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  1. #1
    FEP Member
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    Default Dies after extended driving on hot days. Not overheating

    Hey guys, been having an issue on hot days. After driving around for a long time, then encountering slow traffic, it'll cut out on me. If I try to immediately crank it back up, it'll start ok, then sputter after 5-10 seconds. Feathering the gas will keep it from completely dying for a little bit, but it's obviously not happy. Let it sit for 15-20 minutes, and it's back to normal. I'm almost thinking it's fuel related, but not sure how to verify. Every time it's happened, it's been off the side of a busy street where I couldn't hear the fuel pump. The engine definitely isn't overheating, as the coolant is at a happy temperature, and the fuel system (at least when this isn't happening) is pumping strong as well.
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

  2. #2
    Moderator wraithracing's Avatar
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    Default

    I assume this is your 86 GT since you don't state the vehicle. How old and what condition is your TFI module in? Cap/Rotor/wires/plugs?

    How old is the fuel pump and what size is it? How much fuel is in the tank when this issue occurs? Any idea what the fuel pressure is at the injectors?

    I would check over those area first and see if you find anything. Start with the simple possible problems first and work from there.
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Stalled RestoModification
    1984 SVO Still Waiting Restoration
    1986 GT Under going Wide Body Conversion Currently

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Stalled in TX
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  3. #3

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    TFI for sure would be my first stop. Make sure you use thermal paste if you do replace.

    Ran into the same issue last year, would recommend the motorcraft replacement.

  4. #4
    FEP Super Member erratic50's Avatar
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    Default

    +1 on both points Trey brought up

    At one time my fuel pump sounded fine but my fuel pressure went to crap. Found it by zip tying a harbor freight gauge to a wiper and drove it. Replaced the pump. Stupidly I put in a 255 which caused all kinds of problems with too much fuel pressure with the regulator not in command of it due to return line size.

    Down the road I had the exact same problem you describe with mine years ago, then the ignition stator (electronic pickup inside the distributor) failed completely and the car wouldn’t start or run anymore. Replaced it and things were fine

    Are you using an aftermarket or a factory coil?

  5. #5
    FEP Member
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    It is the '86 in my signature.

    I'm not sure how old the TFI is, but it's not the original.

    Cap/rotor/plugs/wires are either new or verified as good.

    Judging by the page given to me by it's previous owner (also a member here), the pump was replaced about 5 years ago. Not sure what size. I would imagine a GT40P setup with the stock cam is not overtaxing even a stock size pump. Tank was almost full. Injector pressure should be stock.

    I can't remember which brand the coil is, but I do know it was quality. I want to say it was Mallory or Motorcraft.
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

  6. #6
    Moderator wraithracing's Avatar
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    Hate to just start throwing parts at a problem, but my first suggestion would be a new TFI module. Motorcraft would be my first choice, as noted above dielectric grease film before mounting the new one.

    If you want to try to narrow down the issue first, then I would definitely install at least a temporary fuel gauge on the rail to verify fuel pressure while driving and when the car dies. That should help determine if the problem is fuel related or not. Good Luck!
    ​Trey

    "I Don't build it hoping for your approval! I built it because it meets mine!"

    "I've spent most of my money on Mustangs, racing, and women... the rest I just wasted."

    Mustangs Past: Too many to remember!
    Current Mustangs:
    1969 Mach 1
    1979 Pace Car now 5.0/5 speed
    1982 GT Stalled RestoModification
    1984 SVO Still Waiting Restoration
    1986 GT Under going Wide Body Conversion Currently

    Current Capris:
    1981 Capri Roller
    1981 Capri Black Magic Roller Basket Case
    1982 Capri RS 5.0/4spd T-top Full Restoration Stalled in TX
    1984 Capri RS T-top Roller
    1983-84 Gloy Racing Trans Am/IMSA Body Parts

  7. #7

    Default

    How's the fuel filter? Run any fuel system/injector cleaner through the car lately? Any perceived resistance to blowing through one and it's toast, replacement time. I went through five new fuel filters before acceleration bucking and cutting-out finally stopped, after running some such cleaner in an '86 5.0 'Bird...
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  8. #8

    Default

    Yup yup ^^, I was thinking fuel filter as well while people were discussing checking the fuel pressure. I have seen systems maintain pressure but not have enough volume. However, the TFI module would be my first stop.
    1984.5 G.T.350 had since 16y/o
    95 Cobra, Crystal White

  9. #9
    FEP Power Member Broncojunkie's Avatar
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    Default

    Most of the parts stores can bench test your tfi. Make sure they repeat the test a few times. I've seen them pass the 1st test, and fail repeated tests after heating up. Or they can even hit the thing with a hairdryer/ heat gun first to simulate engine temps.

  10. #10

    Default

    I've never had a parts store fail a tri. They always say it checks out good.

    If it is the tfi, its stupid easy to check. You could run codes which should tell you, but people seem to have a hard time coming up with 2 mins and a paper clip, so ill explain how to check it the hard way.

    Unplug an injector. Doesnt matter which one. Check for a 12v pulse while the engine is cranking. That's it. If the tfi is dead, computer doesn't know the engine is cranking and won't fire the injectors.

    Generally I recommend both the tfi and stator in the dizzy get replaced at the same time. They usually go bad at about the same time and they work together.

    I mean there really are quite a few ways to actually know why the car is dying, but if you are dead set on a tfi that all you need to know.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  11. #11
    FEP Senior Member
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    Any chance you can test for spark when the problem occurs? Haystack's advice would be a good quick and easy check as well. I would also grab a fuel pressure gauge and put it in the car so you can check that when it happens. I had a similar issue on my LTD a couple of years ago. A new TFI didn't fix it, and the problem was too difficult to reproduce so I just wound up throwing parts at it. In the end I suspect the pickup in the distributor was flaking out and would no work when it got hot, but resumed working fine after cooling down 15-20 minutes. I also wound up relocating the TFI (Motorcraft for what it is worth) over to the driver's side wheel well to help keep it cool.
    '89 XR-7 5 Speed
    '95 SC 5 Speed
    '91 Crown Vic P72 351W
    '97 Thunderbird
    '85 Ford LTD Squire

  12. #12

    Default

    These are not symptoms of a bad TFI module. A bad TFI module almost never fails when driving because its temperature is constant. It does stop working after the engine is turned off, because its temperature goes up (no coolant flow, no airflow over it). The TFI is the boogyman part in the Mustang world. I've owned three Mustangs with TFI and we have a couple at work. I've never seen one fail, despite very hard track work with all of these cars.

    Your symptoms are of something overheating from a long term thermal issue. I would guess that it is the fuel pump. The temperature of the fuel increases very slowly, the longer the engine is on. This also heats up the pump slowly. If the filter is clogged and/or the pickup hose is collapsing, this will stop fuel delivery.
    Jack Hidley
    Maximum Motorsports Tech Support

  13. #13

    Default

    mine had similar issue turned out to be magnetic pick up in the distributor'but I have duraspark

  14. #14

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    is the tfi on an aod tranny on an 86?

  15. #15
    FEP Member
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    Default

    I replaced the fuel pump (was wanting to upgrade and fix the sending unit anyway), and borrowed a known good distributor. Haven't had a chance to properly test it as it takes a very long time of driving in high heat for the issue to happen. I also bypassed the clutch safety switch, so I can use the starter to get out of traffic instead of having to push again. Burned the crap out of my feet last time because my flip flops came off.

    Manual transmission btw.
    '86 Mustang GT T-top. GT40P swap, Maximum Motorsports subframe/coilovers, MGW shifter
    (RIP) '86 Mustang GT, T-top. Cipher's old car
    '91 Wrangler. 4.0, 5 speed, 8.8 swapped, locked, lifted, caged
    -Steve

  16. #16
    Venomous Moderator Hissing Cobra's Avatar
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    The TFI module in my '89 LX 5.0/5-speed kept acting up and when cold, the car would start fine and run excellent. Every morning on my way to work, I'd get 3 miles from my house to an intersection and it would die out there constantly. It then wouldn't start or run at all. Luckily for me, there's a gas station right there so I could coast into the gas station and let it sit until it would cool down. I'd then be able to make it the next 3 miles to work before it would die again. Once I replaced it, I had no more problems for the final 6 years of ownership. I'd start there!
    Last edited by Hissing Cobra; 05-31-2019 at 10:08 AM.
    Pete Slaney

    1979 Mustang Cobra

    347/T-5/4.30's
    420 rwhp/380 rwt (New Motor)
    11.49 @ 121.86

    306/T-5/4.30's (Old Motor)
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    12.38 @ 111.38

  17. #17

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    My 86GT had a similar issue several years ago. It wouldn't completely die when it got really warm outside, it would stutter and misfire. Once the car was shut off it wouldn't restart until it cooled off. It also wouldn't start if it sat in the sun on a 100+degree day for several hours.

    Ended up being the PIP in the distributor.
    Jeremiah

    1986 Mustang GT 5spd, 3.27's
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