Nissan H15/H20-11/H25 Engine

• Engine power not up to normal (overheating)
  Cause: Insufficient coolant
  Fix: Replenish.
• Engine will crank normally but will not start (low compression)
  Cause: Incorrect spark plug tightening or damaged gasket
  Fix: Tighten to normal torque or replace gasket.
• Engine will crank normally but will not start (fuel system trouble)
  Cause: Lack of fuel
  Fix: Supply.
• Water leakage (engine cooling system)
  Cause: Damaged radiator seams
  Fix: Repair.
• Engine will crank normally but will not start (ignition system trouble)
  Cause: Very high current
  Fix: Inspect for shorted primary circuit.
• Cannot crank / cranking is slow
  Cause: Improper grade oil
  Fix: Replace with proper grade oil.
• Excessive oil consumption (oil leakage)
  Cause: Loose oil drain plug
  Fix: Tighten it.
• Engine power not up to normal (fuel system trouble)
  Cause: Malfunction of choke system
  Fix: Adjust.
• Excessive oil consumption (excessive oil consumption)
  Cause: Cylinder and piston wear
  Fix: Overhaul cylinder and renew piston.
• Poor circulation
  Cause: Restriction in system
  Fix: Check hoses for crimps, and clear the system of rust and sludge by flushing radiator.
• Noisy engine (knocking)
  Cause: Overloaded engine
  Fix: Use right gear in driving.
• Improper engine idling (fuel system trouble)
  Cause: Clogged or damaged carburetor jets
  Fix: Clean or replace.
• Decreased oil pressure
  Cause: Lack of oil in engine oil pan
  Fix: Correct.
• Oil leakage (engine lubrication system)
  Cause: Damaged or cracked pump body cover
  Fix: Replace.
• Noisy engine (others)
  Cause: An improper adjustment of valve clearance
  Fix: Adjust.

Compiled from service & troubleshooting documentation across every generator set that uses the Nissan H15/H20-11/H25.

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.