1985 Eldorado running rough at sustained high speeds

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HT4100 4.1, 4.5, 4.9 Discussion, 1985 Eldorado running rough at sustained high speeds in Cadillac Engine Discussion; I have a '85 eldo with 135k miles on it. I picked it up with 76k miles in 2010. A ...
  1. #1
    twopanjim is offline Cadillac Owners Member
    Automobile(s): 1985 Eldorado,1991 Sedan Deville
    Join Date
    Aug 2014

    1985 Eldorado running rough at sustained high speeds

    I have a '85 eldo with 135k miles on it. I picked it up with 76k miles in 2010. A major tune up at 86k miles plugs,wires, rotor cap,etc. Leaks about a quart of oil between changes. Just replaced the fuel pump and sending unit about 6 months ago.

    I can start the car without touching the gas pedal. It idles fine. I drive it to work about 36 miles of which 24 miles are interstate. Recently, after driving to the highway (10 miles) and about 10 miles on the interstate the car starts a slight vibration above 60 mph. If I accelerate it feels like it is missing. At steady 70mph it feels like it surges and vibrates, if I keep the speed below 60 the vibration and surging stops but the acceleration is still bad. when I get off the interstate, the engine runs rough and accelerates rough for a number of miles, then finally settles down again. This also happens on the return home portion. About 6 months ago a new temperature sensor and new oxy sensor were installed when the check engine light came on. No error codes currently at this time. Just trying to give as complete a picture as I can. Thanks for taking the time to read this long winded narration. Any ideas what is causing this and how to fix it would be greatly appreciated.

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  3. #2
    omston is offline Cadillac Owners Fanatic
    Automobile(s): 1985 Seville
    Join Date
    Jan 2014

    Re: 1985 Eldorado running rough at sustained high speeds

    Could be the tcc solenoid sticking do you have any error codes?


    Yeah sounds like a trans issue to me have you ever serviced it?

  4. #3
    carnut is offline Cadillac Owners Master
    Automobile(s): 1999 BMW 740 Il. 2005 Jag XK8, 1988 Mercedes Benz 300 SE
    Join Date
    Feb 2003

    Re: 1985 Eldorado running rough at sustained high speeds

    Do the same drive in 3rd gear. If it doesn't occur then you need to replace the TCC solenoid in the transmission valve body. Somewhere between 46 to 54 MPH the torque converter is locked up to improve mileage. If it locks up but refuses to unlock these issues you describe happen. Its an additional 15 minute job above changing the trans filter and pan gasket. There are many Cad's running around with the wire harness to the trans unplugged to keep this from happening, but thats not a fix, thats a "redneck" repair!

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