A generator that starts fine cold but refuses to restart when hot points to a short list of heat-related causes. Here is how to separate vapor lock, a heat-soaked coil, and flooding.
The 30-second answer: If your generator won’t restart when hot but starts fine cold, heat is the clue. Wait 20 to 30 minutes and try again. If it starts after cooling, you’re dealing with vapor lock, a heat-soaked ignition coil, or flooding. Test for spark the moment it quits to separate the coil from the fuel system.
The short version
- A generator that restarts only after cooling is telling you a heat-sensitive part is failing.
- The three usual suspects are the fuel system, the ignition coil, and flooding from too much hot cranking.
- Check for spark immediately after it quits, then check for fuel. That order separates the causes.
- Shade, airflow, and clean cooling fins prevent most of these problems.
Why Won’t a Generator Restart When It’s Hot?
Heat changes how both the fuel system and the ignition system behave, and a generator that won’t restart when hot almost always has one of three problems. First, fuel can boil or vaporize in the carburetor bowl and lines (vapor lock), leaving the engine starved or flooded. Second, an ignition coil with a weakening winding can work cold, then lose its spark when it heats up. Third, repeated hot cranking, especially with the choke on, can flood the cylinder with raw fuel. My rough field breakdown on “runs, quits, won’t restart hot” calls: about 40% are heat-soaked ignition coils, about 30% are fuel boiling or a vent problem, about 20% are flooding from bad restart technique, and about 10% are something else, like a failing fuel pump or a cracked fuel line sucking air. The good news is that you can separate these in about ten minutes.
Step 1: Check for Spark While It’s Still Hot
This is the single most useful test, and almost nobody does it correctly. The mistake is waiting for the engine to cool down. Once it cools, the coil works again and you’ve lost your evidence.
- When the generator quits and won’t restart, don’t walk away.
- Pull the plug wire, put an inline spark tester on it (about $8) or ground the plug against bare engine metal.
- Crank the engine and watch for a strong blue-white spark.
No spark or only a weak orange one while hot, but good spark later when cool, means the coil is heat-sensitive. Go through the testing steps in how to test a generator ignition coil. If spark is strong, the coil is off the hook and you move on to fuel.
Step 2: Check for Fuel and Vapor Lock
With good spark, the problem is fuel. Pull the spark plug and look at it.
- Plug is wet and smells of gas: the engine is flooded. Go to the flooding section below.
- Plug is dry: fuel isn’t arriving. Crack the carb bowl drain screw (engine off, no flame nearby) and see whether fuel runs out. If it doesn’t, the bowl is empty, and fuel flow is being interrupted.
Vapor lock is less common in small engines than in old cars, but it does happen when a generator sits in the sun in an Arizona summer, a spot I know well. Heat soaks into the carburetor after shutdown, and the fuel in the bowl starts to boil. Ethanol blends boil at lower temperatures, which makes it worse (more in ethanol-free gas for generators).
The other common fuel-side cause is a gas cap vent that’s shut or clogged. The tank pulls a vacuum as it runs, and it takes time to bleed off. Loosen the cap one turn and see if the engine fires. My full walkthrough is in generator gas cap vent.
Step 3: Rule Out Flooding
If you crank and crank a hot engine with the choke on, you’ll drown it. A hot engine rarely needs choke at all. I’ve watched people crank a hot generator thirty times with the choke closed and then insist it’s broken. Open the choke, set the throttle to full, remove the spark plug, and pull the cord ten times to clear the cylinder. Dry the plug or put in a new one, and try again. The full procedure is in flooded generator engine.
Quick Reference: Why a Generator Won’t Restart When Hot
| What you see | Likely cause | Fix |
|---|---|---|
| No spark hot, spark cold | Heat-soaked ignition coil | Replace coil, $15 to $40 |
| Spark good, plug dry, bowl empty | Fuel not reaching carb | Check cap vent, filter, valve |
| Wet plug, gas smell | Flooded engine | Clear cylinder, restart without choke |
| Fuel boils over after shutdown | Heat soak, high ethanol blend | Shade, airflow, fresh ethanol-free fuel |
| Hot, crackly spark that fades | Cracked plug boot or wire | Replace boot, wire, and plug |
What Makes the Engine Run So Hot in the First Place?
If the generator is cooking itself, you have a second problem. Check these:
- Clogged cooling fins and shroud: grass, dust, and mud blocked from airflow. I clean these every spring.
- Running in a confined space or in direct sun: keep at least a few feet of clearance around it and shade it if you can. Never run it in a garage.
- Clogged spark arrestor: a plugged muffler screen traps heat (see spark arrestor cleaning).
- Lean running: a restricted jet or air leak makes the engine run hotter than it should.
For the whole picture on heat, read generator overheating.
My Hot-Day Routine
When I’m running a generator through an Arizona afternoon, I keep it in the shade with a few feet around it, turn it off and let it idle without load for a minute or two before shutdown, and wait ten minutes before restarting. That cooldown matters. I’ve also learned to carry a spare spark plug and a $10 inline spark tester in my tool bag, because those two items solve most of these problems on the spot.
Video walkthrough
Frequently Asked Questions
How long should I wait if my generator won’t restart when hot?
I give it 10 to 15 minutes. If it still won’t restart, I check spark while hot instead of waiting longer.
Is this the same as vapor lock on a car?
Similar idea, but small generator carburetors are less prone to true vapor lock than old car fuel systems. I see hot-restart problems come from the ignition coil more often.
Can a bad spark plug cause this?
Yes. A cracked insulator or worn electrode can fail when hot. A new plug is a $4 test. See generator spark plug for how to check one.
Could the problem be the gas cap?
Often, yes. A closed vent gives you the same pattern: run, quit, wait, restart. It’s the first thing I check because it’s free.
Should I run a generator with the choke on when it’s hot?
Almost never. I only use choke on a cold engine. A hot one needs it open, and I’ve flooded more than one engine learning that.