What Do You Think It Is??????

Discussion in 'SN95 4.6L Mustang Tech' started by stangrulez, Feb 27, 2014.


  1. stangrulez

    stangrulez New Member

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    I have a 96 gt vert and I was wondering if you guys could tell me what's going on. Whenever the car goes under a quart of tank of gas it begins to shake pretty bad and the rpms move up and down and sometimes shuts of, but when I have over a quarter tank the is no shaking or anything it runs perfect. It's probably been going on for a year and a half. I've replaced the egr valve, fuel filter, fuel pump, and fixed any vacuum leaks I can find. I dont really drive it that much as i bought a new accord coupe i was gong to sell it but i just can't. Any help will be greatly appreciated.
    #1
  2. flstang65

    flstang65 Well-Known Member

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    Bad injector? Needs plugs? Misfire?
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  3. wmburns

    wmburns SN Certified Technician

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    When the fuel pump was replaced, was the gas tank cleaned?

    Are there any DTC codes?

    What is the condition of the EVAP vapor lines? Especially the ones from the TB to charcoal canister in the right hand fender?
    #3
  4. stangrulez

    stangrulez New Member

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    @flstang65 injectors are the original ones. Spark plugs where changed about 40k miles so with the most expensive ones Ford had. And there are no misfires.

    @wmburns the fuel tank was not cleaned when the fuel pump was changed. I haven't ran it for codes but will do tomorrow and I will update. I've changed the purge valve and evap canister as well. As far as the lines in the right Fender my mechanic changed them for me in front of me and the lines were complete garbage.
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  5. flstang65

    flstang65 Well-Known Member

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    Maybe go to a dyno and be checked out?
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  6. wmburns

    wmburns SN Certified Technician

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    my vote is water/debris in the gas tank. This goes double if the filler neck grommet wasn't changed.
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    Kilgore Trout likes this.
  7. trinity_gt

    trinity_gt Advanced Member

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    Not sure about water or debris (gasoline will float on water so water in the tank would likely cause driveability problems all the time).

    But I agree in spirit that the problem is likely in the tank itself and not under the hood. The OP doesn't mention the conditions under which this happens (only that the tank is less than a quarter-full) but I suspect that the driveability issues are accompanied by cornering, braking, accelerating etc. This suggests perhaps a broken baffle or a problem in the pickup.

    I'd take the pump assembly back out of the tank and eyeball the unit as well as the area in the tank in which it sits.
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  8. wmburns

    wmburns SN Certified Technician

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    On 2nd thought, that's a very good theory. IE, that the fuel pump pick-up is out of alignment or not on the bottom of the tank.
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  9. stangrulez

    stangrulez New Member

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    @trinity_gt the problem does only occur when there is less than a quarter tank, any gas over a quarter and there is no issues.

    The problem happened before I replaced the fuel pump, it was the main reason I changed it. I had the pressure tested before changing it and it was 14psi which is no good. I haven't had the pressure tested after I changed the fuel pump.
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  10. trinity_gt

    trinity_gt Advanced Member

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    Right, but does it happen at idle?

    When cruising at steady speed?

    When cornering slowly?

    When cornering hard?

    When accelerating gently?

    When accelerating hard?

    When braking gently?

    When braking hard?
    #10
  11. stangrulez

    stangrulez New Member

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    It only occurs at idle. Usually when I'm waiting for the light to change
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  12. trinity_gt

    trinity_gt Advanced Member

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    Bizarre.

    Can you get a fuel pressure gauge on it? You said before the FP it read 14psi which is, as you note, ludicrously low.

    I suggest you check it again and investigate the fuel pressure regulator and vacuum line running to it. The regulator diaphram may have a pinhole or an internal leak allowing fuel to return to the tank, causing low pressure. What's the pressure at idle? What's the pressure at idle with the vacuum line disconnected?

    Makes no sense though. Why at 1/4-tank and why only at idle?? Hmm....
    #12
  13. stangrulez

    stangrulez New Member

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    Well after it turned off again under the exact same conditions as mentioned above, a friend of mine told me to check the spark plus. I never checked them because the previous owner told me he had just replaced them and this is what I found. Sure hope this is the problem. What do you guys think?

    Attached Files:

    #13
  14. jcgafford

    jcgafford Well-Known Member

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    Looking at this on a cell phone. I would be checking the gap on those plugs. Looks huge on my screen.

    Wow. On second look those things look terrible all over. Pull them all and replace.
    #14
  15. flstang65

    flstang65 Well-Known Member

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    +1 New Motorcraft plugs for sure!
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  16. stangrulez

    stangrulez New Member

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    Went with new motorcraft spark plugs and new wires. Hope that's the problem. And the gap in them was huge.
    #16

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