My Petrol Lawnmower is blowing white/grey/blue smoke

If your petrol lawnmower is producing white, grey or blue smoke from the exhaust, it is usually caused by engine oil entering areas where it should not normally be present. In many cases, the issue is temporary and can be resolved with a few simple checks.

This guide explains the most common causes of white, grey or blue exhaust smoke and the steps you can take to identify and resolve the problem.

Quick Answer

White, grey or blue smoke from a petrol lawnmower is most commonly caused by engine oil burning inside the engine or exhaust system.

Common causes include:

• The engine oil has been overfilled.
• The lawnmower has been tilted onto its side, allowing oil to enter the cylinder, exhaust or air filter housing.
• Oil has been spilled onto the engine during maintenance.
• The air filter has become contaminated with engine oil.

If the mower has recently been tipped over and the oil level is correct, the smoke will often disappear after a few minutes of normal operation.

Before You Begin

Before carrying out any inspection or maintenance:

• Switch off the engine and allow it to cool completely.
• Remove the spark plug cap to prevent accidental starting.
• Work in a well-ventilated outdoor area.
• Wear suitable personal protective equipment (PPE), including gloves and eye protection.

Allow the engine to cool before checking the oil level or removing the air filter.

Steps

Step 1 – Check the Engine Oil Level

Remove the oil filler cap and dipstick.

Check that the engine oil level is between the minimum and maximum marks on the dipstick.

Most petrol lawnmowers require SAE 30 engine oil, or the oil specified in your user manual.

If the oil level is above the maximum mark, remove the excess oil before operating the mower.

Overfilled engine oil can be forced into the combustion chamber or air intake, causing white, grey or blue smoke.

Step 2 – Check Whether the Lawnmower Has Been Tilted

Consider whether the mower has recently been tipped onto its side.

This commonly happens when:

• Cleaning underneath the mower deck.
• Transporting the mower.
• Carrying out maintenance.

Tilting the mower incorrectly can allow engine oil to enter the exhaust, cylinder or air filter housing, causing temporary smoke when the engine is restarted.

Step 3 – Allow Excess Oil to Burn Off

If the oil level is correct and the mower has recently been tilted:

1. Start the engine in a safe, well-ventilated outdoor area.
2. Allow the engine to idle and run normally.
3. Monitor the exhaust.

As the excess oil burns away, the amount of smoke should gradually reduce before stopping completely.

Depending on how much oil has entered the engine or exhaust, this process may take several minutes.

Step 4 – Inspect the Air Filter

Oil contamination can affect the air filter if the mower has been tipped over.

Remove the air filter cover and inspect the filter.

Sponge Air Filter

A clean sponge filter is typically yellow or grey.

To clean the filter:

1. Remove the filter.
2. Wash it in warm soapy water.
3. Rinse thoroughly.
4. Allow it to dry completely.
5. Reinstall the filter and refit the cover.

Do not reinstall a wet sponge filter.

Paper Air Filter

A clean paper filter is normally white or off-white.

Paper air filters cannot be cleaned effectively.

Replace the filter if it is:

• Dirty.
• Damaged.
• Oil-soaked.
• Heavily discoloured.

Fit a new air filter and securely refit the cover.

Step 5 – Test the Engine

Reconnect the spark plug cap and start the engine.

If the smoke gradually disappears, the issue was most likely caused by excess oil entering the engine or exhaust.

If the smoke continues after checking the oil level and air filter, further diagnosis may be required.

Frequently Asked Questions

Why is my petrol lawnmower blowing white, grey or blue smoke?

White, grey or blue smoke is usually caused by engine oil burning inside the engine or exhaust. This commonly happens if the mower has been tilted incorrectly or the engine oil has been overfilled.

Will white smoke stop on its own?

Yes. If the mower has recently been tipped over and the oil level is correct, the smoke will usually disappear after the excess oil has burned away during normal operation.

Can too much engine oil cause smoke?

Yes. Overfilling the engine can force oil into the combustion chamber or air intake, producing white, grey or blue exhaust smoke.

Can a dirty air filter cause white or blue smoke?

A dirty air filter alone is unlikely to cause white or blue smoke. However, an air filter contaminated with engine oil after the mower has been tilted can contribute to the problem.

Is it safe to continue using a lawnmower that is smoking?

If the smoke is caused by excess oil after tipping the mower, it will usually clear after a short period. If the smoke continues, the oil level is correct and the mower has not been tilted, stop using the machine and investigate further, as continued smoking may indicate a more serious engine fault.