Few things are more frustrating than mowing your yard, shutting off your mower for a quick break, and then discovering it won’t restart. If your lawn mower starts fine when cold but won’t fire up again after running—even though the engine turns over—you’re not alone. This problem is common with both push and riding mowers, and it’s usually caused by one or more simple issues you can fix yourself. Here’s your complete guide to diagnosing and repairing a lawn mower that refuses to restart when hot.
Quick Picks: Most Common Reasons a Lawn Mower Won’t Restart After Running
- Vapor Lock: Gasoline vaporizes in the fuel line or carburetor from heat, starving the engine of fuel.
- Flooded Carburetor: Shutting down a hot mower can cause excess fuel to flood the carb, making it hard to restart.
- Weak or Failing Ignition Coil: A failing coil may work when cold but lose spark when it heats up.
- Fouled Spark Plug: A plug that’s oily or carbon-fouled can lose spark, especially after a hot shutdown.
- Dirty or Blocked Air Filter: Heat expands dust and debris, further choking airflow to the engine.
- Low Oil Level: Some mowers have a low oil shutoff that engages when the engine is hot and oil is low or thin.
- Carburetor or Fuel Line Issues: Gummed-up jets or partial blockages can get worse with engine heat.
Buyer’s Guide: What to Check & Tools You’ll Need
- Socket wrench & screwdrivers: For spark plug and air filter access.
- New spark plug: For easy, instant tune-up.
- Carburetor cleaner spray: For clearing fuel deposits.
- Fresh gasoline: Always use fuel less than 30 days old.
- Work gloves & eye protection: For safe handling of hot parts.
- Owner’s manual: Handy for locating the coil and carburetor on your model.
Step-by-Step Guide: What to Do If Your Lawn Mower Won’t Start After Running
1. Let the Engine Cool
- First, wait 10–20 minutes after shutting down before trying to restart.
- Many heat-related problems (vapor lock or coil failure) resolve temporarily as the engine cools.
2. Check for Flooding
- If you smell gas or see a wet spark plug after trying to restart, the engine may be flooded.
- Hold the throttle open (if adjustable) and don’t choke the engine. Try starting again after a few minutes.
- Remove and dry the spark plug if needed.
3. Inspect the Spark Plug
- Remove the plug and check for carbon, oil, or signs of overheating.
- Clean or replace with a new, properly gapped spark plug for best results.
- Ensure the plug wire is tight and free from corrosion.
4. Clean or Replace the Air Filter
- Heat can cause air filters to swell or become even more clogged with dust.
- Replace or clean your filter to ensure strong airflow and easier restarting.
5. Check Fuel System for Vapor Lock
- On very hot days, fuel can boil in the lines or carb, causing vapor lock. Let the mower cool with the gas cap slightly loosened (to vent vapors).
- Refuel with fresh gas and check fuel lines for kinks or signs of softening/melting near the engine.
- Replace old, brittle lines if needed, and keep lines clear of hot engine surfaces.
6. Test the Ignition Coil (If Problem Persists)
- If your mower runs fine cold but loses spark when hot, the ignition coil may be failing.
- Replace the coil with a new one designed for your mower model for a permanent fix.
7. Top Off Oil and Inspect for Low Oil Shutoff
- Check oil level and condition—thin or low oil can trigger a safety shutoff after the engine gets hot.
- Add or change oil as needed, following your owner’s manual guidelines.
Comparison Table: Causes & Solutions for Mowers That Won’t Restart Hot
| Problem | Solution | Difficulty | Parts Needed |
|---|---|---|---|
| Vapor Lock | Let cool, vent tank, replace lines | Easy | Fuel line |
| Flooded Carburetor | Wait, clear throttle, dry plug | Easy | Spark plug |
| Bad Ignition Coil | Replace coil | Medium | Ignition coil |
| Fouled Spark Plug | Clean or replace plug | Easy | Spark plug |
| Dirty Air Filter | Clean or replace filter | Easy | Air filter |
| Low Oil Level | Add or change oil | Easy | Oil |
| Carb/Fuel Line Blockage | Clean carb, replace line | Easy-Medium | Carb cleaner, fuel line |
Frequently Asked Questions
Why does my mower restart fine when cold, but not when hot?
Heat-related ignition coil failure is very common, as well as vapor lock in the fuel line or carburetor. Both prevent a hot restart but may work again after cooling.
How do I know if my ignition coil is bad?
If you lose spark when the engine is hot, but it returns when cold, the coil is likely failing. Replace for reliable restarts.
Can old gas cause hot restart problems?
Old or contaminated fuel can cause both hot and cold starting issues. Always use fresh gasoline and clean your carburetor regularly.
Is vapor lock dangerous?
It’s not dangerous, but it’s frustrating. It happens most in hot weather and is solved by cooling and venting the tank. Keep fuel lines away from hot engine parts to prevent it.
When should I see a professional?
If you’ve checked all these items and your mower still won’t restart when hot, a small engine technician can test ignition and fuel system components more deeply.
Conclusion
If your lawn mower won’t start after running, the fix is usually simple—let it cool, check for flooding, inspect the spark plug, and ensure strong fuel and air flow. Replace any weak ignition parts, and keep up with basic maintenance for easy starts every time, hot or cold!