'92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.
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HT4100 4.1, 4.5, 4.9 Discussion, '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration. in Cadillac Engine Technical Discussion; My '92 Coupe deVille has a really heavy engine vibration/knock/shudder when accelerating. It happens when I put the car into ...
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    Cadillac_Fan's Avatar
    Cadillac_Fan is offline Cadillac Owners Fanatic
    Automobile(s): 92 CdV Spring Edition;92 Brougham d'E 305;98 Seville STS
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    '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    My '92 Coupe deVille has a really heavy engine vibration/knock/shudder when accelerating. It happens when I put the car into drive as well as acceleration at highway speeds. When I let off the gas, the car seems to run smooth and normal.

    It is not unusually loud, it does not hesitate, bog down, stall, or smoke. I've tried gas stabilizers and had the injectors cleaned and that seemed to make no difference.

    Another observation is that I am only getting about 18mpg on the highway at 70mph - plus the car smells like fuel/running rich.

    The only codes it is throwing are E52 (ECM reset) and F48 (climate control fault). I doubt either of those issues are affecting drivability.

    Does this characteristic sound like an emissions, spark/ignition, or *gulp* some kind of major internal engine rod/bearing/cam/valve train? Like I said, it smooths out and stops when I let off the gas but it shudders and knocks like hell upon acceleration.

    As always, thanks in advance for any help, insights, or suggestions.

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    How many miles on it? These 4.9's were brutal on motor mounts...could be what you are hearing/feeling is motor mount related...

    There are 5 motor/trans mounts on these things...

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    Re: '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    164,000. I had motor mount work done at 160,000 a few years ago. (This is a hobby car I seldom drive.) I suppose it is possible the mount(s) failed again? I believe I also did a transmission mount at that time, too.

    It is odd as there is no misfiring, stalling, hesitation or smoking. If I step on the gas, it hustles right up to 70 with ease. (It just vibrates like nobody's business under acceleration.)

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    There are 5..depending on which one(s) you had done, can lead to a ton of problems. I had a shimmy in the front end of my STS (on acceleration) that wouldn't go away for the life of me...till I found the problem was a torn motor mount...



    ----------

    Could also be CV joint...

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    Re: '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    Another peculiar thing it did on several occasions... I was revving the engine in Park (thinking I could kick the carbon out of it) and there were a few instances where it made a hard snap/clank sound and the car actually rolled back in the driveway a couple inches - almost as if it had been out in reverse for a millisecond. I put the shifter in and out of reverse and tried to replicate that scenario again and it did make that awful sound and moved backwards a few inches before stopping itself (all of this happened in Park). Bizarre. I've never revved an engine and had it disengage from Park momentarily.

    ----------

    Many thanks for the trove of info. I had no idea there are five mounts that anchor that engine. I want to speak somewhat intelligently on this when I drop it off at the garage. What I would give to have the technical savvy and expertise! Much gratitude.

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: '92 Coupe deVille (4.9) shudders, knocks and vibrates on acceleration.

    It may not have shifted out of Park, you may just have a sheared fastener that hangs up and finally releases with application of torque. Here is a picture of the lower 4:



    The last one is affectionately called the "dog bone" and is located just to the right of "V8 PFI" on the valve cover in the front of the engine bay. This less a motor mount and more of a torque controller.

    The best way to check is to actually be looking at the engine while it's revving....
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