Thanks for all the suggestions. It looks like I have some homework!
I did check the flap on the AFM and it does not hang up. While I had it out of the car to remove the cover, I also carefully checked it with an ohm meter for any dead spots, or erratic behavior. It checked fine. The only thing I’m not sure about is the air temp sensor. It measured approximately 2,000 ohms at room temperature, but I don’t know if this is correct.
The fuel pump was replaced at some point, before I purchased the car. It is not stock 280, but seems to work normally. I’ve confirmed that both the dead head pressure and pressure at the fuel rail is correct and remains so, even when the engine is running rough. From this I conclude that the pump and regulator are doing their job.
I’m 90% certain that the problem is electrical in nature. The distributor is from a 280zx and includes the ignition module. The stock 280z ignition module was bypassed and removed, as was the ballast resistor. I’ve also replaced the ignition module on the distributor with a new one. Just by chance, I was checking for spark (using a spark gap tester) today, when the engine stalled. I was able to confirm, that even when the engine stalls, the coil produces a strong spark as long as the engine is rotating. Correct me if I’m wrong, but I think this would rule out the entire ignition system, including the switch, as a source of my problem.
As far as the tach is concerned, it is not erratic and does not race, or jump around when the engine stalls. It just follows the actual RPM. I have seen this on a BMW motorcycle, due to a failed Hall Effect sensor on the crank. This is not what’s happening here.
The fuses used to replace the links are the high amperage plastic tab type fuses. The installation is similar to that specified at http://www.atlanticz.ca
. In fact, the distributor upgrade also follows the instruction given at the Atlantic Z site.
As far as “unplugging” the AFM, I did not mean I removed the electrical plug. I was referring to the rubber coupling that goes between the TB and the AFM. In other words, intake air bypassed the AFM and went directly into the TB. The AFM was not grounded and in fact I found the short black ground wire had been disconnected. I reconnected it, but unfortunately, the problem persists.
The problem occurs at all speeds and does not seem to be RPM sensitive. The best way I can describe what’s happening, is to equate it to what would happen if you were running out of fuel. In fact, at first I was sure it was a fuel problem, but I’ve pretty much ruled that out, because the fuel pressure is good even as the engine is stalling.
I have confirmed that the problem is not the CTS. I did this by temporarily substituting a 330 ohm resistor for the sensor. I also installed a new Bosch connector at the CTS and got rid of the old bullet connectors. This did not solve the problem.
I’m starting to think that the ECU itself may be defective, or that the connector is intermittent. I’ve never had it apart, so I don’t know what type of components it uses. If it contains electrolytic capacitors, these are prone to drying out over time and could cause it to fail. I’ve seen this happen in other vintage electronics and in fact, it’s fairly common. I've also seen situations where solder joints between connectors and printed circuit boards fail, causing intermittent connections. I think checking out the ECU may be my next adventure in 280z troubleshooting!