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5.0 V8 99 Mountaineer stumble under acceleration

Discussion in 'Stock 1995 - 2001 Explorers' started by JimM, February 9, 2018.

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    1. JimM

      JimM New Member

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      My 99 Mountaineer 5.0 AWD has been trouble free for the 40K miles that I have put on it's total mileage of 201K. It does exhibit two issues that I have not been able to resolve.

      1. It has a 'stumble' or 'miss' under certain acceleration conditions. If I accelerate hard it performs as you would expect, but I don't normally drive it aggressively. If I accelerate more casually or while pulling a trailer, it will intermittently miss or stumble. It seems to be related to a high vacuum, lower rpm condition.

      2. It has the 'warm restart' issue that I have seen in threads. I replaced the Engine Coolant Temp Sensor based on a previous response, with no improvement. If it is cold it starts right up. If warm, it will hesitate. If I crank it, stop, and then re-crank it will start normally nearly every time.

      I included both issues here in the event they could be related. Thanks in advance for any assistance.
       
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    3. Mbrooks420

      Mbrooks420 High Voltage. Elite Explorer

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      How old are the plugs and wires? No check engine light is on?
       
    4. JonDouglas

      JonDouglas New Member

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      I'm looking for info on this "stumble" (great word for it, actually) as well. My girlfriend's '98 Mounty is doing the same thing. Mostly around 45 to 55 mph under both cruising and acceleration conditions. Occasionally it will happen at lower speeds under acceleration. Thought it was a drivetrain issue, because it bangs when it happens, kinda throws you forward a bit. Checked rear diff, fluid was old but everything checked out, no metal, No change after putting it back together with fresh juice. After looking around here, Im going to test the TPS, check the MAF and IAC and go from there.

      @Mbrooks420, For us, 4mo. old plugs and wires, no CEL,

      As for your crank-stop-crank-start, on ours it was the camshaft position sensor going out. It was pretty hammed when I took it out, would have left us stranded sooner than later. Replaced and first crank starts are back.
       
      Last edited: February 11, 2018
    5. 96eb96

      96eb96 Well-Known Member

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      GFs mounty needs a full mercon V fluid exchange, that is torque converter stumble. OR it could be a misfire.

      JimM:
      Your problem sounds like a MAF(hesitation), clean it first, and an tired IAC(cold soak issue).
       
    6. koda2000

      koda2000 Explorer Addict

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      Typically a "stumble" under acceleration is spark related (plugs/wires). Last time I experienced this was on our '95 Pontiac with the 3.1 V6. It turned out to be that the coil pack had gotten moister in it and had a rusty hole. I cleaned it out and put some dielectric grease in it, I also cleaned the offending plug wire end and the stumble was gone. Afterwards I replaced the coil pack, plugs and wires.

      On Fords, I highly recommend you use Motorcraft plugs and wires, as they seem to be fussy about some brands (light Bosch).

      The warm start problem may be fuel pump related. You might have a split hose inside the tank. This can reduce fuel pressure. When you turn the key off and on multiple times it makes the fuel pump prime again. I wouldn't bother dropping the tank w/out replacing the fuel pump and the submersible hose(s).
       
      Last edited: February 11, 2018
    7. Mbrooks420

      Mbrooks420 High Voltage. Elite Explorer

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      Actually it probably wouldn’t ever leave you stranded. The truck will run without this sensor giving any feedback, and the only imperative part is the bottom gear which drives the oil pump.

      Second the Motorcraft plugs. I bought some expensive double platinums for my first plug change, and it ran poorly after a few months. I replaced them with the factory plugs and ran them for 60-70k.
       
    8. koda2000

      koda2000 Explorer Addict

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      I put Autolite double platinum in all my trucks (V8's & V6's). Never had a problem with them. Autolites are pretty much the same as Motorcraft plugs.
       
      Last edited: April 2, 2018
    9. JonDouglas

      JonDouglas New Member

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      Motocraft plugs and trans fluid. I was already considering getting the trans hooked up to a fluid machine, right track it seems! Thanks folks, I'll be back to talk more after I get it done.
       
    10. JimM

      JimM New Member

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      After action report on my misfire problem: I started with the plugs as the simplest and least expensive step. It was the plugs, badly eroded. I have no idea how long they had been in the truck but at least the 30K miles I've put on it. Solved the misfire problem and picked up an additional 1.5 miles per gallon with the new Autolites installed. Thanks very much for all the input here.
       
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