Lawn Mower: Runs For A While Then Dies

If your lawnmower starts up but then dies in the middle of mowing your lawn, you could have a bad spark plug. The spark plug works with the ignition coil to create a timed spark in the cylinder, causing combustion. The spark plug is really a very simple device. It has an electrode that passes through a ceramic insulator. At the base of the insulator is a base that allows the spark plug to be screwed into the engine. The base also has a second electrode attached to it. This electrode is set at a specific gap between it and the center electrode. This gap is where the spark is produced. The ceramic insulator also isolates the center electrode electrically from the spark plug’s base. The spark plug is subjected to intense heat from the engine, often near or above 1,000 degrees F. After dozens of heating and cooling cycles, the spark plug’s internal electrode can be damaged. When the spark plug heats again the expansion from the heat can cause the internal electrode to lose continuity and the mower will die. Occasionally a cracked ceramic insulator can allow the center electrode to ground to the engine’s frame, also causing the mower to die. Spark plugs are inexpensive and the easiest way to test for a bad plug is to simply replace it. Replacing the plug is easy to do. Remove the plug wire and then use a socket wrench to remove the old spark plug. Before installing the new plug, be sure to set the gap between its two electrodes using a gapping tool or feller gauge. The normal gap for a lawnmower engine spark plug is .030”. Once the gap is set, install the plug and secure it firmly with the socket wrench. Finish by replacing the spark plug wire. The spark plug on your lawnmower should be replaced at least yearly to ensure trouble-free operation.

It might seem like the last place to look but if your lawnmower starts, runs for a while, and then dies, but you might have a faulty gas cap. Most lawnmowers have a vented gas cap. The vent is a check valve that allows air into the fuel tank but keeps gas fumes inside. Sometimes the check valve will fail and no longer allow air to enter the tank. When this occurs, the lawnmower will start and run normally. As you mow your lawn the engine consumes fuel from the tank. As the fuel level drops, a vacuum begins to form in the fuel tank because air can get in to fill the space the consumed fuel occupied. After about 10 or 15 minutes, the vacuum will become strong enough that fuel will stop flowing to the carburetor and the lawnmower will die. If the mower is allowed to sit for a time, usually a few hours, air will seep back into the tank and the mower will start again. Newer lawnmowers have fuel caps that comply with government evaporative emissions requirements. These caps have charcoal filters in them to absorb any fuel that evaporates and gets past the check valve in the gas cap. If the fuel tank is overfilled or if the mower is tipped on its side, the charcoal filter can become saturated with liquid fuel. This will also block airflow to the fuel tank, causing the same tank vacuum issues. Diagnosing a fuel cap that isn’t venting properly is easy. Start the lawnmower and mow your lawn as you normally would. When the mower dies, remove the gas cap and reinstall it. This will allow air back into the fuel tank so fuel can flow again. Try to restart the mower. If the mower starts up again and continues to run you will know that the fuel cap is faulty and needs to be replaced.