My Z dies after it gets warm. After I’ve been driving it for a few minutes and let it get up to temperature it will die. If I keep air moving over it it will stay running a lot longer. Once the Z is warmed up and I stop moving it will usually die pretty quickly. After I let it cool down (10-15 minutes) it will start right back up. Before I parked it for the winter it ran really good other than this problem. This spring it is running really rich, but other then that has a ton of power once you get into higher RPM and boost. There are no codes.

I realize the dying problem generally points to the PTU, so I replaced it.

I have a 1990 300zx TT with about 65k miles. It's deleted, has an aftermarket air filter, and a boost controller (turned all the way down if that matters any). It also has a new temperature sensor that OHMs out correctly. It has a new CAS and new (used) ECU. It has additional grounds going from the battery to the plenum, body, and HICAS. I pulled the motor, put in a new engine wiring harness (probably what is causing the problem), and replaced as many of the hoses as possible. It did not have this problem before I pulled the motor and replaced the wiring harness.

I got it all back together and started it. Drove down the road and then it died. Replaced the PTU (the old one didn’t pass the FSM test), problem remained with the new PTU. Moved the PTU by the air filter, problem remained. Also replaced the temperature sensor. Decided to take it to the local dealer (their head mechanic has been there 20 years and said he's been working on 300zx’s since they were coming off the lot...figured that was my best chance since there are no Z specialty shops around.. I'm in Salt Lake City). He told me it was my PTU. I told him it was brand new. Then he told me it was my CAS (I had already tested the other one and it worked, but oh well), replaced the CAS. Still had the problem, he said it's either the PTU, CAS, or ECU. So I replaced the ECU as well. It still dies. I have double checked grounds several times. I'm not sure what else to do. I've been trying to follow a Code 21 sticky from another forum and have found that my coil ground voltage drops to .38 volts (negative probe on plenum) and .36 volts (negative probe on coil ground wire) when the issue is present. After a few minutes of the engine bay cooling down they go back to battery voltage. I checked the voltage of the ignition coil relay and this is where I’m confused and don’t know what to do next. This is terrible, but I’m going to refer to the relay terminals as if you’re standing at the front bumper and looking at the fuse box with the Ignition Coil relay removed.

When the problem is present (engine is hot) all terminals, except the bottom left, show over 12.3 volts. The bottom left terminal shows .27 volts.
When the engine cools off (and can be started) the relay terminals show as follows: both right terminals are 12.05 volts, the top left terminal is at 1.66 volts, and the bottom left terminal is at .09 volts.
Please help me figure out what is causing the issue, I have no idea what to do next.

Sounds like you have a corroded power wire somewhere in the system or a bad connection on the back of the relay box. Resistance rises with temperature, if your not under load and keeping your alternator producing power (so at idle) the resistance gets too much and the car doesn't stay on. You need to perform a voltage drop test on the suspect wires under hot and cold conditions.

Thanks for the reply.

I'll try to figure out how to do the voltage drop test. The car will also die under load/while driving. But it's pretty much a guarantee if I just let it idle for a bit it will die. Also, if I'm driving the car and get it a little warm and then park it/shut it off (before it dies), something seems to get heat soaked and I won't be able to start the car for a bit.

electrical gremlins are generally hard to isolate especially temperature dependent ones, isolating an area, system, and/or condition will help immensely in your search.