If you’ve pulled the cord and your lawn mower spins easily with no resistance—or you’ve done a compression test and there’s little to no compression—you’re facing a problem that will keep any gas mower from starting. Compression is vital for combustion. Here’s how to diagnose and fix a lawn mower with no compression, using plain English and actionable steps.
Quick Picks: Most Common Causes of No Compression in Lawn Mowers
- Blown Head Gasket: A failed gasket allows air/fuel to escape instead of compressing in the cylinder.
- Stuck or Burned Valve: Intake or exhaust valve is stuck open or burned, letting air leak out.
- Broken or Stuck Piston Rings: Rings seal the piston to the cylinder wall—if damaged or stuck, compression is lost.
- Hole in Piston: Rare, but a piston with a hole will not compress air.
- Loose or Broken Spark Plug: A missing, loose, or cross-threaded plug will leak compression.
- Loose Cylinder Head Bolts: If head bolts aren’t tight, the gasket can’t seal properly.
Buyer’s Guide: What You’ll Need
- Socket set and wrench
- Feeler gauge (for valve clearance checks)
- Compression tester (optional, but very helpful)
- Replacement head gasket or valve parts
- Work gloves and safety glasses
- Service manual for your mower model (optional but handy)
Step-by-Step: How to Fix a Lawn Mower with No Compression
1. Confirm No Compression
- Remove the spark plug and insert a compression tester. Pull the cord and check the reading—below 60 psi usually means a problem. If you don’t have a tester, pull the cord with your thumb over the plug hole. Little or no pressure = no compression.
2. Inspect the Spark Plug and Cylinder Head
- Remove the spark plug and look for damage, cracks, or if it’s not sealing properly. Tighten or replace as needed.
- Check if the cylinder head bolts are tight. Loose bolts can prevent the head gasket from sealing.
3. Check for Blown Head Gasket
- Remove the cylinder head (usually a few bolts). Inspect the gasket for blackened, cracked, or missing sections. Replace with a new gasket and torque bolts to the spec in your manual.
4. Inspect Valves for Sticking or Burning
- With the head removed, look at the intake and exhaust valves. If stuck open, gently tap to free them, clean, or replace as needed.
- If valves are burned (edges look rough or missing), they’ll need to be replaced or reground by a professional.
- Check and adjust valve clearance if you have feeler gauges.
5. Examine Piston and Rings
- Look for visible scoring or damage on the piston. If you see a hole or severe wear, the piston will need replacing (a big job for most DIYers).
- If you suspect stuck rings, pour a teaspoon of oil into the cylinder and try another compression test. If compression rises, the rings may be stuck or worn.
- Piston ring replacement usually means a full engine teardown—consider professional help unless you’re mechanically experienced.
6. Reassemble and Test
- Install new parts as needed (gasket, valves, plug). Reassemble carefully, torque bolts evenly, and check that all connections are tight.
- Re-test compression before replacing the spark plug. If it’s back to normal, try starting the mower.
Comparison Table: Causes of No Compression & Solutions
| Problem | Solution | Difficulty | Parts Needed |
|---|---|---|---|
| Blown Head Gasket | Replace gasket, torque bolts | Medium | Head gasket |
| Stuck/Burned Valve | Clean or replace valve | Medium-Hard | Valve |
| Bad Piston Rings | Replace rings (engine teardown) | Hard | Piston rings |
| Hole in Piston | Replace piston | Hard | Piston |
| Loose Spark Plug | Tighten or replace | Easy | Spark plug |
| Loose Head Bolts | Tighten bolts | Easy | — |
Frequently Asked Questions
Can a lawn mower run with low compression?
No—a mower with very low or no compression will not start or will run extremely poorly and die quickly.
Is fixing no compression a DIY job?
Head gasket, plug, or valve issues are reasonable for a handy DIYer. Piston or ring problems usually require engine teardown—consider a repair shop for these.
How do I prevent compression loss in the future?
Use clean fuel, change oil regularly, and keep the air filter clean. Avoid overheating the engine or mowing with a dirty filter.
Conclusion
No compression means your mower can’t run, but with some basic tools and patience, many causes can be fixed at home. Focus first on the head gasket, valves, and plug. For major engine damage, professional repair may be your best bet.