How to Test for an Engine Misfire With a Spray Bottle of Water

Written by chris stevenson
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How to Test for an Engine Misfire With a Spray Bottle of Water
Although water can cause an electrical short in an engine, it also can be a mechanic's best friend in finding a problem. (Jupiterimages/Pixland/Getty Images)

Automotive engines can exhibit some very peculiar, intermittent symptoms when it comes to engine miss and component malfunction. A vehicle can act up at odd and unpredictable times, and a mechanic may not be able to verify the problem because it never shows up when expected. Electrical shorts can be the most baffling issues to troubleshoot, sometimes occurring when the temperature or humidity changes. Actually, a direct correlation exists between moisture and electricity. A competent, do-it-yourself mechanic can use a spray bottle of water to diagnose an electrical fault in his vehicle.

Skill level:
Moderately Easy

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Things you need

  • Multimeter
  • Tachometer
  • Spray bottle
  • Water
  • Masking tape
  • Rags

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  1. 1

    Park your vehicle in a semi-dark location such as a garage. Leave the door partly open and the vehicle's exhaust pipe pointing outside. Shift into park or neutral, depending on your vehicle type. Set the emergency brake and open the bonnet. Place the positive lead of a multimeter on the positive terminal of the car battery. Place the negative lead of the meter on the negative battery post.

  2. 2

    Adjust the scale to "Volts" and read the meter. Any reading of below 12 volts indicates low battery voltage. Charge the battery to bring it up to maximum charge. An undercharged battery will not send proper voltage to the sensors, fuel pump, injectors and ignition system, which can cause an engine miss -- thus, it's important to rule out the battery before going on to other troubleshooting steps.

  3. 3

    Hook up a tachometer lead to the negative post on the ignition coil or to the trigger signal post on your coil pack, if your vehicle is equipped with electronic ignition. Refer to your owner's manual for the exact location of the coil pack wire. Hook the other tachometer lead to an engine ground source. Fill a spray bottle with water. Start the engine and let it reach normal operating temperature.

  4. 4

    Start at one plug wire location near the spark plug and spray water over the entire length of the plug wire up to the distributor cap. Look for any blue-white arcing sparks coming out of the wire and listen for electrical crackling or popping noises. Watch for signs of the engine cutting out or missing. See if an rpm drop occurs on the tachometer gauge. Proceed to the next wire, wet it down and repeat this step.

  5. 5

    Spray test each plug wire, one by one, following the procedure described in the preceding step. If you find a wire that arcs and shows a miss, turn off the engine and wrap a piece of masking tape around the wire to mark it. Start the engine again. Spray water on top of the distributor. Look for arcing sparks between the distributor poles or from the pole necks to the distributor cap body. Any sparks indicate a carbon crack in the distributor casing. Use tape to mark any pole that emits an electrical arc, but shut the engine off first.

  6. 6

    Spray a mist of water over the top and sides of the ignition coil or coil pack, depending on your vehicle. Ignition coils typically crack and emit sparks at the top pole neck, just under the rubber boot. Look for arcs on the coil pack between the plug wire connectors.

  7. 7

    Spray water over the tips of the fuel injector heads. If the fuel-injector wire connectors have cracks, they will transfer voltage to ground and emit blue-white arcs. You will see a noticeable drop in rpm on the tachometer and notice an engine miss. Replace wires, connectors and components evidencing obvious electrical shorts. Use rags to wipe up any puddled water on the engine surfaces.

Tips and warnings

  • Water is an excellent conductor of electricity. It will travel through any cut or frayed wire to a ground source. Check all of the rubber boots on the spark plug wires for cracks and proper fit. Some boots melt from extreme exhaust heat. Replace all cracked and melted plug wire boots.
  • Although inspecting for electrical shorts in a semi-darkened garage is helpful for seeing electrical arcing, make sure you have proper ventilation for exhaust gases from the tailpipe of your vehicle. Use a large box fan to expel carbon monoxide and other dangerous fumes.

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