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

[Patcon]

I hate to add a cook to the kitchen but is it possible the fuel rail has an internal restriction? I don't remember the vehicle history, but if there were a restriction near the return port that would keep fuel pressure high through all these test. Varnish, rust, contaminant...

[Captain Obvious]

18 minutes ago, mbz said:

Can I just put a valve in the vacuum line between the manifold and the FPR so I can get quick readings without having to replace the hose every time?

No. Not unless it's a three way valve and vent the regulator diaphragm to atmosphere when you move the switch to eliminate the vacuum. In other words, it's possible to do this, but not with your typical two port valve.

However... You can use a longer piece of tubing with a curly-loop in it ("pig tail") to make it easier to get on and off for testing. I have found that you can bend the two metal nipples so they aren't pointing exactly at each other and fit a longer piece of tube in there temporarily.

[mbz]

4 minutes ago, Captain Obvious said:

You can use a longer piece of tubing with a curly-loop in it ("pig tail") to make it easier to get on and off for testing. I have found that you can bend the two metal nipples so they aren't pointing exactly at each other and fit a longer piece of tube in there temporarily.

I'll give this a try.

29 minutes ago, Zed Head said:

Hose on - pressure reading.

Hose off - pressure reading.  Repeat a few times.

Tell us a story. Your pressure readings are so erratic throughout the thread that you need to do those comparison within seconds and tell what happened.

How many pressure readings do you think is sufficient? 10 on 10 off? More? Less?

I'm going to go hook up a longer hose.

[mbz]

14 minutes ago, Patcon said:

is it possible the fuel rail has an internal restriction?

It is possible, however the PO's Mech's notes indicate that the injectors were re-sealed with new mount holders and all fuel hoses replaced in July of 2018.

Point being, it's not like It's a 40+ year old fuel rail that has not been serviced recently.

[Dave WM]

he is talking about the rail itself, being steel it could rust internally. If you have vacuum and the FPR is still not responding and the return line was disconnected and directed into a bucket and you still have high pressure, then about the only thing left is rust in the rail. The only way to know about that would be to bypass the see if the return leg has any pressure build up. You will need to rig up a test line to see if fuel passes easily thru that metal hard line. Essentially replace the FPR with a T connector and run the pump, pressure should be quite low since there is no restriction

 

Edited April 13, 2019 by Dave WM

[Zed Head]

20 minutes ago, mbz said:

I'll give this a try.

How many pressure readings do you think is sufficient? 10 on 10 off? More? Less?

I'm going to go hook up a longer hose.

Enough to be confident that you see or do not see a difference.

You're kind of to the point where you should have a better feel for how things work, than you did when you started the thread.  Since the engine does run, you can poke around, take measurements, and see what happens.  If you just run through the list of things to do your progress will be very slow.

[Zed Head]

On 3/27/2019 at 12:31 PM, mbz said:

Ok got a chance to test fuel pressure today (just the fuel pump this time.... engine NOT running).

35.5 psi




Sent from my iPhone using Tapatalk

Let's review...

[Zed Head]

On 3/31/2019 at 9:45 AM, mbz said:

OK Guys, here's the results, ALL of the readings below are with the NEW BATERY installed:

Checking the vacuum on the fuel pressure regulator (FPR) as it affects the fuel pressure while the engine is running.

0). New Battery Voltage Reading: Everything OFF = 12.48 Volts, Engine Running = 12.12 Volts

1). Fuel Pressure PUMP ONLY = 29 psi. (see photo 1) [This seems VERY ODD considering last test with old battery was 35.5 psi]

2). Fuel pressure reading while the engine is running = 41 psi (see photo 2) [PRESSURE UP]

3). Fuel Pressure with FPR Vac. line disconnected = 44.5 psi (see photo 3) [PRESSURE UP]

4). Fuel Pressure while Sucking on the vacuum hose from the FPR = 40 psi (see photo 4 & video) [PRESSURE DOWN]

5). Fuel Pressure while Blowing on the vacuum hose from the FPR = 44.5 psi (see photo 5) [PRESSURE UP / NC]

 

And here...

[Zed Head]

Reread those posts and the ones right before them.  You're back to the same type of situation.  The same things apply.

[mbz]

24 minutes ago, Dave WM said:

Essentially replace the FPR with a T connector and run the pump, pressure should be quite low since there is no restriction

You mean something like what I drew in this picture?

[mbz]

21 minutes ago, Zed Head said:

Reread those posts and the ones right before them.  You're back to the same type of situation.  The same things apply.

I get it, sort of.

Re-run the fuel pressure tests several times to "try" and get consistent data.

However, only do it with the vac line to the FPR open or closed..... yes?

If so, do you think this rig will work to quickly check fuel pressure with vac and without vac? (see photo)

Where the VAC is open (or ON) if the hose clamp is open and the VAC is closed (or OFF), if the hose clamp is closed.

[Zed Head]

If you cut the line where I marked and have some sort of connector it would work.  But the clamp alone won't work because there will be vacuum inside the FPR.  You have to release that vacuum.

So, you'd clamp the hose, then release the vacuum.  That would close the potential vacuum leak, but let the FPR vacuum out.

You could also put a T fitting in the cut section and block the third connection.