Defective pressure gauge
Improper valve timing
Excessive valve guide clearance
Clogged fuel filter
Defective pressure relief valve
Fuel filter clogged
Worn out liner
Fuel leakage
External leakage of oil
Defective thermostat valve
Defective thermostatic valve
Low compression
Loose battery terminals
Water in the fuel system
Clogged air cleaner
Worn out of piston rings
Low oil level