Bogging/hesitation At Highway Cruising Speeds

Discussion in '1994 - 1995 Specific Tech' started by sixt9coug, Jun 27, 2013.


  1. sixt9coug

    sixt9coug Member

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    Recently I have noticed while after driving the car for a good period of time on the highway, I've felt that I've been getting a "pulsing" out of my car while steady-state cruising at highway speeds. It seems to happen while around 1500rpms-2000ish rpm with steady, or very light throttle inputs.

    I replaced the distributor a while back for a seemingly unrelated problem, so I don't think that's it (ya never know though) and I have no CEL on. The problem only seems to happen after the car has been driven for a while (45 minutes or longer) and it never feels like it's going to stall, but there's a definite pulse/miss/bog to the way it feels, and you can see the tach respond to the motions too so it's not just road bumps or anything.

    Any ideas of where to start?
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  2. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    Vacuum leak.

    Edit: I guess I should elaborate. I had what turned out to be a vacuum, leak one time. I searched for weeks to find the problem. If I had to describe the things the car was doing I would do so exactly like you have said.

    The car started and ran great. Pulled hard at WOT. It was when I put it in 5th gear and was keeping a steady speed on the highway when I noticed the sputter and hesitation at slight throttle inputs. It drove me nuts. Finally we put a vacuum gauge on the car and it showed like 9 inches of vacuum. It took another week to track it down, but once I fixed the problem I had 15" of vacuum and the problem went away.

    Really hope this helps.
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  3. sixt9coug

    sixt9coug Member

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    Nice. I hate tracking them down lol, but it makes sense.

    My AIR injection system is throwing a code too, so I wonder if the leak could be there. Where was your leak? How did you troubleshoot it? I doubt mine is in the same place, but hey, it's worth looking.
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  4. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    I am a machinist. I picked up a 65MM TB off of a Ford Lightning. The EGR spacer I had was my stock one and if I remember right it was a tad smaller. So I bored it out on a Bridgeport mill. I noticed when I did that the cut I was taking got thicker at the bottom meaning the bore was tapered. I didn't figure it mattered. But as it turns out machining that taper out reduced my vacuum in the system by 6". I asked the question if that would have been the problem when I first noticed I had low vacuum. Everyone said no, that wouldn't cause it. After a week of chasing a leak that didn't exist I replaced the EGR spacer and magically I had 15" of vacuum.
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  5. sixt9coug

    sixt9coug Member

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    Ahhh. Fair enough. It looks like I'm gonna go out and get some carb cleaner then and see if I can track this down.

    This only happens when the car is warm. I mean, not operating temp, but driving around for almost an hour warm. The idle get's a little "hunty" too. It will idle ~750rpm, then jump to around ~900rpm, then back down. It sounds like a vacuum leak would make sense too, but the fact that it's not ALL the time, but only when it's "hot" keeps throwing me off.

    For example, drove it to work this morning (32 miles, almost all highway, 35 minutes) and it was absolutely normal. I am betting on the way home though (32 miles, likely heavy traffic in spots, closer to an hour drive) I will probably get the symptoms near the end of the drive.
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  6. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    May be electrical then.
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  7. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    Vacuum gauges are cheap ;) If you check your vacuum let me know what it is. Depending on your cam you should be upwards of 15". I would check it cold, and warm to see if there is a difference. Maybe a fitting is heating up and expanding a hose enough to leak? Just a shot in the dark but ya never know.

    A coil that is going bad will cut out when it gets warm also. I had one that took a couple weeks to die. I wasn't sure what the problem was until it died. I started replacing the cheaper things and that was the first thing I tried. Other than that I am not much good at electrical problems.

    Good luck. I am sure you will find it. I am curious to know what it is.
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  8. sixt9coug

    sixt9coug Member

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    No problem, I'll update when I figure it out. With the dizzy being new, I doubt that's it, but you never know. I might try a coil if I can find a known good one.

    I also have a vacuum gauge here too so I'll take a reading this weekend and see what it is. Other than a CAI kit, the car is stock so I would hope it pulls better than 15". My 69 Cougar with a stock cammed 351 would pull 21" when I had it on the road and tuned well.

    FWIW, here's a video of it driving. You'll see the tach get a bit jumpy when the car pulses.

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  9. toyman

    toyman SN Certified Technician

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    After 10 minutes of driving the engine is at "operating temperature". That operating temperature may vary from car to car, with changes in ambient temps and different T-stats but it is the "operating temperature".
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  10. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    I had the F303 cam. 15" was about normal I think. But yeah, that is right at the bottom end.

    It may be an intake gasket leaking. That may explain it getting worse after it warms up. Sorry, just throwing random ideas out here. I am still leaning toward a vacuum problem only because a sensor should throw a check engine light. But it doesn't rule out a spark problem.
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  11. toyman

    toyman SN Certified Technician

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    As vacuum leaks can be hidden and be anywhere in the vacuum system, the only truly effective way to detect them is with a smoke tester. Another way of testing is to make an adapter that will fit onto the TB which can be pressurized with a bicycle pump. If the pressure holds you could assume it's leak free. If not you might be able to hear where it's leaking.
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  12. SableSal

    SableSal Member

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    From what I see in that video, it looks more like a tach signal issues/misfire from the distributor. I would replace that distributor. This happened to me last year. Hope this helps.
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  13. sixt9coug

    sixt9coug Member

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    Is it bad that I'd love to try this out just so I can build that device? That sounds cool lol.
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  14. 5.0 Ford Guy

    5.0 Ford Guy Founding Member

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    Yeah, it's bad. You should seek professional help then let me know who you went to. Because I want to build one also.
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  15. creektrack

    creektrack Member

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    Super easy and cheap to make...I had to make one for a boost leak before...but I recommend doing it as close to the air filter as possible, as that's the only spot air is supposed to come in...just go to home depot and get some plumbing thats the same size as your intake with some hose clamps, a cap for the plumbing, and a valve stem...I used one from a car tire, just drill a hole through the cap for the stem to go through...in fact I used a 90° elbow to be able to access it easier...

    Sent from my SPH-L720 using Tapatalk 2
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  16. sixt9coug

    sixt9coug Member

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    I was thinking about that this weekend, and I thought of something almost exactly like that. Glad to know that it will likely work.
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  17. sixt9coug

    sixt9coug Member

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    I swapped my old OEM distributor in yesterday and so far it seems to be better. However, I will keep an eye on it and see if it's really gone.

    Damn Richporter distributor must have been failing on me.

    Also, I hooked up a vacuum gauge to it and it pulls around 16-17in at idle.
    #17
  18. SableSal

    SableSal Member

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  19. fordtech28

    fordtech28 Member

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    I had this same issue on my cobra it would be fine when cold and when I drove it in traffic it felt jittery but it wasnt constant.Found one of my wires arching on the header on the passenger side the one next to the EGR tube I had installed new wires months prior and I had forgot that particular wire had to be a 90 degree boot it had the 45 degree boot and it only had 1/4 of clearance between the boot and the header tube.
    #19

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