Lawnmower Won’t Start When It’s Hot? (We Have a Fix)


Lawnmower Won't Start When It's Hot

Ideally, when using your lawnmower, all you have to do is start it up, and it will be able to cut your whole lawn in one sitting. However, there are times when you have to stop the mower, such as to clean out the grass bag or move to another area of the yard. In some cases, after doing so, the lawnmower won’t start again until it has cooled off. Why does that happen?

There are multiple reasons why a lawnmower might not turn on when it is hot. The most common culprits are a bad ignition coil, an overheated engine, or a faulty spark plug. If this happens regularly, you will want to inspect your mower and see which of these components is not working correctly. From there, you can either try to fix it yourself or take the machine to a repair shop.

Because there are many reasons for your lawnmower to stop working when it’s hot, we will dive into each option and discuss what you can do to fix it.

Top Reasons Why Your Lawnmower Won’t Start When It’s Hot

The Problem: A Bad Ignition Coil

The spark plug and coil are two pieces that control the electrical output of your mower’s engine. As the machine runs, electricity runs along the coil, helping control the voltage and ensuring that the motor continues to operate efficiently. It does this by moving the flywheel with magnets.

Ignition coils are made of copper, and they are built to withstand high temperatures. However, over time, the metal will wear down, meaning that it won’t be as resistant to heat. When that happens, the electrical system won’t work until the coil has cooled down.

Another problem with a faulty coil is that the motor may continue to run, even after releasing the lever. If this happens, you will have shut the engine down manually, which usually means disconnecting the ignition wire from the spark plug. In either case, you will have to replace the coil because it cannot be fixed.

The Solution: Replace the Ignition Coil

This process is relatively easy if you have the right tools. Fortunately, all you really need are some work gloves, a screwdriver, a socket wrench, and a feeler gauge. Alternatively, you can use a business card instead of a gauge. Before starting, we highly recommend referring to your owner’s manual for specific instructions. Here is a brief overview of how to change the coil.

  • Step One: Disconnect the Coil From the Spark Plug
  • Step Two: Loosen the Starter Pull Cord From the Handle
  • Step Three: Remove the Flywheel/Blower Housing
  • Step Four: Rotate the Flywheel – The magnets should be facing away from the ignition coil
  • Step Five: Remove the Ignition Coil
  • Step Six: Place a New Ignition Coil – Be sure to set it as far away from the flywheel as possible before tightening the bolts.
  • Step Seven: Rotate the Flywheel Back to Its Original Position
  • Step Eight: Allow the Ignition Coil to Stick to the Flywheel – Place the feeler gauge (or business card) between the two so that there is a small gap between them.
  • Step Nine: Tighten the Bolts on the Ignition Coil
  • Step Ten: Replace the Housing and the Starter Pull Cord
  • Step Eleven: Test the Mower

The Problem: An Overheated Engine

If the engine is too hot, it will not start. There are several reasons why the motor can overheat, so you will have to inspect each component to see what is causing the problem. The most common issues are a dirty carburetor, dirty cooling fins, or an incorrect amount of oil (too much/too little). Here is a breakdown of how to fix each of these elements.

Solution #1: Clean the Carburetor

This component is what controls the mixture of fuel and air inside the engine. If the carburetor is dirty, it won’t create an efficient mixture, causing the engine to overheat. Fortunately, you don’t have to remove the carb to clean it. For more detailed instructions on how to do this, click here.

Solution #2: Clean the Cooling Fins

Usually, these fins will get clogged with grass and other debris while you mow. Over time, this dirt will clog the fins, which doesn’t allow air to flow through as easily. When that happens, the engine overheats. Locat the fins and remove them from the lawnmower. They should be held in place with screws, so you just need a screwdriver. Be sure to wait until the mower has cooled off before starting this process.

Depending on how old the fins are, they may also be rusted. If that is the case, we recommend buying and installing new fins so that they will work more efficiently. Otherwise, scrape off any visible debris and then scrub the surface of each fin as much as possible. They don’t have to be sparkling clean, but there shouldn’t be any visible grass or dirt. Once they’re clean, you can put them back on the engine.

Solution #3: Check the Oil

Your engine’s oil is crucial for its performance. Too much, and it could cause a leak, which can wind up in your lawnmower’s exhaust. Too little, and the engine will not be properly lubricated, so it will overheat. If you haven’t changed the oil in a while, it is likely time to do so. First, though, you will want to check the oil level to see if it is adequate.

To check the oil, you will need to pull the dipstick out, wipe it off, and then stick it back in. This step is necessary so that you can get an accurate reading. If there is enough oil, you might just want to change it. If there is too much in the engine, you can simply drain some of it and retest the dipstick. If there is too little, you can add some until you get the right amount.

To change your oil, you will simply have to find where the oil drain is and place a drip pan underneath. Open the valve and wait for all of the oil to come out (this could take a little while). Afterward, close the valve and fill the oil. Be sure to regularly check the dipstick so that you don’t put too much in the engine by accident.

The Problem: Bad Spark Plug

Your mower’s spark plug is an essential part of the engine, as it is what controls the starter and the air/fuel mixture. If the spark plug is faulty, damaged, or dirty, it can cause problems when starting the lawnmower.

The Solution: Clean or Replace the Spark Plug

The first thing to do is to remove the plug to see how it looks. You will need a specialized socket to do this, as a standard socket could damage the internal components. If the plug is filthy, cleaning it might make it work better. However, if you notice any electrical corrosion or cracks in the ceramic, it will have to be replaced.

When replacing a spark plug, be sure to find the exact make and model for your lawnmower. Also, you will need to adjust the gap slightly so that it will work more efficiently. You can purchase a gap tool at most auto parts stores.

The Problem: Bad Choke

The choke on your lawnmower’s engine is what controls the amount of air and fuel that get mixed inside the carburetor. Typically, this component will stay closed and gradually open as the engine heats up. When the mower is already hot, it might allow too much fuel to go into the carb, causing motor failure. Newer models will have an automatic choke, while older lawnmowers have a manual version.

The Solution: Repair or Replace the Choke

Usually, the choke gets stuck in the open position because of dirt or grime. All you have to do is locate it and clean it out with a wire brush and some cleaning spray. If the mower is really old, it is likely time to replace the choke altogether. Because each machine has a different layout, we recommend taking your mower to a repair shop to get this done. Otherwise, you will need to find specific instructions for your make and model.

Related Questions

What if my mower doesn’t start at all?

If the machine doesn’t start even after cooling down, the problem is likely the spark plug, the ignition coil, or the starter. If it never starts again after you’ve shut it off, the battery is likely completely dead.

Does ambient temperature affect the motor?

Unless the mower is sitting in direct sunlight on a 100+ degree day, ambient air temperature shouldn’t affect the engine. Lawnmower motors can get blisteringly hot by themselves and work just fine, so there is no reason to suspect that a hot day is causing any problems.

Upgraded Home Team

We are a team of homeowners and home improvement enthusiasts who enjoy sharing decorating, gardening, home improvement, and housekeeping tips with other homeowners! Whether you're looking for advice on furnishing your living room or the next outdoor DIY project, we've got you covered.

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