4.9 hesitates on highway
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HT4100 4.1, 4.5, 4.9 Discussion, 4.9 hesitates on highway in Cadillac Engine Technical Discussion; hello all, i own a 1994 Cadillac Deville with the 4.9. i'm not quite sure how to describe what it's ...
  1. #1
    louisinchains6 is offline Cadillac Owners Member
    Automobile(s): 1994 Sedan DeVille
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    4.9 hesitates on highway

    hello all,
    i own a 1994 Cadillac Deville with the 4.9. i'm not quite sure how to describe what it's doing, but here goes. When on the highway or when traveling at speeds above 50 mph my car hesitates or jumps even while i'm holding the gas steady. When I'm accelerating it doesn't hesitate at all. It feels like the gas is surging randomly. this also happened with my 1990 Sedan Deville 4.5. so needless to say i've tried replace almost every emissions part there is. thanks for any advice.

    louis

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  3. #2
    kdcing is offline Cadillac Owners Fanatic
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    Re: 4.9 hesitates on highway

    With key in ignition and ignition turned to On/Run (engine off or on....doesn't matter), press the "OFF" and "Warmer" buttons on the HVAC control for about 4-7 seconds. Check for codes.

    If P039 shows, there is an issue w/ the VCC inside your transmission. The VCC locks up the torque transmitted from your engine to your transmission via a mechanical clutch. The VCC locks up only at steady state speeds in 4th gear...I do not believe that the VCC is designed to engage in any other gear but 4th gear. During acceleration and decelerations, the VCC unlocks.

    Although, I have not driven a vehicle w/ a defective VCC, I have read other posts that have indicated symptoms similar to those that you've described (i.e. engine bucking at speeds high enough for transmission to be in 4th gear).

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.9 hesitates on highway

    Try driving at 50 mph with your left foot slightly on the brake pedal. This will disengage the VCC, if the problem goes away, you have a transmission problem, if not, let us know.

  5. #4
    caddydaddy's Avatar
    caddydaddy is offline Cadillac Owners Connoisseur
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    Re: 4.9 hesitates on highway

    How recent is the tune up? My '94 did something similar, and a set of fresh spark plugs and wires fixed it.

  6. #5
    cadillac_tech is offline Cadillac Owners Fanatic
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    Re: 4.9 hesitates on highway

    My first thought is your A/C compressor cycling on and off. This would be a normal condition.

  7. #6
    caddydaddy's Avatar
    caddydaddy is offline Cadillac Owners Connoisseur
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    Re: 4.9 hesitates on highway

    Quote Originally Posted by cadillac_tech View Post
    My first thought is your A/C compressor cycling on and off. This would be a normal condition.
    I've never had a car do that from the A/C compressor cycling. My '94 Deville doesn't surge as he described with the A/C on.

  8. #7
    cadillac_tech is offline Cadillac Owners Fanatic
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    Re: 4.9 hesitates on highway

    It all depends on your definition of "is", "that", "hesistate", "buck", "surge", "jump", etc.....
    Only 'louisinchauns6' knows what's up and , ... "i'm not quite sure how to describe what it's doing, but here goes" ....... leaves us all open to our own ideas.
    'louisinchauns6', someone needs to ride with you to figure out what is actually being felt, then it can be isolated.

  9. #8
    Tailfin's Avatar
    Tailfin is offline Cadillac Owners Enthusiast
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    Re: 4.9 hesitates on highway

    I don't think this is as likely, but might want to check the fuel pump out. A pressure tester isn't much, and there's a fitting for it on the fuel rail. With the car running, it should be 35-38ish, and throttling it shouldn't make it drop below that, or at least not much.

    I would agree that you should check your ignition components first, though. I have had the feeling you describe from bad misfiring, and it happened with slight throttle because putting the pedal down at all gave it plenty of fuel to ignite.

  10. #9
    louisinchains6 is offline Cadillac Owners Member
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    Re: 4.9 hesitates on highway

    thanks guys, i haven't had much time to try anything this week. i'll try everything today. thanks so much for the help. i'll let you know how it works out.

  11. #10
    louisinchains6 is offline Cadillac Owners Member
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    Re: 4.9 hesitates on highway

    ok, so i tried pressing the brake pedal slightly while it was hesitating and it totally went away. so i guess i have a problem with the vcc. is it hard to fix/expensive? i'm not sure i could do it myself. thanks a lot.

    -louis

  12. #11
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.9 hesitates on highway

    Well, don't jump to conclusions yet...it could be the VCC engaging/disengaging, or you could just have one cylinder misfiring when you're at speed and the VCC engaged simply magnifies its effect.

    When the VCC is engaged, you nearly have a direct connection between the engine and the driveline--similar to a manual transmission car--so you will feel misfires MUCH more than without it engaged.

    It's hard to tell what your problem is just by the description.

    Here's where I'd start: buy a new set of plugs (you can even buy AC Delco regular plugs for about $1.50/each) and put them in. Then, if you have a multimeter, check the resistance of each of the plug wires, or just buy a new set.

    I'd say with about 60% certainty that doing this will solve your problem. The plugs will only cost you about $12, so why not try?

  13. #12
    louisinchains6 is offline Cadillac Owners Member
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    Re: 4.9 hesitates on highway

    i'm hesitant to do this as the plugs and wires are only about a year old. if i do replace both of these, what is a good reading from the miltimeter?

  14. #13
    behind-bars is offline Cadillac Owners Connoisseur
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    Re: 4.9 hesitates on highway

    Quote Originally Posted by louisinchains6 View Post
    i'm hesitant to do this as the plugs and wires are only about a year old. if i do replace both of these, what is a good reading from the miltimeter?
    What brand are they? I had 7 no name brands and a bosch platinum in mine, all fairly new, and it had similar problems as you.

    I pulled the bosch and one other out and put ac delco rapid fires in and it cured it a bit. Problem is back again so i may need to do the other 6 when i get time.

    Does yours sorta hesitate if you floor it whikle moving? If im running 30-40 and floor it their is a few second gap before it really takes off, just wanted to know if i had the same symptoms as you

  15. #14
    louisinchains6 is offline Cadillac Owners Member
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    Re: 4.9 hesitates on highway

    i dont have any problems when i floor it. its mainly when i'm just cruing at 50 and over. i always thought that the hesitation during that kind of acceleration was the overdrive kicking in. btw, i have all bosch platinums. the three tipped ones.

    -louis

  16. #15
    peteski is offline Cadillac Owners Enthusiast
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    Re: 4.9 hesitates on highway

    If you were in the diagnostic mode display while driving on the highway, there is an indicator that lights up when TCC engages. You could watch it to see if the engagement corresponds to the roughness. I don't know what the TCC indicator is on a 1994 car. But it is there. There is also an indicator for the 4th gear.

    Peteski

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