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Thread: Engine knocking upon startup

  1. #1
    REDELDO is offline Cadillac Owners Member
    Automobile(s): 97 Eldorado, 01 Deville
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    Engine knocking upon startup

    Help....My 97 eldo with northstar engine has been towed to the dealership. I did not drive the car Sunday and when I started it up Monday morning it had a knocking noise and a very rough idle. I cut the car off immediately, waited a few minutes and started it again. After a few minutes of idle it smoothed out but when I gave it gas there was a high pitch knocking noise. When tow truck arrived 2 hours later I cranked the car again and there was a hesitation, it turned over a couple of times before starting. Again the edle was rough and the knocking was there. I've noticed for the past couple of weeks that there is a slight stick when pressing the gas pedal. Also, when accelerating it seemed to have a slight lose of power (almost like the gas flow was interrupted briefly). May not be related. What can it be?


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  3. #2
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    Sticking pedal is a dirty TB. Just needs a simple cleaning. Knocking on start up sounds like "cold carbon rap". Needs to be taken out and flogged aka WOT. Have you had the fuel rail recall done? A misfire afterwards is not uncommon. It's caused by moving the old wires around. New A/C Delco wires usually solves the problem. May as well do the plugs as well as it is probably due. Do a search as all of the above have been discussed before in great detail. None of it sounds serious.

  4. #3
    REDELDO is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    Thank you Ranger. The Dealership is slammed this week and it's taking forever. Last update I got they said black smoke was coming out of the exhaust so they put some sort of solvent in and will let it set for 2-3 hours before cranking again. If it runs smooth after that they will keep another night to be sure it's ok when cold. I have my fingers crossed. I will respond with the results once I hear.

  5. #4
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    Go to the top left of this page, click on Technical Archives and read up on oil consumption & full throttle accereration. That would be a cheaper, easier and more fun fix.

  6. #5
    FredMaxwell Guest

    Re: Engine knocking upon startup

    Seafoam it and drive the hell out of it a little!!

  7. #6
    REDELDO is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    OK....this is the diagnosis from the dealer. After letting the solvent sit for a few hours they blew out the engine which produced a good bit of black smoke. They held it over night and itís running smooth on a cold start up. Still has a slight knock upon start up and I'm being told it's a piston slap that goes away when it's warm and is not anything to worry about. Total cost with oil change is $180 plus towing. Ranger....you were on the mark with the "cold carbon" diagnosis, awesome. Any thoughts on the piston slap, should I use something like a slick 50???

  8. #7
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    Piston slap aka cold carbon rap. Slick 50 is snake oil. Just give it a regular dose of WOT once a week or two.

  9. #8
    JimHare's Avatar
    JimHare is offline Cadillac Owners Connoisseur
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    Re: Engine knocking upon startup

    Any thoughts on the piston slap, should I use something like a slick 50???

    Ackkk!!!! Ok, we'll let ya slide cause you're new here. Our departed and sadly lamented technical guru advised against adding ANYTHING to your Northstar except Gasoline, Oil, and 50/50 Dexcool.

    the "Piston Slap" is probably just the "cold carbon rap"...a few Wide Open Throttle adjustments should cure it.

  10. #9
    REDELDO is offline Cadillac Owners Member
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    Re: Engine knocking upon startup

    Will do....thanks to all for the info. Since I need to give my car hell this weekend, anyone up for a race

  11. #10
    FredMaxwell Guest

    Re: Engine knocking upon startup

    Don't forget to:
    Seafoam it and drive the hell out of it a little!!

    All sources so far say that Seafoam makes short work of carbon!


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