open and closed freely as it should?

[Zed Head]

43 minutes ago, mbz said:

You mean like the clear plastic connector in this photo?

The clear one would work.  You could clamp the hose with your clamp and break the connection at the clear connector.  The vacuum in the FPR will be released and pressure will rise.  The clamp will prevent a vacuum leak at the manifold.

[mbz]

JONATHANRUSSEL:

Yes the AFM flapper moves easily.

ZED HED:

I ran a set of 10 tests and got consistent results:

Fuel Pressure Engine Idling:

With Vac. Connected to FPR: 41 psi (every time)

Without Vac. Connected to FPR: 44 psi (every time)

I did not think of trying it with the hose clamp to prevent vac leak at manifold, but will try that next.

<------------------------- START SIDE NOTE ----------------------->

I am starting to believe that the PO's Mech may have adjusted the AFM to pass smog (his notes state that he "leaned out" the mixture when trying to pass smog.... but this does not explain why the car is running rich).

Take a look at these videos.... the first one shows the AFM while idling WITH the Vac hose connected. The second one shows the AFM WITHOUT the Vac hose connected.

Also, when the vac line to FPR is connected, if I hold the AFM flap in position (lining up the white marks) and do not let it fluctuate, the idle returns to normal and the fuel pressure drops to 34 psi (acceptable level).

However, when I let it go, the pressure rises back to 41 psi and then the symptoms return (rough idle), fuel pressure rises, and then engine dies.

<------------------------- END SIDE NOTE ----------------------->

[Zed Head]

You've found some good correlations but I think that the cause-effect might be off. Closing the AFM vane makes it act like a throttle blade, cutting off air (lowering idle speed), and also causing intake vacuum to increase (causing fuel pressure to decrease).

Maybe the vacuum leak guys were right, but your bad fuel pressure gauge is making things confusing.  You might have a big vacuum leak, causing low vacuum to the FPR which increases fuel pressure, a bad pressure gauge, causing the fuel pressure look high when it isn't, and and AFM that might be adjusted (except that it appears to be adjusted the opposite way to make things leaner).

You have a lot of bad information.  Bad in produces bad out.  Notice that your fuel pressure reading with no vacuum is now the highest it's ever been.  44 psi.  It keeps changing, nobody knows what it really is.  If it was mine, I'd get a new gauge first thing.  You can unscrew the gauge from the hose, you don't need a whole new setup.  Any oil pressure gauge will work also.  A 0 - 50 oil pressure gauge will probably screw right on there.

[Zed Head]

Unscrew gauge here - 

[mbz]

10 minutes ago, Zed Head said:

You might have a big vacuum leak, causing low vacuum to the FPR which increases fuel pressure

OK so maybe I should get a vacuum gauge and test the vacuum AND, get a vacuum puller to pull the proper vacuum on the FPR to ensure it is functioning properly? IF the proper vacuum is applied to the FPR and it functions properly AND the fuel pressure goes down when proper vacuum is applied to FPR, then it stands to reason that there is a vacuum leak somewhere causing the FPR to not function properly.

At that point it seems that checking vacuum at various points will give more info as to if and where a vac leak may be present.

Regarding the fuel pressure gauge, I'm not defending it, however at worst I think it's maybe 1-2 psi higher than it should be.

That last set of fuel pressure data was pretty consistent (albeit high).

Edited April 13, 2019 by mbz

[mbz]

Correct me if I'm wrong, but isn't there supposed to be an anti-stall dash pot right there?

[Dave WM]

with the consistant 44psi with no vacuum you have to figure out why that is. I would seem the vacuum is doing something.  so either the gauge is wrong or the return hard line is plugged up.

Not sure what the the adj of afm to white line means, did you manually close it (lean out)? if so yes the mixture is way rich and manually clsoing will improve the A/F ratio for better running more eff better vacuum etc... but its not the fix or at least the thing to go to yet.

Peeps aways want to monkey around with the AFM. That should be the last thing to mess with, there really is nothing to go out of spec, only issue could be worn traces. but that would not indicate any reason to adjust the tension.

Edited April 13, 2019 by Dave WM

[mbz]

1 minute ago, Dave WM said:

return hard line is plugged up.

OK so if the return hard line is plugged up, how do I clear it out?

Why not just test that and clear it out?

This would eliminate the clogged return from the puzzle.

It seems that I would want to disconnect the fuel line going into it and coming out of it, then flush it out with something?

Or is there a snaking wire I could run down the length to clean it out?

[Dave WM]

well its a possiblity, its pluged since I can't think of any other reason for the high pressure. you need to feed fuel from the pump thru the rail with the FPR out of the equation. You will have to rig up some way to bypass the FPR, so the fuel goes from the injector rail then directly into the return line where the FPR sits. Remove the FPR and rig up some pluming. Just make sure you do have a return path, you dont want to dead head the pump, they can make a LOT of pressure if there is no means of return, enough to burst hoses and spray fuel everywhere. with no FPR restricting the return flow the pressure should be very low.

Edited April 13, 2019 by Dave WM