intermittent miss and other weird happenings
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HT4100 4.1, 4.5, 4.9 Discussion, intermittent miss and other weird happenings in Cadillac Engine Technical Discussion; Hello everyone, I have a weird problem that has just started recently I have a 1983 deville with the ht4100 ...
  1. #1
    partyharty is offline Cadillac Owners Member
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    intermittent miss and other weird happenings

    Hello everyone, I have a weird problem that has just started recently

    I have a 1983 deville with the ht4100 engine in it. The problem is that it just started developing a miss in the lower rpm ranges. This miss is intermittent (it only happens about 1/3 of the times that we have driven it). The only time we notice it is when we are sitting at a stoplight (the car shudders almost like someone is holding the gas and the brake at the same time). A few minutes later you can never tell there is a problem.

    I am leaning towards a sensor problem because this problem is intermittent and the problem started after we had a alternator die. When the alternator "died", I believe it sent a surge through the system (my wife was driving and said every light came on on the dash and it started acting "weird" (whatever that means)).

    The alternator killed the battery so the car now has a new battery and a new alternator on it. The miss started after the new battery and alternator was installed. I have checked and the belt to the alternator is not too tight (nor is it binding up the engine that I can see).

    So does anyone have any idea's what this can be? I would appreciate the help. I have worked on vehicles in the past however I have never worked on a cadillac.

    Thanks,
    Chris

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  3. #2
    carnut is offline Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
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    Re: intermittent miss and other weird happenings

    This engine was prone to plug wires being mis routed and shorting on exh. manifolds. With it running spray a mist of water on the wires and see if it develops a miss. Also an over tightened belt will never 'bind" up an engine. Just the opposite, This year of car was noted for the smog pump belt being too loose causing the alternator belt to slip causing weak batteries and misdiagnosed alternator failure! With the engine off, by hand, try to rotate the alternator pulley. If it rotates then the smog pump belt is loose. It must be tightened from under the car. Also a cracked insulator on a spark plug can do this as well. Sometimes holding the brake pedal firmly and slowly adding throttle will produce the miss which helps verify plug wires or cracked spark plug.

  4. #3
    partyharty is offline Cadillac Owners Member
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    Re: intermittent miss and other weird happenings

    Thanks for the response.

    The alternator was dead (pretty much as dead as it could be). The backplate on the alternator was so hot that it was tough to pull off almost six hours later . We put it in a box and took it in and got it tested (autozone), It failed all of the test impressively :>. I also checked once we put the new battery on and it wasnt putting out much (if any) juice. With the new battery on (car running) I tested 12.2v with a multimeter.

    I will check the plug wires when it gets dark. I am wondering if the coil may need to be replaced (obviously its not dead yet but it is a cheap fix and would rule it out).

    This is our daily driver. It has 62,000 original miles on it (and its an 83). This just came up out of the blue so I was leaning towards something we may have done while changing out the alternator or something that the alternator did just prior to its death).

    Thanks,
    Chris

  5. #4
    carnut is offline Cadillac Owners Master
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    Re: intermittent miss and other weird happenings

    With car running, you should see 13 to 14 volts at the battery terminals. Car off 12.5 to 13 across the terminals. The true test of an alternator is how many amps it produces under load. Load tested, it should be able to produce 50 to 60 amps. A cracked plug wont always show up in the dark.

  6. #5
    partyharty is offline Cadillac Owners Member
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    Re: intermittent miss and other weird happenings

    I posted the multimedia readings to support the fact that the old alternator was dead (hence the 12v reading with a new battery installed). It now puts out between 13.2 and 13.8v with the new alternator installed.

    They load tested the bad alternator at Autozone (they have a machine to do it). Like I said before, the alternator was toast.

    I will check out the plugs and wires tonight (I will see if I can find some arcing tonight and I will go from there). I would think a cracked plug would be unlikely (what will crack a plug?), This started when the alternator died (the car worked fine prior to that, the only indication we had that anything was wrong was the charge light came on for a few seconds the night before and then all of the lights came on as my wife was driving it). It is also very intermittent (I would think physical damage (cracked plug) would do it all the time). But then again I have been wrong before.

    Thanks,
    Chris

  7. #6
    carnut is offline Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
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    Re: intermittent miss and other weird happenings

    Cracked insulators usually happen upon installation. The socket hits the side and cracks it. Again, that reason for the miss is rather remote. Plug wires are very common. Does this miss happen as you are just about to come to a complete stop or as you are sitting still waiting?

  8. #7
    partyharty is offline Cadillac Owners Member
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    Re: intermittent miss and other weird happenings

    we only notice it when we are sitting still (like at a stoplight/stop sign). I do not notice it any other time (WOT, etc). It is very rythmic regardless of the rpm's (you can sit still and rev the engine and the miss stays the same, this tells me it probably is not the spark plugs or wires as I believe if it was them they would speed up/slow down with the engine).

    We did get to looking today and the pipe between air cleaner and the part underneath (almost looks like an exhaust hookup) was not together. I put it back together and we didnt have any problems with it. I am unsure that this has corrected the problem (as it only did it 1 out of every 3 times we drove this car (approx)). Then again I am not sure what this part was (it is corrugated pipe (almost looks like dryer vent pipe) and it hooks between the arm of the air box and a exhaust looking port underneath.

    One thing is for sure, I have got to get a chiltons or a haynes manual for this thing. :>

    Thanks for the help and if you can think of anything else, by all means let me know (I appreciate the help).

    Thanks Again,
    Chris

  9. #8
    carnut is offline Cadillac Owners Master
    Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
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    Re: intermittent miss and other weird happenings

    The pipe you refer to simply provides warmer air upon warm up to atomize the fuel and prevent ice forming in the throttle body. Has nothing to do with the "miss". I think you might be describing something out of balance driven by one of the fan belts. Try using the car without the A/C compressor on for a few days to compare.

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