Too Much Oil In Lawn Mower? Read Our Easy Fix It Guide!

What happens with too much oil in the lawn mower? Well, too much of a good thing can be bad for you! Right? Well, the same law applies to lawn mowers and engine oil. An overfilled lawn mower oil tank will lead to performance problems, failed starts, or an oily overflow mess. And much worse!

So, what other engine issues arise from putting too much oil in a 4-stroke lawn mower? And are these issues easy to fix? 

Let’s find out!

Too Much Oil In Lawn Mower

Overfilling a lawn mower oil tank will negatively affect engine performance and possibly prevent the mower from starting. Too much oil in a lawn mower can easily clog the air filter, foul spark plugs, and potentially cause a hydro-lock, which could bend the connection rods in a multi-cylinder mower. 

The way 4-stroke oil works in a 4-stroke walk-behind single-cylinder mower or a multi-cylinder lawn tractor is surprisingly straightforward:

• Lawnmower engine oil lubricates the engine and helps to keep it cool. 
• The oil tank on a lawnmower feeds oil into the crankcase, where it is placed under pressure by the down-stroke of the piston during the combustion process. 
• The air pressure forces the oil upwards to lubricate the piston and cylinder, as well as the crankshaft and con rod (piston push rod).
• The crankcase has a ventilation valve (breather) that releases pressurized vapor, which forms an oily mist. 
• A rubber hose connects the ventilation valve to the mower’s air filter housing and carburetor air intake. 
• The crankcase vapor passes through the air filter to the carburetor, where it mixes with the gasoline that fuels the engine.

What Happens When You Overfill the Oil In Your Lawnmower?

Too much oil in a lawn mower crankcase causes the vapor released via the ventilation valve to become oil-rich, which clogs the air filter, creating an overly rich air-to-fuel ratio that fouls the spark plugs and causes the engine to smoke and run poorly. Extreme over-oiling will stall the engine.

With too much oil in the mower’s oil tank, an excess amount of oil feeds to the crankcase, effectively reducing the volume (air space) of the crankcase, which increases the pressure in the crankcase during the piston down-stroke.

• The increase in pressure will force the excess oil through the ventilation valve into the air intake. From there, it will clog the air filter.
• The oil-rich vapor (potentially pure oil in extreme cases of over-filling) will enter the carburetor and blend with the gasoline that powers the engine.
• The overly rich air-fuel mixture will enter the combustion chamber and foul the spark plugs, causing the engine to sputter and stall. 
• A severely over-filled lawnmower oil tank (and crankcase) will cause a hydro-lock, where the piston cannot move due to excess oil filling the combustion chamber (between the cylinder head and the piston crown).
• A hydro-lock has a similar effect to a seized engine – the engine stalls and won’t restart.
• Attempting to crank the engine of a multi-cylinder mower when hydro-locking has occurred could bend the con rods (piston push rods).
• Hydrolocked single-cylinder lawnmower engines generally don’t suffer con rod bending. 

How Do You Know If You’ve Put Too Much Oil In Your Lawn Mower?

You’ll know you’ve put too much oil in your mower when:

• The oil on the dipstick is above the upper indicator line. 
• Excessive smoke emits from the exhaust.
• The engine runs roughly and sputters.
• The engine stalls and won’t restart.
• The spark plug is oily.
• The air filter is oily.

Can You Put Too Much Oil In Your Lawn Mower?

Yes! You can put too much oil in a lawn mower if you fail to limit the volume of oil poured into the oil tank to the amount specified by the mower manufacturer. And filling oil into the mower directly from a large oil can without checking the dipstick as you fill the tank can lead to over-filling.

Note: Consult your lawn mower owner’s manual for the correct oil volume and grade.

Oil volume ballpark – Lawn mower oil volumes generally vary between 15oz to 20oz, ranging from single-cylinder walk-behind mowers to larger multi-cylinder ride-on mowers.

What Are the Risks of Overfilling a Small Engine With Oil?

The risks associated with overfilling a small engine with oil include the following.

• Bent con rods – which may require expensive engine repair!
• Your lawn mower air filter may spoil.
• Your lawn mower spark plugs risk soiling.
• Wasted oil – the ultimate sin for thrifty homesteaders!

Read More!

What to Do When You’ve Put Too Much Oil In the Lawn Mower? Easy Fix!

The best way to fix an overfilled lawn mower is to drain the engine oil from the oil tank, crankcase, and combustion chamber. Remove the air filter and spark plug and clean them to remove all traces of oil. Crank the engine several times with the spark plug removed to purge residual engine oil.

• Equip yourself with your owner’s manual and the correct tools for the fix!

How to Fix a Failed Mower Engine Due to Oil Overflow?

Do you need to fix a lawn mower that’s stopped running due to oil overfilling? Then follow these steps.

1. Get the Right Tools, Including the Following:

• A jug or can of the specified oil for your mower.
• A spark plug wrench.
• A screwdriver or wrench. These tools help to remove the air filter.
• A wrench! Wrenches are perfect for removing the oil drain plug.
• Pliers to remove the ventilation hose.
• A solvent. It helps to clean the lawnmower spark plug.
• Detergent! Warm water with grease-cutting soap works fine. It helps to clean the air filter.
• A plastic funnel.
• An oil drain pump – but only if the mower lacks an oil drain plug.
• An oil drain hose – is critical for ride-on lawn tractors.
• An oil drain pan.
• A measuring jug.
• Paper towel.

2. Troubleshooting Your Lawn Mower – Step-by-Step

1. Disconnect the spark plug boot and remove the spark plug from the engine.
2. Remove the air filter cover and ventilation hose.
3. Remove the air filter.
4. Clean the spark plug.
5. Clean the air filter and dry it with a paper towel. 
6. Lightly oil the air filter to prevent it from drying out and perishing.

3. Drain All the Oil From the Crankcase and Oil Tank – Step-by-Step

1. Remove the oil drain plug (on the side of the engine or under the deck) and drain the oil into an oil drain pan (large mowers may need an oil drain hose to attach to the oil drain valve).
2. Pump oil out of the oil tank (for mowers without an oil drain plug) into an oil drain pan or disposable bottle.
3. Tip the mower on its side with the oil tank cap removed (for mowers without a drain plug). And drain oil from the oil tank and crankcase into an oil drain pan.
4. Crank the engine several times to vent oil vapor from the spark plug hole and crankcase ventilation hose. 
5. Let the mower stand with the spark plug, oil drain plug, and air filter removed for 45 minutes to evaporate oil-vapor residue.
6. Refit the cleaned spark plug, air filter, and ventilation hose.
7. Screw in the oil drain plug.
8. Pour the manual-specified amount of oil into a measuring jug (you can DIY a used canned fruit tin or similar).
9. Fill the oil from the measuring jug into the oiling tank via a funnel.
10. Allow the oil to settle for two minutes.
11. Screw in the dipstick and oil cap.
12. Unscrew the dipstick and check the level. Top up if necessary. But don’t go over the upper marker line on the dipstick.
13. Screw on the oil tank cap.
14. Crank the engine. The mower should start.
15. Allow the mower to idle for a few minutes.
16. Smoke will emit from the exhaust as the engine burns away the remaining oil residue.
17. Stop the mower and check the dipstick. Top up the oil if necessary using the measuring jug.
18. Cut the lawn!

Conclusion – Re-Oiled and Ready to Mow

If you’ve overfilled oil in your lawn mower, don’t beat yourself up – it’s a common mistake! And, the remedy needn’t cost much more than the price of a new can of oil.

Irrespective of what type of mower you own, having the right tools for the job and following our step-by-step oil overfill fix will get your mower back into the field. Pronto!

In the meantime, let us know if you have more questions about what to do if you put too much oil in the lawnmower.

We have tons of experience tinkering with lawn mowers, tractors, engines, and small farmyard equipment.

And we’re always happy to help troubleshoot.

Thanks again for reading.

Have a great day!

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Too Much Oil In the Lawnmower References, Guides, and Works Cited: