Intermittent Lurch—91 DeVille 4.9
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HT4100 4.1, 4.5, 4.9 Discussion, Intermittent Lurch—91 DeVille 4.9 in Cadillac Engine Technical Discussion; ...
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    gdwriter's Avatar
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    Intermittent Lurch—91 DeVille 4.9

    I have been having a very intermittent stumble or lurch for the past couple of months. It had gone away for a couple of weeks, but now it's happening again.

    Although the conditions vary, it happens most often at between 40 and 55 MPH when I'm coasting with my foot off the gas or immediately when I put my foot back on the gas and start to accelerate after coasting. The car will cough, stumble or lurch a couple of times, then it runs fine again.

    I don't believe it's the lockup torque converter. I've run the car in diagnostic mode when it happens and the lights indicating it's in 4th gear and the VCC are engaged stay on. Also, I had the transmission rebuilt last summer.

    I'm thinking it's ignition related. I checked for codes yesterday and got these (not sure which ones are history and which ones are current):

    E30 - ISC RPM Out Of Range
    E41 - Cam Sensor Signal Problem
    E55 - Closed Throttle Angle Out Of Range
    E98 - High RPM P/N to D/R in ISC Range

    I've already replaced a number of ignition/tune-up parts. Here's the rundown:

    7/05: Tune up—new plugs, wires, cap and rotor
    10/05: New fuel injectors (Bosch from 5-0 Motorsports)
    11/05: New camshaft position sensor (hall effect switch), ignition control module and coil
    12/06: New ISC motor

    I put in the new ISC motor myself after getting the E30 code repeatedly when I'd idle at longer stoplights. However, I could not figure out how to adjust for the new ISC motor as described in another thread.

    Any clues as to what the problem might be? Since it's so intermittent, I don't want to take it to a mechanic until I have a better idea what the problem might be.

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    Disturbed_One is offline Cadillac Owners Member
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    Re: Intermittent Lurch—91 DeVille 4.9

    I too am having ISC troubles and have the E30 code.
    I just hope I dont end up having to replace it

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    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Intermittent Lurch—91 DeVille 4.9


  5. #4
    gdwriter's Avatar
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    Re: Intermittent Lurch—91 DeVille 4.9

    Quote Originally Posted by behind-bars View Post
    Thanks! That's helpful. I probably won't have a chance to try it until this weekend, but once I do, I'll post the results.

    Damn, I love this place.

  6. #5
    behind-bars is offline Cadillac Owners Connoisseur
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    Re: Intermittent Lurch—91 DeVille 4.9

    Quote Originally Posted by gdwriter View Post
    Thanks! That's helpful. I probably won't have a chance to try it until this weekend, but once I do, I'll post the results.

    Damn, I love this place.
    Ive been meaning to do this aswell, mine does lurch as much like yours, but mine it will not decelerate when i take my foot off the gas lol, Idling at 25 is annoying.

    Maybe I can do it this weekend aswell when I flush the power steering fluid.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Intermittent Lurch—91 DeVille 4.9

    Sounds like your EGR tubes are plugged or your throttle butterfly valves are carboned...your EGR valve may also be stuck. Your TPS and idle screw may also be out of adjustment. First, push up on the bottom of the valve and test if it moves smoothly with a slight amount of resistance. If not, replace the valve. If the EGR valve is not fully closing at idle, your computer will try to compensate by raising the ISC motor. The clunk you feel is most likely the fuel cutoff after your foot is off of the gas for about 2 seconds. Because your idle speed is out of whack the fuel cutoff is happening while the rpm is low, but the computer is feeding the engine more fuel than normal to keep the idle rpm up. When the fuel cuts out, this creates a more dramatic drop off.

    One of the above issues is most likely your problem.

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    gdwriter's Avatar
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    Re: Intermittent Lurch—91 DeVille 4.9

    Well, I ran the diagnostics to check my idle speed driving home last night, and the ISC definitely needs to be adjusted. At a stoplight in Drive, it's idling at 800-850 RPM. When I put it in Park, the idle jumped to 950-1050 RPM.

    Quick question. Is the torx nut on the plunger of the ISC motor the same thing as the minimum air screw? If not, can somebody take a picture or will it be obvious when I pull off the air cleaner to access the throttle body?

    Thanks for the tip on the EGR valve. I'll check that, too.

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    Re: Intermittent Lurch—91 DeVille 4.9

    Quote Originally Posted by gdwriter View Post
    Well, I ran the diagnostics to check my idle speed driving home last night, and the ISC definitely needs to be adjusted. At a stoplight in Drive, it's idling at 800-850 RPM. When I put it in Park, the idle jumped to 950-1050 RPM.

    Quick question. Is the torx nut on the plunger of the ISC motor the same thing as the minimum air screw? If not, can somebody take a picture or will it be obvious when I pull off the air cleaner to access the throttle body?

    Thanks for the tip on the EGR valve. I'll check that, too.
    I do not think your ISC motor plunger is torx. Read up this thread, you'll find the pictures of home-made ($0.01 plus tax) and commercially available inexpensive ($4.00 or so)tools you need for the job.
    http://www.cadillacforums.com/forums...tml#post940618

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    Re: Intermittent Lurch—91 DeVille 4.9

    If your idle is that high, your ISC motor may be stuck or inoperative. Make sure to go through the TPS adjustment procedure and idle screw adjustment procedure.

    If you engine is idling too high when the fuel cutoff takes place, and the torque converter is locked up, you will definitely feel a big lurch.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Intermittent Lurch—91 DeVille 4.9

    To check if we've indeed pinpointed the problem, try this: with the car warmed up, accelerate to about 50 mph (or whatever spped you feel the lurch) and lift your foot off the gas (no need to do it abruptly). Make sure you are doing this coasting downhill. After about 2 seconds you should feel a lurch.

    Now, repeat the same test, but after you release the gas pedal, apply a slight amount of pressure to the brake pedal to disengage the TCC...you should not feel a clunk.

  12. #11
    MachX's Avatar
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    Re: Intermittent Lurch—91 DeVille 4.9

    Last I checked, the torx screw on the ISC is non adjustable. If you look, there is also a little placard under your hood that explicitly claims that there is no provision for adjusting your idle speed/mixture or something along those lines. Your idle speed issues are from another problem which needs to be diagnosed. If your idle is that high, it could be flooding your engine and giving you the lurches.

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    awadecki is offline Cadillac Owners Enthusiast
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    Re: Intermittent Lurch—91 DeVille 4.9

    The ISC plunger is absolutely adjustable...just because you move the plunger doesn't mean that you change the idle speed (it's set at a fixed TPS and RPM). Nonetheless, you need to have a gap between the ISC and the throttle stop screw when its fully retracted.

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    Re: Intermittent Lurch—91 DeVille 4.9

    my throttle stop screw is locked into place, if yours isn't, it's been broken loose by somebody at some point. the point of electronic engine controls is to automatically adjust your ISC plunger to the correct idle speed. if you bypass that by breaking loose the throttle stop screw, you are bypassing what is another problem somewhere in the engine.

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    Re: Intermittent Lurch—91 DeVille 4.9

    Quote Originally Posted by MachX View Post
    my throttle stop screw is locked into place, if yours isn't, it's been broken loose by somebody at some point. the point of electronic engine controls is to automatically adjust your ISC plunger to the correct idle speed. if you bypass that by breaking loose the throttle stop screw, you are bypassing what is another problem somewhere in the engine.
    "the throttle stop screw" is called minimum air screw. The original question was quite confusing..."Is the torx nut on the plunger of the ISC motor the same thing as the minimum air screw?" that's the reson the following discussion did not make much sense.

  16. #15
    MachX's Avatar
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    Re: Intermittent Lurch—91 DeVille 4.9

    aye, but it seems that he is wanting to 'turn down' his idle speed, which isn't possible as the screws on the throttle stop, and ISC are non adjustable. the only screw there that IS adjustable is the minimum idle screw, which never actually does anything unless your ISC completely fails and collapses, in which case the car will at least run until you replace the ISC. To answer the original question, if his idle is that high, he should try doing an idle speed re-learn, and barring that maybe make sure his ISC is retracting like it's supposed to.

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