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car stalls only when gas tank is half full

When gas tank gauge is half full and I come to a hard stop and sometimes when moving, my car stalls, acts as though it was out of gas. I had a fuel tank sending unit replaced and it still does it. My car is a 1988 Honda Prelude SI with 170,000 miles.


  • edited November 2010
    Two possibilities come to mind.

    1) a weak fuel pump. You'll need to test the line pressure. A full tank could be providing enough "head pressure" (from the weight of the gas) on the pump pickup to help it be able to maintain fuel flow, but ehen the tank gets low and the head pressure drops the pump is insuffciently strong to maintain presusure on its own.

    2) a saturated carbon canister. Your gas tank breaths in through the charcoal bed in the charcoal canister. If that's saturated, it cannot do so and a vacuum can develop in the gas tank's airspace as the pump draws the gas down, preventing the pump from pumping out fluid. This can lead to a failed pump. And it can be caused by "topping off" the gas if by chance you are doing this I'd recommend changing that practice.

    The problem is not the sending unit.
  • edited November 2010
    I don't know the protocal about answering but here goes..thank you for your intelligent and quick answer. I have been to three mechanics and the dealer and have not gotten any diagnosis or solution that was anything but, at best, a guess. Now I will try and find a mechanic that will run this down properly in that I feel armed with adequate information. I thank you so much. Sincerely, Judy
  • edited November 2010
    Let us know how you make out. We do care.
  • edited November 2010
    I would be happy to keep you posted, again, many thanks.
  • edited November 2010
    Are you aying that 3 or more mechanics duplicated this condition,were allowed to spend, lets say one hour looking the car over and not one had any idea what was wrong? In my day letting a car with a sympton that was both so significant and easily duplicated get away from you not repaired was cause to hang you head in shame.

    I can't imagine this happening. I have worked for people who would have fired me on the spot if I was not able to present a list of items that I had ruled out as being the cause of this condition.

    I can only attenuate my exasperation by relating that the car is an 1988 and perhaps all 3+ mechanics are used to having more help with the diagnosis. It is a shame these guys gave up so easily.
  • edited November 2010
    How do you get the car restarted after the engine quits? If it is a case of a blockage not allowing air into the tank as the fuel is used, you might try opening the gas cap. You may hear a rush of air into the tank. That would at least narrow the list of possible problems,
  • edited November 2010
    I'm confused: you said "car stalls only when half full." Literally meaning, it runs fine with more fuel, or less fuel--it only acts up at precisely 1/2 tank.

    Assuming you meant "car stalls only when half full or less," that, plus the fact that it stalls when decelerating, makes me think that the fuel sock, fuel baffle, or whatever means Honda uses to mitigate fuel sloshing is acting up.

    This would explain a problem that starts around 1/2 tank (and gets worse as the tank is continually drawn down), but it doesn't explain a problem that happens ONLY at 1/2 (not at 3/4 or 1/4 or 1/8).

    1. Did you mean (a)"only at half tank," OR (b) "only at half tank (or less)"?
    2. If you continue to drive below 1/2 tank, does the problem get worse?
    3. In addition to occurring when decelerating, does the problem occur when making extended turns (such as a "cloverleaf" on- or off-ramp)?
  • edited November 2010
    The part about changing the fuel tank sender is a bit fuzzy and puzzling to me. A sender only provides a signal to the gas gauge; it does not have anything to do with the way the car runs.

    Other than a potential fuel pump issue, there are a number of things that could cause this.
    Idle Air Valve problem, failing ignition switch, problem in the spark ignition system, and while my memory on this is very hazy it seems to me this model used a deceleration fuel cut-off relay or module.
    Maybe this fuel cut-off is acting up. After 23 years and high miles it's feasible anyway.
  • edited November 2010
    I presume that the fuel pump and sending unit are a a single unit, right? That eliminates the possibility that it is a pump, a pickup sock, or tubing inside the tank.

    Does this car have a 'split' fuel tank with half on either side of the spare tire cavity? If so, there must be some sort of mechanism for transferring fuel over to the fuel pump side when the tank is low. I would look at that transfer mechanism.
  • edited November 2010
    I have only done about 7 or 8 fuel pump, sending unit, or internal line replacements but I have never seen one where the pump, lines and sending unit were not all separate.
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