What to do when your engine starts knocking Main Image

Knock Knock, who’s there? Oh no! It’s your small engine. An engine knock is a sign that some form of engine maintenance is needed before a serious problem arises if it hasn’t already. Luckily there are some simple things you can do to troubleshoot your small engine and determine whether or not more serious repairs are going to be needed.

You’ll need a few things from around the garage to start your diagnostics, but before you gather those, let's start here: if your engine has been running, power it down and let it cool off. You don’t want to possibly burn yourself on a hot engine. Once your mower is turned off, use this time to gather your tools. You'll need an old (but clean!) rag, a drip pan, and a socket set.

Checking the oil level

The first thing you want to check is the oil level. Low engine oil is one of the main causes of engine knocking so you’ll want to check this before assuming you need extensive repairs. Once your mower has sat long enough for the engine to cool, you'll want to remove the oil fill cap and dipstick. You're going to take the rag and wipe the dipstick clean, re-insert it and remove it again to get a reading of your oil level. If it reads low, get some motor oil (based on the requirements from your engine owner's manual) and add it until you’re at an adequate fill level. Make sure to continually check the oil level as you're adding it, as you do not want to overfill the engine. Keep checking it with the rag and dipstick after adding small amounts of oil. If the knocking sound persists after adding oil or if the oil level was adequate, then you’ll want to check a couple other things.

Oil Fill Level

Changing the gas

Another common cause of engine knocking is old gas. Once again, you'll want to make sure your mower engine is cool. While you're waiting for it to cool is a good time to get your drip pan handy. After the engine is cool to the touch, you’ll want to place the drip pan near the fuel shut off switch on your engine. One way to drain the old fuel from your engine is to disconnect the fuel line from the fuel cut-off switch and position the fuel line so it drains into your drip pan. After the old gas has been drained from the engine reconnect the fuel line to the fuel shut off switch. If you’re not comfortable doing this and would prefer a more straight-forward approach, you’ll want to place the drip pan in an area where tilting the machine on its side can drain the old fuel into the pan. Make sure you always tilt your mower with the air filter on the top/high side. Now that your pan is in place we can remove the gas cap, lean our mower on its side, and empty the old gas into our drip pan. When the old gas has been fully removed you can stand your mower back on all four wheels, fill the tank with fresh gas and re-secure the gas cap. If the knocking sound still persists after changing your gas, there are still a few things you can check.

Changing Old Gas
Disconnecting the fuel line from the fuel shut-off switch to drain old gas.


Checking the air filter

The next stop in your diagnostic testing is to check the air filter. To do this, you'll need to remove the wing nut, screw, or clip-on cover that holds the air filter cover in place. This will allow you to remove the top half of the air filter cover. Once the air filter cover has been removed you'll remove the air filter and inspect it. If the filter looks extremely clogged, dirty, or deteriorated it would probably be a good idea to replace it with a new one. You should also check the air filter cover and housing for debris. Be cautious when cleaning the air filter housing because debris can easily enter the engine without a filter. Gently clean the housing with a clean rag. After your filter has been inspected or replaced, insert it into the air filter housing, reattach the air filter cover, and re-secure it. Now that you've inspected the air filter (and possibly replaced it), you’ll want to start the mower. If your engine has quit knocking then you solved the problem! If it’s still knocking, you'll want to take a look under the mower deck.

Checking air filter
Opening the air filter cover to inspect the air filter


Checking the blade / checking under the deck

After your mower has been turned off and it has cooled down, lay the mower on its side with air filter on the high side. This will keep oil from getting into the air filter. Take a look at the blade under the mower deck. Sometimes the knocking sound you hear could be the result of a loose, bent or wobbling blade. If you’ve recently removed or replaced the blade and it suddenly started knocking, this is quite possibly the cause of your issue. Begin by checking to see if the blade retaining nut is tight. If this is loose, it could cause the blade to wobble so you'll want to tighten it with your socket set. Please note that we do recommend attaching blade nuts with an impact wrench whenever possible. If the nut is tight but the blade looks out of place or looks like it isn’t seated correctly, you're going to want to remove the retaining nut with your socket set or impact, pull the blade off, and then re-seat it properly onto the shaft/blade adapter. Once the blade has been properly re-seated, you can tighten the retaining nut to secure the blade in place. If your blade looked excessively dinged up or bent to the point of making contact with other objects under the deck, you’ll want to replace that blade as soon as possible. Having a blade that is making contact with other objects in your mower deck is very dangerous and it should be replaced immediately. If your knocking hasn’t stopped after inspecting the blade and replacing it, then there is one last thing we can check.

Removing lawn mower blade
Carefully removing lawn mower blade from under the deck.


Checking the spark plug

Sometimes engine knocking can be caused by the engine not firing in sync. This could be caused by a bad or misfiring spark plug. To test this you’ll need a spark plug wrench to remove the old spark plug. Take it with you to our local lawn mower service/repair shop to get your replacement spark plug. When replacing the spark plug, always remember to hand-thread the plug into the cylinder to make sure you don't strip the threads. After hand-turning, tighten with your spark plug wrench and start up the mower. If the engine stopped knocking, you’ve solved your problem. If you're still hearing the same engine knocking sound then you need to take your mower to a certified mechanic or lawn mower service center and have them look it over. It could be a much more serious issue and the more often your mower is ran with this issue, the more damage you could be doing to it.

Removing spark plug
Removing spark plug with spark plug wrench.


Hopefully one of these steps has solved your engine troubles. Engine knocking can be the sign of a serious issue with your small engine and if these steps didn’t solve your problem, your mower needs to be looked at by a certified mechanic. The mechanic will be able to provide you with an estimate of repair costs and can inform you about the extent of the damage as well as if it may be easier (more cost efficient) to simply replace the entire engine on your mower.