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1978 Datsun 280Z that won't start after a fuel filter change is likely suffering from an airlock in the fuel lines or an improperly seated filter. The car's pre-existing rough running condition could also indicate a separate, underlying problem with the fuel delivery or ignition system. 
Fix the airlock and check the new filter
  1. Prime the fuel system. A simple and effective way to purge air from the fuel lines is to cycle the ignition key.
    • Turn the key to the On position (just before starting) and listen for the electric fuel pump to hum for a few seconds. The 1978 280Z's fuel pump is located near the passenger-side rear wheel well.
    • Once the humming stops, turn the key to the Off position.
    • Repeat this "key on-and-off" process 5 to 6 times. This forces the pump to build pressure and push the air bubbles out of the system.
  2. Inspect for leaks and check connections. While priming, carefully check the new fuel filter and the associated fuel lines for any signs of leaking.
    • Make sure the filter is oriented correctly according to any flow arrows on its housing.
    • Confirm that all hose clamps are tight and that the hoses themselves are not cracked or brittle.
  3. Try starting the car. After priming, crank the engine. It may take longer than usual to start, and the idle may be rough initially as any remaining air is purged. 
If the engine still won't start
If priming the fuel system doesn't work, consider these other issues.
Fuel delivery problems
  • Fuel pressure loss: The old, clogged filter may have been masking a failing fuel pump or a leaky fuel pressure regulator. Now that the old restriction is gone, the system may not be able to build enough pressure. Check the fuel rail for a schrader valve (like a tire valve) and test the pressure with a fuel pressure gauge.
  • Clogged fuel lines: The debris from the old filter may have been pushed further into the fuel system, clogging a line or the fuel injectors. If the car ran roughly before, it could have been due to a restricted fuel flow. 
Ignition system issues
  • Weak spark: A struggling fuel system can put stress on the ignition. A pre-existing issue with a spark plug, coil, or ignition wire could now prevent the engine from firing. You can check for a spark by using a spark tester.
  • Failed electrical component: The fuel pump is run by a relay. Look for the fuel pump relay near the strut tower and check its operation. A broken or loose wire connection could have been disturbed during the filter change. 
Other potential issues
  • Vapor lock: If the fuel filter or lines were exposed to a heat source, it's possible for the fuel to vaporize, causing an air bubble in the line.
  • Timing belt: While unlikely to have been affected by a fuel filter change, a broken or slipped timing belt would prevent the engine from starting. You can quickly check this by removing the distributor cap and confirming that the rotor spins while someone cranks the engine. 
 
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