Fuel System Related (most likely culprits):
* Fuel Pressure Regulator: A failing regulator might not maintain sufficient pressure after the initial start. This is a strong suspect given your observation. Testing fuel pressure is crucial here. You'll need a fuel pressure gauge to confirm pressure is adequate both at initial startup and during running.
* Fuel Pump: The fuel pump might be weak, unable to deliver enough fuel consistently. It could be failing to prime properly or struggling to keep up with demand. This would also manifest as a car that runs for a bit then dies.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms. This is a cheap and easy part to replace, so it's a good idea to check it first.
* Fuel Injectors (if equipped): While less likely on a '75 280Z (which likely uses carburetors), if it has fuel injection (unlikely but possible depending on the model year and region), clogged or faulty injectors could be the problem.
Ignition System Related (also high probability):
* Ignition Coil: A weak ignition coil might provide enough spark initially but then fail as the engine runs.
* Distributor Cap and Rotor: These components can wear, leading to poor spark distribution. Cracks or corrosion in the cap or rotor can cause misfires and stalling.
* Ignition Switch: A failing ignition switch could intermittently cut power to the ignition system.
* Pick-up coil in the distributor: This component plays a key role in triggering the ignition system. If faulty, it won't consistently supply ignition signals.
Other Possibilities:
* Air Intake System: A vacuum leak in the intake manifold or elsewhere can disrupt the air/fuel mixture, causing the engine to stall.
* Crankshaft Position Sensor (CMP sensor): If equipped with electronic ignition, a faulty CMP sensor might prevent the engine from running consistently.
Troubleshooting Steps:
1. Check the fuel filter: This is the easiest and cheapest thing to check first. Replace it if it's dirty.
2. Test fuel pressure: This is absolutely crucial. Rent or borrow a fuel pressure gauge to see if the pressure is sufficient and consistent.
3. Inspect the distributor cap and rotor: Check for cracks, corrosion, or worn contacts.
4. Check for vacuum leaks: Carefully inspect the intake system for any cracks or loose connections. Use a propane torch (carefully!) to sniff for leaks near the intake.
5. Examine the ignition coil: It may not be easily visually inspectable; testing the coil's output would be needed if suspicion remains.
If you're not comfortable working on the car yourself, take it to a qualified mechanic specializing in classic cars or Japanese vehicles. They'll have the experience and tools to diagnose the problem effectively. The symptoms you describe point to a fuel or ignition issue, but pinpointing the exact cause requires systematic troubleshooting.
