Fuel system vacuum pump, what does it do?

Discussion in 'Clarity' started by ken wells, Feb 16, 2019.

To remove this ad click here.

  1. ken wells

    ken wells Member

    I couldn't find threads related to this. The manuals remark in a couple of places about a vacuum pump associated with the gasoline fuel system. Can someone tell me what the pump does? Where does it pull (vapor?) from and where does it push it to?
  2. To remove this ad click here.

  3. insightman

    insightman Well-Known Member Subscriber

    Depressurizing the gas tank keeps your gas fresh longer. Some people advocate using a gas additive product like Stabil to add another level of confidence in the freshness of the gas that you haul around month-after-month while driving on EV power.

    If you're one of those people who have added gas to your Clarity (or someone who just tries all the buttons), you'll have noticed the delay in opening the gas-filler door. I don't know why the Clarity can't simply open the door and release the vacuum without the delay. Would a sudden release in the vacuum cause gas to spill out if you opened the gas-filler door when the tank is already filled (or over-filled)?
    Last edited: Feb 17, 2019
  4. ClarityDoc

    ClarityDoc Active Member

    A vacuum cannot be released, since it's an absence. Relieving the vacuum, i.e. sucking air into the tank, might be something they engineered - and the fill port might be the wettest (i.e. worst) path for that. Not sure but I hope they might have a dry, perhaps filtered, path for air to enter when repressurizing (on opening).

    Sent using Inside EVs mobile app
    BobS and insightman like this.
  5. neal adkins

    neal adkins Active Member

    My camry hybrid did had the same delay. I wonder if this coild be a safety precaution to avoid the possibility of a spark from static electricity or because of the batteries being close to fuel tank.
  6. ken wells

    ken wells Member

    In an ICE car, warm days cause the gasoline vapor pressure to increase. To prevent escaping vapor pollution, the gas tank is attached to a carbon granule canister. The carbon absorbs the gasoline vapors. When the engine is run, the intake manifold develops a vacuum. It is attached to the canister, and it sucks the vapors our of the carbon and they get burned by combustion. Since the engine runs frequently, it keeps the carbon from getting saturated.
    A PHEV may go long times without the ICE turning on to empty the canister of absorbed gasoline vapors. I'm wondering if the vacuum pump is for emptying (or filling) the canister. But if the vapor pressure builds up over time, and the vacuum pump empties it, where does the pump exhaust go? You can't release it to the air, you can't burn it in the engine if the engine doesn't run. So, what is is connected to (input and output) and when does it run? And why?
  7. To remove this ad click here.

  8. Walt R

    Walt R Active Member

    Are we sure it is under partial vacuum? I got the impression it was retaining pressure. This makes sense to me, as when the vapors are created with temperature rise, rather than venting them or chemically absorbing them, they can just be retained at higher vapor pressure, and will re-condense into the gasoline if the temperature falls. If gasoline is burned then the free space in the gas tank will increase, reducing the vapor pressure without actually losing any vapor.

    Keeping the vapor to re-condense also makes sense since it would keep the gasoline mixture from changing - more volatile compounds escape to vapor more readily, and this is more of an issue if more time elapses before adding fresh gasoline. Under this theory vapor would end up being vented when depressurizing to refuel, but not between refuelings, and the tank has the most free vapor space if it is refueled near empty.

    This could be all wrong - I'm not a chemist, but I am a physicist.
    insightman likes this.
  9. Groves Cooke

    Groves Cooke Active Member

    "I got the impression it was retaining pressure."

    You are correct, there is no vacuum; the fuel system is pressurized. Here is a quote frpm the owners manual:

    "This vehicle has a pressurized fuel system.
    When the fuel vapor pressure inside the fuel tank is
    high, it takes about 10 seconds for the vehicle to vent
    the fuel tank. Once the pressure is vented, the driver
    information interface changes from Please Wait to
    If the filler nozzle keeps turning off when the tank is
    not full, there may be a problem with the pump's fuel
    vapor recovery system. Try filling at another pump. If
    this does not fix the problem, consult an authorized
    Honda Clarity Plug-In Hybrid dealer.
    Do not continue to add fuel after the filler nozzle has
    automatically stopped. Additional fuel can exceed
    the full tank capacity.
    The filler nozzle automatically stops to leave space in
    the fuel tank so that fuel does not overflow as a result
    of changes in air temperature."
    MNSteve and insightman like this.
  10. ken wells

    ken wells Member

    Hello fellow physicist. Gasoline vapors in the tank will be under a relative vacuum or pressure depending on ambient temperature. In this weather, my plastic gas cans crumple inward. In the summer, if left in sunlight, they bulge outward. So, it depends.

    From the owners manual, describing sounds unique to the Clarity: "Noise from Under Vehicle: This noise is caused by the vacuum pump inside the fuel evaporation leakage check module (ELCM). Depending on conditions, the pump will come on for about 15 minutes about 5 to 10 hours after engine shutoff. This noise is just normal vehicle operation and don’t indicate a vehicle problem."

    I don't know of other cars using a vacuum pump as part of the fuel system. My original question stands: What does this pump do? What is it connected to on the input? What is it connected to on the output?
    KentuckyKen likes this.
  11. neal adkins

    neal adkins Active Member

    Wouldn't it make sense to suck it from the canister and put the vapors back into the fuel tank? This would also repressurize the fuel tank after the the fuel condenses after cooling down at night. Im definately not an expert though.
  12. To remove this ad click here.

  13. ken wells

    ken wells Member

    SOLVED, nothing unique or special. Full explanation of similar system here https://www.motor.com/wp-content/uploads/2016/02/6_EvapSystem_0216.pdf

    TL;DNR: When I read that the vacuum noise was "unique to the Clarity", I assumed this fuel system was unique, perhaps because the gasoline might stay unused for so long in the Clarity. Nope. This is just a pretty standard way of checking for leaks in a car's fuel vapor system as required by current emission regulations. What a disappointment. OTOH, I learned how fuel vapor leak detection worked, so that's fun.
    KentuckyKen likes this.

Share This Page