engine surge at cruise speed

[Serg]

A new situation has presented itself; while cruising (not under acceleration)at a constant speed (not any one set speed) there is a variation in the revolutions per minute that the engine turns. It is apparent in the tach and it can be felt by my professional seat of the pants buttmeter. The variation is slight (between 100 and 250 rpm). I did a search for this issue but only found items related to idle and this is at speed. Any thoughts. Dirty MAP?, leaking vacuum line? ...

Thank you in advance.

BTW: 99 cab trip with about 70k

Edited July 5, 2011 by Serg

[whall]

Serg,

My guess is that what you are feeling/seeing is the torque converter "lock-up clutch" which locks the torque converter to minimise slippage.

If I understand the transmission's inner workings and hidden mechanisms properly, when you feel/see the decrease in RPM the the lock-up clutch is engaged; when you feel/see the increase in RPM the lock-up clutch is disengaged allowing the transmission fluid to again couple the engine to the transmission.

I'm sure this rather simplistic explination can be improved upon by the experts here at RennTech.

Bill

[Serg]

Serg,

My guess is that what you are feeling/seeing is the torque converter "lock-up clutch" which locks the torque converter to minimise slippage.

If I understand the transmission's inner workings and hidden mechanisms properly, when you feel/see the decrease in RPM the the lock-up clutch is engaged; when you feel/see the increase in RPM the lock-up clutch is disengaged allowing the transmission fluid to again couple the engine to the transmission.

I'm sure this rather simplistic explination can be improved upon by the experts here at RennTech.

Bill

I understand your explanation of specific reason for the on/off feeling, but this leaves the solution aspect of the question still hanging. Thanks though.

[valley996]

I have the same problem on my 99. I assume you have the tiptronic? I have replaced about everything that I thought it could be but I am getting ready to replace my torque converter. I believe others on here can tell you a similar story.

[valley996]

See if this helps any. Look at the video on page 2. It's a BMW but is is doing what mine is then look at the replies.

http://www.renntech....__surge__st__40

Edited July 9, 2011 by valley996