motor idles eratically at start and then dies
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Northstar Engines and System Technical Discussion Discussion, motor idles eratically at start and then dies in Cadillac Engine Technical Discussion; when started engine revs up once or twice on it's own and then dies. did a fuel pressure test- 40 ...
  1. #1
    t.nuernberg is offline Cadillac Owners Member
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    Angry motor idles eratically at start and then dies

    when started engine revs up once or twice on it's own and then dies. did a fuel pressure test- 40 lbs of pressure. Could the fuel pump be the problem or is it something deeper than that? 1993 sts . Ran fine until i let the fuel get low. Changed fuel filter, no difference.

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    t.nuernberg is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    help

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    Re: motor idles eratically at start and then dies

    Did you check the fuel pressure when cranking and when it started and stalled??? A static fuel pressure test is kind of useless as it takes very little for the pump to make pressure when there is no demand on the system.


    Did you check the fuel pressure regulator for leaking?? You will have to take the top cover off the intake manifold and energize the fuel pump (key on ...do NOT start with the silver cover off) and watch the vacuum nipple on the FPR for leakage. To remove the top cover take just the perimeter row of screws out of the cover...not the 4 recessed bolts.

    Have you cleaned the throttle body bore behind the throttle blade?? Possibly deposits are thick enough to cause the idle speed control to loose gain when starting.

    Try a test starting the engine where you force the throttle a few degrees...lightly depress the accelertor prior to cranking and hold it there while cranking. See if that makes it start better. If so, that may indicate a problem with the idle speed control motor not actuating the throttle correctly for start.

    And/or...hold the throttle wide open by fully depressing the accelerator before cranking and hold it there during crank to enable clear flood. See if that makes it start better. If so, it will indicate a rich condition for diagnosis.

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    t.nuernberg is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    We checked out and cleaned out an already clean motor and found no problems with carbon build up anywhere. We have however found these codes in the computer, po32,po71, and po95 and are all related to the map sensor. Could this be the problem with the eratic running of the motor? Thanks, we appreciate any help as we are new to the northstar.

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    Re: motor idles eratically at start and then dies

    Yep,

    The MAP Sensor inputs to the PCM help control the air/fuel mixture. I don't know if it would cause the engine to quit, but, sure could affect idle.

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    t.nuernberg is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    Quote Originally Posted by OlManRivah
    Yep,

    The MAP Sensor inputs to the PCM help control the air/fuel mixture. I don't know if it would cause the engine to quit, but, sure could affect idle.
    Thanks for the input,I will be changing out the map sensor today and let everyone know how it went. Thanks again.

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    t.nuernberg is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    Quote Originally Posted by t.nuernberg
    Thanks for the input,I will be changing out the map sensor today and let everyone know how it went. Thanks again.
    Changed map sensor and no change,know what?

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    Re: motor idles eratically at start and then dies

    Even though the MAP does aid in idle, your OBD-I diagnostics aren't as specific as the OBD-II.

    In your description, your fuel pressure was stated as 40 psi. That's lower than specs. Put a fuel pressure tester on the rail and monitor as the engine fails. I'm not sure about a '93, but I would think it should be around 47 or higher and maintain the pressure.

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    Re: motor idles eratically at start and then dies

    For the 1993 N* .. 40 PSI is dead on, charts show that if its under 40, replace fuel pump.. over 40 replace pressure regulator

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    Re: motor idles eratically at start and then dies

    Does the fuel pressure hold steady when you apply a load (brake torque) or does it drop? It should hold relatively steady.

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    The Director is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    Damn...same thing happened to me today. I was driving and all of a sudden it starts to hesitate (I can also feel my gas pedal move down and then up repeatedly) for a while then the engine stops. So I put the car on neutral and pull off to the side of the road. I crank the engine, it starts for like two or three seconds and dies. I kept on trying to start it several times but it wouldn't stay on (btw- I heard a misfire somewhere around the spark plugs area). So I let it sit for about 20 minutes and tried again...this time it starts and stays on with out hesitation and I drove home without a problem. Now I'm afraid to drive it now until I get whatever is wrong fixed. Somebody please HELP ME OUT. I also have a 93 STS.

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    zonie77 is offline Cadillac Owners Connoisseur
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    Re: motor idles eratically at start and then dies

    How full is the gas tank when this happens? Does it only happen with a low tank?

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    Re: motor idles eratically at start and then dies

    Quote Originally Posted by t.nuernberg
    when started engine revs up once or twice on it's own and then dies. did a fuel pressure test- 40 lbs of pressure. Could the fuel pump be the problem or is it something deeper than that? 1993 sts . Ran fine until i let the fuel get low. Changed fuel filter, no difference.

    Alright, so the problem started after the gas got low and has been that way since? how low are we talking?

    The Reving up after engine start makes me think of ISC motor. (personal experience) .. the 'after running low on gas' makes me think a clogged fuel injector. I have the service manual.. can help if i know what to look for.

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    The Director is offline Cadillac Owners Member
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    Re: motor idles eratically at start and then dies

    Nah, This happened while running on a full tank.

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    Re: motor idles eratically at start and then dies

    When I stated the fuel pressure is below specs, I was speaking of a 97 Northstar. I wouldn't have thought they would change that much in components from 93 to 97. Even my 99 Jeep I6 wants 50+.

    But, I have seen stranger. Didn't mean to mislead you.

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