HT4100 Running BAD when warm.

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HT4100 4.1, 4.5, 4.9 Discussion, HT4100 Running BAD when warm. in Cadillac Engine Discussion; ...
  1. #1
    472's Avatar
    472 is offline Cadillac Owners Fanatic
    Automobile(s): 55 60S, 81 DeVille (Diesel), 83 Coupe (507), 84 Wood (425)
    Join Date
    Dec 2004

    HT4100 Running BAD when warm.

    Hey folks, I recently acquired a 1984 Fleetwood Brougham with the HT4100; on the bright side it only has 82,000 miles. It sat for about 5 or more years, but it fired up and I drove it about 20+ miles home. I checked the coolant anticipating a flush but it looked perfect, so I only changed the oil.

    It ran decent for about 50 miles, but on its last trip it barely made it home; it got progressively gutless, sounded like it was firing on about 2 cylinders, sputtering and popping, and under the hood it sounded like a gang of typewriters clanging away. So I let it sit for a couple days, and then checked the oil which was fine so I started it up again. This time the motor was completely quiet, a little rough and the “Service Soon” light came on (which went away with some throttle).

    I pulled the code through the ECC and got two TPS errors. I checked the voltage at the TPS and it was way high, so I adjusted it (it appears to have been “adjusted” recently). After adjustment, it ran great! But after about 2 miles of driving SAME problems; popping, backfiring, sputtering, and barely limped it home. I would think “no problem, new TPS is in order” but it sounded BAD under the hood; the same “dry” sounding commotion from the motor.

    I let it sit for about an hour rechecking the TPS values which seemed high again and took it for another test drive. At first it was good; ran right, no noisy engine, until I drove it about a mile when the noise returned, and soon after it ran rough again.

    So my guess is that it’s overheating (thermostat stuck) AND a bad TPS? Or bad O2 that gives bunk readings once warmed up? I think it may be a combination of things due to the mixed symptoms of really noisy engine with the sputtering and popping, YET fixable since they only pop up once warmed up.

    I appreciate the help and don’t want to lose another Fleetwood to the 4100! Please let me know what you think.


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  3. #2
    Melrose is offline Cadillac Owners Fanatic
    Automobile(s): 1999 Deville, 1987 Eldorado
    Join Date
    Apr 2005
    North Carolina

    Re: HT4100 Running BAD when warm.

    See my post under HT4100 Backfiring

  4. #3
    awadecki is offline Cadillac Owners Enthusiast
    Automobile(s): 76 Eldo (54k), 76 Eldo (52k), 78 Eldo (9k), 85 Deville (43k)
    Join Date
    Jun 2005

    Re: HT4100 Running BAD when warm.

    This is a toughie. I'd start with the thermostat (cheap) and 02 sensor (cheap and easy). 02 sensors are only read by the computer when the engine is hot, so this makes sense. You may also want to check your fuel pressure to see if it drops off as the engine warms. You could try a new TPS too if it appears to keep going out of spec. Aside from that, I'm clueless.

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  6. #4
    peteski is offline Cadillac Owners Enthusiast
    Automobile(s): 85 Eldo Biarritz
    Join Date
    Dec 2003
    Northeast USA

    Re: HT4100 Running BAD when warm.

    As far as overheating goes, you can display the coolant temperature (and lots of other parameters) while in diagnostic mode. So instead of guessing, just check and see what coolant temperature the car shows. You can also display and check the TPS angle value. Another LED indicator shows if the car is in a closed loop mode. If that corresponds to the rough running then the O2 sensor could be bad (as the O2 sensor only gets used while in closed loop mode - when the engine warms up).


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