Most likely causes (in order of likelihood)

  1. Varnished carburetor — by far the most common cause, especially on Onan units that sit unused.
  2. Clogged fuel filter restricting flow once the carb bowl empties.
  3. Weak or failing fuel pump — pumps just enough to start, not enough to sustain.
  4. Low oil shutdown engaging after oil pressure drops at idle.
  5. Faulty governor causing the engine to over-rev and trip protection.
  6. Bad fuel — stale or contaminated gasoline.
  7. Air in the fuel line on diesel generators.

Diagnostic steps (in order, free/cheap before expensive)

  1. Check the oil first. If it's borderline, the low-oil sensor will let the engine catch on residual fuel pressure, then shut down a moment later.
  2. Inspect the fuel filter. If it's been more than 100 hours or 2 years, replace it as part of diagnosis.
  3. Listen for the fuel pump. With ignition on (don't crank), you should hear a brief priming buzz. No buzz means a dead pump.
  4. Spray a small shot of starting fluid into the air intake during the next start attempt. If the generator runs as long as you spray, fuel delivery is your problem. If it still dies, look at ignition or governor.
  5. If the generator has been sitting more than 30-60 days, plan for carburetor cleaning. Drain the old fuel, fill with fresh, add Sta-Bil or Sea Foam, and run it for 20+ minutes. Many varnished carbs will clear on their own with this treatment.
  6. If symptoms persist after carb cleaning, fuel pump replacement is the next step.

DIY vs. call a tech

Oil check, filter replacement, fuel additive treatment, and starting fluid diagnosis are DIY. Carb cleaning is intermediate. Fuel pump replacement is intermediate-to-advanced depending on access — many Onan units require generator slide-out for service.

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