Vapor Lock - an overlooked cause of engine failure

Rubber fuel cap vent on a Bolens woodsplitter

So you read our previous article and you followed all the startup steps for your snowblower and it’s purring like a kitten, chewing through the snow.  You’re golden now, or so you think as you hear the RPMs begin to drop only a minute or two after start up.  The engine slowly winds down, begins to sputter, then quits.  You pull on the pull rope, but nothing happens.  You repeat the start up sequence, but still no luck.  You can feel your blood pressure climbing as you fight to get the motor start for ten or more minutes.  Just when you’re about to give up, it starts again, but only briefly before slowly sputtering out again.  Defeated, you reach for your shovel in disbelief. 

I’ve seen this happen multiple times in the past several years, so much so that it’s one of the first things I look for when a customer complains of an engine that starts, then dies.  It’s called vapor lock and is extremely simple to diagnose at home.

The principle is simple enough:  As the fuel flows from the fuel tank into the carburetor, air must have a way to fill the empty space in the fuel tank to take its place.  If air cannot enter the tank, then fuel can not leave, starving the engine of fuel.  The way air enters the tank is through a fuel cap vent.

Fuel cap vents can have a variety of different designs.  Some of the older Briggs and Stratton lawn mower caps have three small holes in them to allow venting.  Other models have rubber one-way valves that perform the same function.  Some styles are more elaborate than others, but the principles and concepts remain the same. 

Sometimes these components fail, leaving us in a bind called vapor lock.  The holes can get plugged with dirt, snow, or ice.  I’ve seen the valves fall off the cap and into the tank causing fuel flow restrictions at the vent and the fuel hose.  I’ve also found vents to be just plain missing and the cap sealed tight again the tank. 

A quick and simple way to inspect the vent cap is to remove the cap and shake it.  If it rattles, it’s generally still good.  If not, inspect it for a rubber vent or look for the holes and make sure they are clear.  Not all cap vents rattle, but a lot of them do.  Another quick diagnostic check you can perform at home is after engine failure.  If the motor is in vapor lock, unscrew the cap and leave it off for thirty seconds and then screw it back on tight.  If the motor starts and repeats the same symptoms, you can be quite certain the issue is likely the fuel cap.  To confirm that this is the issue, remove the fuel cap completely and start the machine.  If it runs without issue, you know you’ve found the culprit. 

Safety note: It’s not generally recommended to do this with a full tank of fuel, as the vibration of the motor can cause fuel to splash onto the muffler or hot engine block and start a fire. 

You can find fuel caps online or at your local parts store and they are generally inexpensive.

In closing, vapor lock isn’t the end of the world if you know what you’re looking for.  If you find yourself struggling to diagnose a failing motor, or if you’re struggling to replace or repair your fuel cap, give Polar Tree and Engine Care a call at 907-414-0600.  We are here for all your small engine service needs.