My FPR is stock. I think the error is more likely in the inline gauge but I do have a spare stock FPR and can try that one to be sure . I haven’t heard many good things about the aftermarket ones. Hard to avoid the Chinese crap:
I like Jim’s idea to test to contacts to the temp sensor at the ECU connector.
I have a hunch this is why I’m running rich and the plug fouling is why it’s running like crap.
I have the valve cover off now and will redo the valve lash while cold. Most were about + .001 off while warm. Cold it might be worse. I noticed the oil I had a very black tint and excessive for only 500 miles of driving. More confirmation of a rich running engine.

What coil do you have?

Edited April 16, 20232 yr by siteunseen

My first guess would also be the temp sensor unit.

Pull the big connector off the ECU and measure resistance between (harness side) terminals 13 and 16. That should be temp sensor and ground. Looks like this:

Compare the resistance measurement result to this.

Thanks for the chart Bruce.   I printed it out and plotted the 4 sensors I had including the one I pulled from the head.  
The one that came off the car had the worst/richest curve of all of them.   
So I decided to install one of the other 280z ones into the thermostat housing.   That was a small disaster.   First none of my extras fit im that housing.   This is something that Nissan did that ticks me off.   All the different lower thermostat housings and a myriad of odd and non consistent openings across the years thread size/type and opening sizes…..unbelievable!  Well once I found out non of the holes worked and about a quart of radiator fluid on the garage floor I cracked one of the openings by over tightening it.   Dummy! 
so I had to remove it and make more of a mess and roll into one of my back-up housings in the box of parts.   Have two other and they both have different opening and threat types.   Are you serious!  I finally got a combo together and filled the extra holes with needless sensors.   Got it all re-installed and put it all back together.   Then cleaned the garage floor before my dogs drank the anti freeze.  
Job is not complete and ran out of time. Here is the temp sensors graph I made putting them in hot water on the stove.  
 

Blue was the one installed on the car.  

Edited April 17, 20232 yr by Av8ferg

Here is Nissan's documentation from the 1980 EFI book.  Since you're mostly looking at a warm engine, the resistance at 180 is probably the most relevant.  I'd guess that their spec is 250 to 275 ohms, by eye (edit - probably more like 300 to 350 on the high end since the line is 500.  Of course, they are just lines on a piece of paper [pixels on a video screen]).  Seems to match what CO's plot shows.

Yours all seem rich.  Did you measure at the sensor pins or at the ECU?  Might have resistance at your bullet connectors in the sub-harness oe at the EV1 connector itself.

Edited April 18, 20232 yr by Zed Head

I read more of your details and see that you have 180 marked.  How did you get the water temperature set?  Must have taken some time.  Could be some error there.

You might jump the pins at the sensor connector in the harness at the thermostat housing then measure continuity at the ECU connector between 13 and 16 like suggested.  It should be very low, like under 5 ohms.  If it's good you won't have to mess around with the wires.

Zed, thanks for the FSM ref. I put all the sensors in a pan of hot water with a thermometer in the water and heated the water to various temps on my stove (wife was at work [emoji6]). I will say the sensors react very rapidly to cooling.
I wanted to get a baseline of the sensors before I put them in the car.
I think I’m going to order a couple additional ZX sensors and see what I find.

Well I think I’ve sorted this out. Here’s what I did.
- New spark plugs
- valve lash check and adjust
-move temp sensor to thermostat housing and replace.
-oil change (needed anyway only I looked under valve cover)

She’s running great again and not rich anymore. No more spitting black mist out the tailpipe.

Nice, I like stories with a happy ending..

Good work!