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OK, I am going to separate this in to three separate issues...that is probably what they are. The RPM oscillation is likely a vacuum leak or some issue with the oil separator, possibly even a mass airflow sensor. What you need to do is check the vehicle for faults, and check the fuel trim TRA, and FRA numbers. That way you can determine if the car is running rich, lean or if the problem might be somewhere else. The Tiptronic needs a service PERIOD. Make sure that the shop OR you use the factory recommended fluid, and filter...and utilize a scan tool to reset the transmission adaptations. This will make sure that if the Tiptronic control unit has picked up some strange shifting characteristic over the years it will be reset and will hopefully not re-learn that hesitation between gears.
Is there any risk of coolant getting in to the gearbox, yes but you will see that when you do the transmission service. It will look like chocolate or strawberry shake coming out of the gearbox and you will have some external leakage as a result of the fluid level going above the max fill level due to the added coolant.
As for the corrosion, you might have a tough time saving those fasteners. Buy all new ones, and try the FILL plug first. There is no point in taking the pan off if you will not be able to refill the trans... SNAP-ON makes a bolt extraction socket set which should get the pan bolts off relatively easy. You might need to buy a new pan if the fill plug will not come out.
- Casey
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