98 STS misfires. Pretty badly. Unless it's raining.
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Cadillac Seville / Cadillac Eldorado Forum Discussion, 98 STS misfires. Pretty badly. Unless it's raining. in Past Cadillac Vehicle Discussion; My 98 STS is misfiring constantly. Does ok at constant highway speed, but under load it bogs right down. Very ...
  1. #1
    kjd03842 is offline Cadillac Owners Fanatic
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    98 STS misfires. Pretty badly. Unless it's raining.

    My 98 STS is misfiring constantly. Does ok at constant highway speed, but under load it bogs right down. Very rough idle to the point of stalling.

    When it rains, or the road is wet, it runs WAY better. Almost perfect while driving. Slight misfire at idle, might not even notice if I weren't paying attention.

    The only code is P0300-engine misfire. Go figure. I put in new plugs & wires recently. I have 4 new coils sitting in a box in my trunk, soon to be installed.

    The rain issue makes me think, grounding or other electrical. But it's still a guess.

    Any ideas?

    Thanks folks
    KD in NH

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    Zorb750 is offline Cadillac Owners Connoisseur
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Coil packs going bad can be aggrivated by high humidity. Never heard of the reverse though... Perhaps the sound of the rain masks the misfire?

    Anyway, plugs, wires, fpr, check for vacuum leaks, then coil packs, then throw the car away if it still doesn't work.

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    kjd03842 is offline Cadillac Owners Fanatic
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Quote Originally Posted by Zorb750
    plugs, wires, fpr, check for vacuum leaks, then coil packs, then throw the car away if it still doesn't work.
    Yeah I don't think it's the coils, I might return them, but there they sit...

    Definitely not rain masking the noise, the performance difference is significant. Like it's not even the same car.

    91k, don't think I'll throw it away just yet. But I might trade it in. On a rainy day.

    Thanks for the reply.

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    kjd03842 is offline Cadillac Owners Fanatic
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Oh yeah, I replaced the FPR and fuel filter recently too. And the fuel pump, but that's another story. I think.

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    Ranger is offline Cadillac Owners Member
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Pull the coils off and see if there is any signs of arcing on the underside. Listen to them with a stethescope while idling. If they are arcing internaly you should be able to hear it. As a last resort, borrow a coil from another car and swap them, one at a time to see if, and which one will make a difference.

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    kjd03842 is offline Cadillac Owners Fanatic
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Thanks Ranger

    I've got a scope, I'll give them a listen. I also have four coils ready to install, guess I've got nothing to lose by putting them in. Bought and paid for, probably had them too long to return by now.

    It's just this rainy day thing, why so much better I can't understand.

    Another idea, what's the MAP sensor's role in all this?

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    kjd03842 is offline Cadillac Owners Fanatic
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Last night I changed the coils and re-seated the map sensor, which may have been askew. It's running pretty good, but then again, it's still raining.
    There's still an occasional miss. I really don't think I've found it yet.

    Map sensor is $60, MAF is ??. EGR valve is probably in that mix somewhere too. This is getting costly, and aggravating. I may actually have to take it to a shop.

  9. #8
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Don't replace the EGR. Just clean it. See Tech Tips section for a "how To".

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    irebroff is offline Cadillac Owners Member
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Misfiring could be a bad injector wire,

    do you have a P0201, 0202 .. 3..4 .. 5.. 6.. 7...8 Code, too?

  11. #10
    kjd03842 is offline Cadillac Owners Fanatic
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    Re: 98 STS misfires. Pretty badly. Unless it's raining.

    Hi Irebroff, thanks for replying to this old post.
    No such codes. IIRC, the only one close was "misfire detected", I think it was p0301.

    I eventually followed up in another thread, it's pretty much back to normal now. EGR was the main, but not only, culprit.

    Thanks again
    KD in NH

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