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RPM drop while driving on Highway

Discussion in 'E38 (1995-2001)' started by mcanha, Jan 25, 2011.

    • Member

    mcanha

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    I have 2000 740i w/sport package. Noticed today while driving at 70mph (approx 2200rpm) that the RPM would drop by approx 100rpm and recover. This occures continuously at 4 sec intervals. Car idles perfectly. Only at higher rpm's do I see this and feel a slight hesitation when it occurs.
    Has anyone else experienced this?
    Any thoughts?

    Thanks all for your inputs!
    Mike

    cwbiii guest

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    lockup

    My guess is that the lockup mechanism for the torque converter has an issue...
    or you've got some kind of vacuum leak issue.

    One way to check is to see if the transmission freewheels. If you let up slightly on the gas when the idle is higher does the vehicle coast or does the transmission
    try to slow the car down. It should try to slow it down rather than letting it freewheel.

    One quick fix (attempt) is to disconnect the battery(in the trunk) and push in a cigarette lighter to make sure any capacitive charge is drained down... then wait a few minutes before hooking it back up. This resets the transmission (and everything else) and the transmission will re-learn your driving habits for the next 30 or so miles of driving.

    I've had this correct some odd stuff... if it doesn't then you will probably have to plug it in to a service computer for a diagnosis.

    Chuck
    • Member

    mcanha

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    Thanks Chuck for the inputs. I'll disconnect the battery as you suggested and give that a try - easy enough. The transmission does not seem to freewheel as if I do let up on the gas it does try to slow the car down - at least what I can tell.
    Mike

    dmottram guest

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    Have the injectors checked. It might be one leaking.
    • Member

    mcanha

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    Just in case others have this similar problem - it turns out that the problem with mine was a bad torque converter - ugh. I'm debating now to have that replaced - shop is concerned if they pull the pan and drain the fluid if they see debris then a new tranny may need to be done. Big $$
    • Member
    • Technical Service Advisor

    mooseheadm5

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    There are many cheap E38s out there that need transmissions due to assorted failures causing it to go into limp mode. How much money is big money compared to a manual conversion?
    • Member

    CRKrieger

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    Completely unrelated. The ONLY WAY the engine speed can drop while the car continues at the same speed is a transmission issue.

    cwbiii guest

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    Not many other choices...

    With a bad torque converter you really don't have many choices... and the longer you wait to fix it the more likely the need to replace the tranny. It's quite a bit of money either way unless you have the knowledge to do it yourself. Just the lifetime tranny fluid change is big $... but don't skip it. At least dropping the pan to take a peek and see if it only needs a torque converter isn't all that much work... then you know if you can get away with only a torque converter. Or if you can find a reasonably priced used one as a standby then that can make it less expensive as well... but the transmission has to be dropped to replace the torque converter so there is a significant labor hit either way.

    Chuck

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