HT4100 Misfire Question
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HT4100 4.1, 4.5, 4.9 Discussion, HT4100 Misfire Question in Cadillac Engine Technical Discussion; Hi everyone-- So my car (85 Deville with 32,000 miles) runs perfectly now for the most part except an issue ...
  1. #1
    awadecki is offline Cadillac Owners Enthusiast
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    HT4100 Misfire Question

    Hi everyone--

    So my car (85 Deville with 32,000 miles) runs perfectly now for the most part except an issue that still continues to plague it that I've finally been able to rule out a few issues as the culprit:

    Basically, my car runs just fine now around town and at anything up to about 3/4 throttle and 3500 rpms. After that, if I really lay into it, I'll get popping in the intake, like an exhaust valve is sticking or not sealing. There's no ticking ever, so I don't think it's a lifter...but what do you think? Bad valve spring? Rocker arm pivot? Bent pushrod? Flat cam?

    I have an early 85 model with the more flimsy rocker arm pivots so I was wondering what the symptoms are of a car with such a problem.

    As I said, the car has good power up to about 3500 rpms, but the compression is low on one particular cylinder (#7).

    Timing is spot on (jumps around a bit because of the low compression, but not much), all new ignition system (the entire thing--distributor, plugs, wires, etc.).

    Any opinions?

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  3. #2
    NickySantoro is offline Cadillac Owners Fanatic
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    Re: HT4100 Misfire Question

    First guess would be fuel supply issues. Fuel pump output? fuel pressure regulator, fuel filter ?

  4. #3
    carnut is offline Cadillac Owners Master
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    Re: HT4100 Misfire Question

    I agree, the fuel filter is probably original! if an engine runs lean, it will pop thru the air intake. Change the filter. You can verify if the filter was plugged by simply blowing thru it once removed.

  5. #4
    awadecki is offline Cadillac Owners Enthusiast
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    Re: HT4100 Misfire Question

    I already changed the fuel filter when I got the car and the pressure is always a good 12 psi. It's definitely a mechanical issue. Both injectors work perfectly too.

  6. #5
    NickySantoro is offline Cadillac Owners Fanatic
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    Re: HT4100 Misfire Question

    Does it exhibit the same symptoms just revving in neutral or do they appear only under load?

  7. #6
    awadecki is offline Cadillac Owners Enthusiast
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    Re: HT4100 Misfire Question

    Good question. If I really rev it in neutral, it'll do it too. It's definitely some mechanical issue...catalytic converter has also been changed, so it's not a back pressure issue.

  8. #7
    NickySantoro is offline Cadillac Owners Fanatic
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    Re: HT4100 Misfire Question

    Does it appear worse under load or begin at lower RPMs?

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