Diesel Generators

Maintenance & Troubleshooting for Portable and Standby Diesel Generators

Trouble Starting Portable Generators

What to do when you can't start a portable diesel generator.

Trouble Starting Generator

Diesel engines are very simple machines. When they fail it is often a fuel problem. Less common is some mechanical engine failure that has resulted in a loss of compression. Loss of compression can be caused by a cracked engine, broken valve, warn piston rings or more commonly just some valves out of adjustment and that is easily fixed. First here is what you should check if you are using a single cylinder Yanmar type diesel engine.

  • Use diesel motor oil only. 15w40. Do not use motor oil for gasoline vehicles. You will cause engine damage.
  • Use regular automotive diesel fuel. Other fuel type have a different flash point and may cause you problems.
  • Keep in mind that very cold fuel will gel and clog up fuel lines and filters.
  • Make sure you are under 3000 feet elevation. Air density is about 10% less at 3000 feet and it will be hard to start your diesel.
  • Battery must be charged. Slow cranking results in to little heat and compression to ignite air fuel mixture.
  • Throttle should be in the run position. Inside the engine it is connected to the fuel pump and closes a valve causing no fuel to enter the pump


  • Another common reason for the pump not to work is because you ran out of fuel and air reached the pump. It can not pass air, only fuel. Air must be removed from the fuel lines all the way from the fuel filter and all the way to the fuel pump. You can do this by removing the hose at the pump and letting the fuel flow free into a container until all air is removed. Air in your fuel lines is the most common cause of failure. It will also happen every time you let the generator run out of fuel. Trucks have the same problem. Just pull off the rubber hose and let the fuel drain out into some container until all the air is removed.
  • If no fuel is flowing with the hose removed check that all fuel valves are open. There may be a valve on on the fuel filter and under the fuel tank.
  • Some fuel lines from the tank may be too long or too short and either trapping air in a high spot that reaches the pump after a few minutes or kinking the fuel line preventing fuel from flowing down to the pump. It works up to this point by gravity only.
  • Check the electric fuel valve mounted on the fuel pump if you have one. It has one wire going to it and when it has no power the fuel passage way is closed. Power is turned off to this valve when there is low oil pressure or you turn the key or remote off. There should also be fuse in line with it. It is controlled by a circuit board that may have failed but unlikely. This is how most engines are turned off. Just take power away from the switch and the fuel supply stops. Often this valve will get stuck or may burn out if you left the key on when it was not running. You can unscrew it and remove the only two parts. A sparing and a plunger. After doing this the only way to turn off the engine is to shut off the fuel supply and that is done by releasing the throttle. (Normally it is locked in the run position)

Electric Fuel Switch

  • To confirm your fuel pump is working, loosen the metal fuel line. When you crank, fuel should start running out. It is not gushing, its between running and dripping. If you are not sure then it is easier to remove the fuel injector with the lines still connected and watch for fuel spray when cranking. (Protect your eyes and fingers. It is under high pressure.) Also check that the fuel injector sprays in all directions the same.
If fuel is not the issue then you should check the rocker arms and valve adjustment screws. Many people neglect doing required valve adjustments resulting in engine failure. The valve lifters and push rods are mechanical not hydraulic like on cars today. As they ware the play increases and valves don't open as much as they should resulting in hard starting, smoke and in advanced stages knocking and engine damage. What happens is the rocker arms rather then making contact with parts they should be moving they work like hammers and eventually break off an adjustment screw. Have a comment ? Please post it below.
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