How to Check and Test a Gasoline Vapor (Fume) Detector

Look at the helm for a small panel labeled "fume," "vapor" or "gasoline" detector, usually with green, yellow and red lights and a test or reset button. Note the brand and model and download the manual. Testing and sensor replacement vary by maker, so the manual comes first.

Most units use a similar pattern (check yours):

• Green: powered and working.
• Yellow (fault): a wiring problem or a disconnected or failed sensor.
• Red with horn (danger): vapor at the alarm level. On many units, the horn can be silenced for a short time but comes back while vapor is still there. A red light means the problem is not solved.

With the detector powered (often whenever the battery switch or ignition is on), press the test button. On many units the lights cycle and the horn sounds. No horn or no lights means the display, fuse or power wiring needs attention before you trust it.

Open the engine hatch and find the sensor (a small round or square puck on a cable). BoatUS advises mounting it in the engine-space bilge as low as possible, but above the normal slosh height of bilge water, because the sensor is not waterproof. One maker's rule of thumb is no lower than the starter solenoid. Also check that:

• It's away from the hottest parts of the engine, such as exhaust manifolds, which can damage it.
• It's not where washdown water or spray hits it.
• If the boat has a second sensor (some have one under a cabin sole or near the tank), it follows the same rules. A sensor mounted high on a bulkhead will smell vapor late. If yours is badly placed, have it moved.

Follow the sensor cable to the display. Look for chafe, pinched spots and corroded plugs. BoatUS notes the sensor cable usually can't be cut or shortened, because its length is part of the calibration; coil extra cable neatly instead. Many units show a yellow fault light if the sensor is unplugged; that is one way your manual may tell you to check the fault circuit.

Use only the test method in your manual. Some makers say to hold an unlit butane lighter next to the sensor and press the button to release a little gas; within seconds the red light and horn should come on, then clear about a minute after you take it away. Do this with the engine off and nothing else running in the space, and never use gasoline. If the alarm doesn't trip, the sensor is not doing its job.

• BoatUS suggests testing vapor alarms monthly, using the maker's procedure.
• Sensors wear out in the marine environment. BoatUS advises replacing them after no more than five years; one maker recommends every three to four years. Follow your manual.
• Replace a sensor right away if it has been underwater or soaked. Mark the install date on the sensor cable with a tag.

Some detectors switch on the bilge blower automatically when they sense vapor. If yours does, the alarm test in step 6 should start the blower too. The blower must be ignition protected (made not to spark), as all electrical gear in a gas engine space must be.

The detector is an extra layer, not a replacement. Federal rules require boats with gasoline inboard and sterndrive engines in closed compartments to have a blower and a label near the ignition telling you to run it for four minutes and check the bilge for vapor before starting. BoatUS adds: sniff the engine compartment and bilge, especially around the tank, after every fueling and before every start. Detector makers say the same thing: their units supplement normal safety practices.

First assume the alarm is real and follow the safety box. Once you are sure there is no fuel leak, common causes of nuisance alarms include:

• Solvent, paint, adhesive or degreaser fumes after work in the bilge.
• Bilge cleaners, alcohol or aerosol sprays.
• Hydrogen given off by batteries while charging; some sensor makers warn against mounting sensors near or above batteries.
• Exhaust fumes drawn into the engine space.
• A wet, dirty or old sensor, or a weak connection. Ventilate, clean up the source, and test again. If alarms keep happening with no cause, replace the sensor or have the system checked. Never unplug a detector because it is "annoying".

Follow the safety box. BoatUS advises shutting down the engine, turning off the battery switch, opening the engine compartment and looking for the source. A blower can help clear vapor from a small leak, but it won't remove liquid gasoline. With a large spill at the dock, get everyone off and call 911; don't run anything electrical, including the blower. On the water, use a cellphone or a handheld VHF away from the fumes rather than a fixed radio, keep everyone in life jackets toward the bow, and call for help.

One more warning sign, even with no alarm: an engine that refuses to start right after fueling. BoatUS explains that a leaking fill hose can make the bilge vapor too rich to burn inside the engine; as fresh air mixes in, for example when someone opens the engine hatch, it drops into the explosive range and a spark from the starter or a switch can set it off. Stop cranking and check for a spill. See Fuel Fill and Vent Hoses. BoatUS reports that boats with older fill hoses have exploded right after refueling, including a cruiser that blew up leaving the fuel dock when the owner restarted an engine that had stalled. Its fueling routine helps the detector do its job: engines, motors and stoves off and the battery switch off while fueling, hatches and ports closed so fumes stay out, then open everything, air out and sniff before starting.

Write the test date and result in your boat log each time. It's a simple habit that shows when the sensor is getting old and gives a surveyor or insurer proof that the system works.