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

[Dave WM]

31 minutes ago, mbz said:

When at idle, NO / negligible.

When RPMs are increased, YES.... LOTS of BLACK smoke.

you want to make sure the TPS sensor is working per the FSM, its on at idle, then off just off idle,  until about 1/2 throttle movement, then on again . think its pin 2 to ground (on at idle, off as you move away from idle) and then pin 3 to ground (on about 1/2 and up to full throttle).

Edited April 14, 2019 by Dave WM

[Zed Head]

The checklists in this book will tell you a lot.  Good news on the FPR, although you do have an extra one now.  Weird that you know what I'm cooking for dinner...

 

[Zed Head]

Has anybody confirmed that the injectors are the factory green or tan NA injectors and not the purple/brown turbo injectors?  Always worth confirming.  

[mbz]

29 minutes ago, Zed Head said:

Has anybody confirmed that the injectors are the factory green or tan NA injectors and not the purple/brown turbo injectors?  Always worth confirming.  

Zed, I knew you'd be back!

I have GREEN INJECTORS.

My 280z is a 1976 (built in 1975)..... it's right there in the title of this very, very, very, very long thread. (which at least 2/3 of it can be attributed to the bad fuel pressure gauge). ?

I have both the FSM & FI Bible (actually printed them both and put them in a 3 ring binder as well - I was feeling ambitious and bored).

Are you having waffles with your chicken? (it's very good)

Edited April 14, 2019 by mbz

[Zed Head]

Nah, just plain old chicken.

There's a lot of good stuff in the thread that you should recheck, all the stuff that people mentioned.  The EGR and the AFM glue blobs and the CSV and the coolant temperature sensor and the TPS (actually TVS).  The AFM and the sensor are the most likely.  Take a magnifying glass and see if you can tell if the glue blobs are not where they started from.  Check the coolant sensor circuit at the ECU connector first, not the actual sensor.  Make sure the TVS isn't on WOT.  Check, check, check...

[Captain Obvious]

Well those numbers look a lot better than the measurements you were getting with the previous gauge. I'm less concerned with the absolute numbers than the fact that it seems to be reacting to manifold vacuum now. Just changing the gauge shouldn't have had any effect on that, but whatever....  Now it just looks like most other old 280Z's that are running either really rich or really lean because of a whole bunch of interrelated issues like dirty connectors, improperly placed connectors, and vacuum leaks.

Back calculations from the pressure differential to calculate what the manifold vacuum is in psi... Do the numbers make sense?

16 hours ago, mbz said:

Here's the results:

Fuel Pressure with Fuel Pump ONLY: 36-38 psi (hard to tell from angle of camera, but well within spec)
Fuel Pressure with Engine Idle: 34 psi (RPM = fluctuates between 400-500 at idle)
Fuel Pressure with Engine Idle and OIL CAP OFF: 32 psi (RPM = fluctuates between 600-1,000 at idle)

With the filler cap on and the engine struggling at 400-500 RPM... Decrease of 2 psi converts to 4 inches of mercury.

With the cap off and the mixture more conducive to a smoother and higher idle... Decrease of 4 psi converts to about 8 inches of mercury.

Did I do that right, and does it make sense?

[mbz]

16 minutes ago, Captain Obvious said:

With the filler cap on and the engine struggling at 400-500 RPM... Decrease of 2 psi converts to 4 inches of mercury.

With the cap off and the mixture more conducive to a smoother and higher idle... Decrease of 4 psi converts to about 8 inches of mercury.

Did I do that right, and does it make sense?

Yes the conversion looks correct, I checked it here:

https://www.convertunits.com/from/in+Hg/to/psi

[Captain Obvious]

OK... So the more important question.

Does it make sense?   

[mbz]

45 minutes ago, Captain Obvious said:

Does it make sense?

Short answer is.... I don't know. (I'm not afraid to say it)

I don't think we need to assume that I'm ignorant after the fuel pressure gauge debacle.... ?

But kidding aside, I really don't know if that makes sense or not.

How is it supposed to perform?

[Captain Obvious]

Haha! Well a quick look on the web found the following:

Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 17" and 21" HG. A quick opening and closing of the throttle should cause vacuum to drop below 5" then rebound to 21" or more.
http://www.gregsengine.com/using-a-vacuum-gauge.html

So I'm not sure.

If a normal engine is supposed to be 18 in-Hg, and calculations indicate you're getting 4 in-Hg instead of 18? A massive intake manifold leak could maybe account for that, but it would be running way lean, not way rich.

I'm hoping someone can chime in and show me that I screwed the calculations somehow.

[mbz]

CO, I think you are right on all accounts and what I'm dealing with is a number of problems inherited from the PO, where they just kicked the can down the road on these issues to be dealt with later.... well later for the PO seems to be now for me.

I have read through the service notes from PO's Mech (pretty extensive), and it indicated that the engine was running rich (failed smog), and that there was a vacuum leak that was repaired, which would have made it run lean (although I do not have details about that yet - going to put a call into the PO's Mech and see if I can get more info about this). However PO's Mech. indicated that he leaned out the engine to pass smog.

The car did pass smog and was driving OK (I've put about 150 miles on it since purchased at end of January 2019, half of which was driving the car home after the initial purchase), up until this problem just presented itself when the car refused to start while out for lunch about 5-6 weeks back.

DaveWM's advice has been pretty spot on thus far, so I'm sticking with the next round of tests he indicated yesterday, which is:

1). Clamp off the fuel supply to the cold start valve to be sure it is not providing excess fuel into the engine if it is malfunctioning.

2). Check Water Temp. Sensor at 36 pin connector.

3). Check AFM air temp sensor and the Static reference resistance readings.

4). Check that TPS sensor is working per the FSM.

I'm reading up on these tests now trying to educate myself so as to be not quite as ignorant, and once I have the knowledge and the time (hopefully by this coming weekend or sooner), I'll be able to perform these checks and report back the findings.

Edited April 15, 2019 by mbz

[Zed Head]

45 minutes ago, Captain Obvious said:

If a normal engine is supposed to be 18 in-Hg, and calculations indicate you're getting 4 in-Hg instead of 18? A massive intake manifold leak could maybe account for that, but it would be running way lean, not way rich.

I'm hoping someone can chime in and show me that I screwed the calculations somehow.

I remember a past thread, maybe several, where intake vacuum was being used to diagnose an engine problem and they had a really low number.  My premise at the time was that intake vacuum only really showed the quality of the pistons, rings, and valves and wasn't good for much else.  Then they found a giant vacuum leak.  The lesson I got from that was that intake vacuum is really only good for the fine details.  I could be wrong, of course, but I don't get much out of intake vacuum readings when they are way off.

In theory, if the injectors are squirting enough fuel, you could have a leak so big right at the injectors themselves, that intake vacuum could be close to zero and the engine would still run.  Air and fuel get sucked in by the pistons, they don't care where it comes from.

Maybe the PO's mechanic didn't find the main vacuum leak and adjusted the AFM to compensate.  Might be time to be squirting some carb cleaner or similar at the usual leak points.  The base of the injectors is a good start, since they were worked on.

The big positive here is that the engine will idle steadily.  So you can run a bunch of tests.  Take good notes and don't assume anything.  Get numbers.