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How to check for a blown head gasket on a lawnmower, Step by Step

Lawnmowers are convenient machines to maintain your lawn. If you bought one some years ago, you probably used it a lot. But recently, you notice that the performance of the mower has downgraded a bit. It eats up a lot of engine oil or gives a lot of white smoke from the engine, or maybe the engine runs a little erratically. Your head gasket may be the problem. But as it is a more complex repair and time-consuming fix, you should first diagnose it before starting.

You may confuse the symptoms with other lawnmower issues, like using the wrong fuel type, a lousy sparkplug, or maybe a faulty carburetor. But if you have repaired these issues repeatedly to alleviate the problem, and the operation remains problematic. Well, in that case, the problem might be different, and your lawnmower might have a blown head gasket. To fix a blown head gasket is not the easiest fix, but you should definitely try it if you are up to it. This blog shows a step by step approach to replace the head gasket on your lawnmower.

How to check for a blown head gasket on a lawnmower, Step by Step:

  • Step 1: Check for general symptoms
  • Step 2: Remove the engine cover
  • Step 3: Disassemble the carburetor, filters, and dipstick from the engine
  • Step 4: Locate and unscrew the sparkplug
  • Step 5: Disassemble the engine head
  • Step 6: Check for and scuffs or marking or blows around the head
  • Step 7: Look for carbon deposits of valves and piston
  • Step 8: If diagnosed as blown, replace the gasket. If not, reassemble all the components
  • Step 9: Start the lawnmower and check if any problem persists

Before diving deep into the step-wise diagnostic approach, let us get an insight on a few technicalities. These will be helpful not only in this repair but also for any future problems. In case you do not want to indulge yourself here, skipping the next portion and referring back when required is an excellent way to go as well.

Head Gasket – cause and effects of a blown head gasket

Head gasket is a seal usually made of layered metal. It is fixated and compressed between the engine block and head. This gasket aims not only to make sure there are no fluid leakages but also no pressure from the engine compartment leaves out. Oil and coolant are avoided to enter the combustion chamber

● Causes of a blown lawnmower head gasket:

A blown head gasket could exist due to lousy combustion timing, terrible fuel, and knocking while running. All of these cause high pressures inside the cylinder and cause critical damage to the head gasket.

● Effects of a blown lawnmower head gasket:

A blown head gasket’s effects include pressure leakage, oil seepage to the combustion, and if coolant is present, the coolant also enters and combines with the fuel.

If not replaced timely, a blown head gasket may prove to be very costly—the running cost of lawnmower increases. The oil is burned that adds up to the expense. The sparkplugs get wasted easily, piston head may get damaged, might even lead to scratches on cylinder liner and require honing. And to add further, the engine might become unusable and require a complete rebuild.

Checking for a blown head gasket on a lawnmower, Step by Step:

Before starting with the fix itself, check if you have the needed tools for the job:

● Needed tools:

  1. Socket wrench
  2. Wire brush
  3. Sandpaper
  4. Engine cleaner
  5. Rag
  6. Collector pan
  7. Grip pliers
  8. Clips/ stopper

● Step 1: Check for general symptoms

Before getting your hands dirty, it is good to recheck the symptoms to make sure you do not fix something that is not the problem. With a blown head gasket, oil spreads inside the filter cover, exhaust smoke, usually white or blue (less often), are the main clues. If the engine has a radiator system, checking the reservoir for the coolant’s foamy or milky appearance can be something to check. In case the engine starts and is erratic in its operation, shows smoke, or even heavy smoke clouds, it is a clear indication. It is also possible that smoke direct seeping out from the engine headcover.

Checking the sparkplug condition is also a fundamental step. If the head gasket is blown, you will find carbon deposits on it.

The above mentioned are a few general symptoms that you should check, but each case will be slightly different.

● Step 2: Remove the engine cover

Whenever working on a lawnmower engine, the First step is to remove the covers to access the engine and related components.

  • Step 2.1: Start by locating the bolts and checking for the socket size.
  • Step 2.2: Using a socket wrench, remove the bolts
  • Step 2.3: If the cover is not spring-loaded, just 2 to 4 latches of grooves will be holding it. Remove by pulling the body off
  • Step 2.4: Look for any electric connection with hood fan or light and remove it from the allocated connection clip

● Step 3: Disassemble the carburetor, filters, and dipstick from the engine

Before opening the engine head, it is vital to all the complementary parts present over the engine.

  • Step 3.1: Locate fuel supply line, carburetor, oil pump, and filter and oil pipes
  • Step 3.2: Start by removing the fuel line. Clip the pipe or close using a stopper
  • Step 3.3: Look for carburetor connection and remove it along-with its gasket and keep the screws safe.
  • Step 3.4: Remove the oil filter, pump connection, and pipe going to the oil sump from the engine
  • Step 3.5: If the engine has a cooling system, remove those pipes as well
  • Step 3.6: Take the precaution of putting rag or collector pan while performing these to avoid spills all over the place

● Step 4: Locate and unscrew the sparkplug

The next step is to locate the sparkplug wire and disconnect it. Unscrew the sparkplug using a torque wrench.

● Step 5: Disassemble the engine head

The engine head is usually bolted down onto the main engine block. The easiest way to locate it is that it is close to the location of the sparkplugs:

  • Step 5.1: Locate and Unscrew the bolts present on the valve cover
  • Step 5.2: Put a rag or collector pan below to collect any lubricant oil present in the head
  • Step 5.3: Please take a look at both the valves and their respective pushrods. This Step is very crucial
  • Step 5.4: Remove the pushrod and rocker mechanism smoothly and carefully
  • Step 5.5: Unscrew the screws present around the head of the engine and pull it off carefully
  • Step 5.6: Clean any spills of fuel or lubricant 

● Step 6: Check for and scuffs or marking or blows around the head

There are usually three signs of a blown gasket.

  1. Any blown scuff present around the area covered by the gasket
  2. Blown away or cracked layer of the gasket itself
  3. Marks of oil or coolant seepage into the tank

● Step 7: Look for carbon deposits of valves and piston

The inside of valves and the piston’s top is the place where you should find most of the oil and carbon deposits.

● Step 8: If diagnosed as blown, replace the gasket. If not, reassemble all the components

If the gasket is proved, or you have indications that it is blown, replace it. If you think it is not blown, you should consider replacing it as you can access it now easily. Follow these steps and complete reassembly.

  • Step 8.1: Clean the surface of the piston
  • Step 8.2: Clean the outer by using engine cleaner or only gasoline
  • Step 8.3: Reassemble carefully, and every fit should be fair and tight
  • Step 8.4: Before reassembly, clean all the components like pumps and pipes, carburetor, etc.

● Step 9: Start the lawnmower and check if any problem persists

Start the lawnmower and check again. If the problem were just a gasket and cleaning, the regular operation would continue.


These are some things you should consider before carrying out a head gasket repair:

  • Protect yourself: Use safety goggles and gloves to protect yourself as well as delicate engine parts and surfaces from scratches
  • Safe location: Try to carry all task in the open air and away from fire sources
  • Do not overtight bolts: Try not to overtighten any bolts, and it will lead to permanent damage to threads
  • Air compressor: If using compressed air for blowing particles of dirt and carbon, use safety and try to keep pressure nominal
  • Sparkplug: Gap the sparkplug before checking

Related Questions

1. Is there any easier way or instrument to check for leaking compressional pressure?

The use of a pressure gauge at the exhaust of the engine is an alternative. And comparing the value to average values can be used as a diagnosis. This alternative might seem more comfortable but require a significant amount of experienced observation and knowledge.

2. Can WD 40 be used as an engine cleaner?

Yes, WD40 can be used as a cleaner for outer surfaces and piston head. A wire brush should not be used on the body where the gasket is placed.

Final Remarks

A smoothly running engine is a treat to work within the lawn. If a problem, as stated, arises, replacing the head gasket is a perfect option. If your engine is older and gives smoke and does not seem to have the same power as before, gasket replacement can prove beneficial. Checking for a blown gasket and replacing it is a step you should know for proper repair and engine maintenance and saving it from more significant losses that will require professional assistance or rebuilding the engine.